November, and the same two hollows fill again
A back lawn behind a 1970s estate house: sixteen metres from the patio to the bottom hedge, eleven across, falling away from the building by perhaps a quarter of a metre over its whole length. From May to October it is a lawn. By the second week of November there is standing water in the same two hollows as last year, footprints fill as fast as they are made, the sward has gone over to annual meadow-grass and moss, and soft rush has thickened along the bottom fence where nobody mows. The client photographs it in January, gets three quotes in February, and expects a machine on it in March — the worst possible month to drag anything tracked across saturated clay.
The reflex quote is a scheme: laterals, a main, stone-filled trenches, a soakaway in the bottom corner. It is right often enough to get written without checking. But a large share of gardens that stand in water have no missing pipe at all. They have a lid — forty to a hundred millimetres of dense, structureless soil in the top spit, left by a rotavator run in wet ground, by turf laid onto a builder's scraped subgrade, or by a tracked machine that came through during the extension. The water is not failing to leave the site; it is failing to get down the first two hundred millimetres, and a perforated pipe six hundred below that will spend its life dry.
So the fork comes before the takeoff. Is the water sitting on something, or has the ground beneath it filled up? The two look identical from the kitchen window, cost different orders of magnitude to put right, and are separated by an afternoon with a spade and an auger plus a week of leaving a hole alone. Every number further down this page assumes that question has been answered, and answered in a wet month, because a hole bored into cracked clay in August tells you nothing about February.
Compaction or a water table: the hole decides it
Cut two slots, not one — three hundred millimetres deep in the worst of the standing water, and a second in ground on the same lawn that stays walkable. Every difference between the two faces is a candidate and every similarity is eliminated. Cut them after rain rather than in a dry spell; you are reading where the water went, and there has to have been some. Lever the slot open for a clean vertical face and read it downward: where the roots stop, where the colour changes, where the spade suddenly worked harder.
Colour is the part most people walk past and the most reliable evidence on site. Soil saturated for long periods loses its iron oxides and turns grey or blue-grey; soil saturated only seasonally develops ochre mottling along old root channels against a greying matrix. The depth at which mottling begins records how high the water has stood, written over years rather than over one wet fortnight, and it is described formally under redoximorphic features in the USDA Natural Resources Conservation Service Field Book for Describing and Sampling Soils. A profile uniform brown all the way down, no grey and no mottle, but which the spade rings against at a hundred and fifty to two hundred and fifty millimetres, says the opposite: the water has never been down there because it has never got down there.
Then bore a hole and leave it. A hand auger takes a seventy-five to hundred-millimetre bore to a metre or so in most garden soils. Sleeve it with slotted pipe, cap it against pets and children, bail it dry and go away for a week of ordinary winter weather. A hole that comes back overnight and holds a level is a genuine water table within reach of the turf, and that level is the number the rest of the design answers to. A hole still bone dry while the surface is a puddle is perched water on a pan, and no pipe beneath the pan will ever see it. An afternoon and a week of patience separate a two-day aeration job from a scheme with a machine, ten tonnes of muck away and a consented outfall.
Percolation testing gets reached for here and answers a different question. The falling-head test in a trial pit set out in BRE Digest 365 is a soakaway design method: it measures how fast subsoil accepts water at the depth a soakaway's base will sit. It gives no lateral spacing and is no substitute for watching a bored hole through a wet week.
| What the ground shows | The fault it points at | What it changes about the job |
|---|---|---|
| Uniform brown to 300 mm with a dense platey band at 150-250 mm the spade rings against | A cultivation or traffic pan holding water perched above it | Break the pan first; a lateral laid under it will never be reached by the water |
| Grey or blue-grey subsoil with ochre mottling along old root channels from about 400 mm | Seasonal saturation up to roughly the depth the mottling starts | Set laterals at or just below that horizon rather than as deep as the machine will go |
| Cased hole bailed dry, standing at 350 mm again the following morning | A working water table within reach of the rootzone | There is something for the pipes to drain; the outfall level now governs everything |
| Cased hole still dry after a week while the surface holds water | Perched water with no table beneath it | The purchase is aeration and surface fall, not pipe, stone and a machine |
| Rush, sedge and creeping buttercup thickening along one line only, not across the lawn | A spring line, a collapsed old field drain, or a leaking service | Trace and prove that line before laying anything across it |
When the fault is a pan, the trencher is the wrong purchase
Pans are made, not inherited. The usual culprits are a rotavator run on wet clay, which slices a glazed floor at the bottom of its arc and packs the soil above it; turf rolled out over a subgrade the housebuilder scraped flat and never lifted; and years of mowing and playing at the same moisture content. The pore structure is destroyed rather than merely squeezed, and destroyed structure does not recover on its own under a permanent sward, where there is no cultivation and little frost heave to help it.
The remedies run in a clear order of cost. A deep-tine or vertidrain machine heaves below the pan without lifting the surface — the least invasive thing that works, and a repeat treatment rather than a cure. Hollow coring, cores removed and sand brushed into the holes over successive autumns, slowly builds vertical routes through it. A winged subsoiling tine shatters a pan across a whole garden in an afternoon where a machine can get in and nothing is buried in the way, at the price of a mess that takes a season to settle. Where the pan is thick and the soil above it is fifty millimetres of imported skim, the honest answer is a re-profile.
The two faults also live together, and that is where pipe schemes earn their bad name. With both a pan and a table, laterals alone underperform badly because water still cannot cross the pan to reach the stone. The answer used on every clay-built sports pitch is secondary drainage: sand slits or gravel bands cut at close centres across the line of the laterals, connecting the surface directly to the stone below. USDA NRCS Conservation Practice Standard Code 606, Subsurface Drain, does not prescribe that pairing, but it sets the condition the pairing exists to satisfy: the practice applies to areas suitable for the intended use after the other required conservation practices have been installed, and the soil must have enough depth and permeability to permit an effective system in the first place. A scheme quoted without that, on a garden whose slot face shows a pan, will disappoint in its first winter and be blamed on the installer.
Setting out laterals, and why the spacing is not a lookup
Geometry first. Laterals run down the line of steepest fall so each one cuts across the direction water is moving; the main runs along the bottom, across the fall, and takes them all to one outfall at the low corner. Where a garden falls in one clean direction that gives a parallel layout; where it falls into a valley or a corner it gives a herringbone, laterals coming in at forty-five to sixty degrees from both sides. The layout is decided on the ground with a laser and a bag of pegs, in winter, with the water showing you where it wants to go.
Spacing is where the trade wants a number and the physics refuses to supply one. Spacing, depth and the soil's hydraulic conductivity are bound together; the steady-state relationship every drainage handbook works from is Hooghoudt's, set out with the rest of the method in the USDA NRCS National Engineering Handbook, Part 650, Engineering Field Handbook, Chapter 14, Water Management (Drainage). Deeper drains may be spaced further apart because a deeper drain draws down a wider cone — but the conductivity term collapses in clay, which pushes you the other way, towards shallower laterals at closer centres. Working ranges in garden and sports work land around three to five metres apart at four hundred and fifty to six hundred deep in a heavy clay, and five to eight in a loam. Those are trade practice figures, not a published limit; BS 4428, the code of practice for general landscape operations excluding hard surfaces, governs the workmanship around them.
Set the layout out on the ground and the schedule falls out of it. On the lawn above, three laterals at four-metre centres running fifteen metres down the fall is forty-five metres of pipe, the main across the bottom eleven, the run to the outfall another three: fifty-nine metres. Eighty and hundred-millimetre corrugated perforated coil comes in twenty-five or fifty-metre lengths; rigid stick is typically three metres, and the American equivalent, three to six-inch corrugated polyethylene tubing, is specified under ASTM F405. Count fittings separately: one junction per lateral, a bend at the low corner, and a rodding eye brought flush to the surface at the head of the main and at every change of direction. Land drains silt, and a scheme with no access gets dug up rather than jetted.
Total the laterals, the main and the outfall run as one length, then set the segment field to the length your merchant stocks, a stick or a twenty-five or fifty-metre coil alike. The answer is what goes on the order, before junctions and rodding eyes are added by hand.
The total perimeter length of the foundation to be drained.
The stock length of one pipe segment as sold.
Pipe segments needed
15 segments
They open the calculator with your figures already in it
Perimeter Weeping Tile Pipe Segment Calculator: 15 segments — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The perimeter asked for is a single number, and the pipe circuit is not the wall line. Weeping tile is laid outside the footing at the level of its underside, so on a rectangular plan the circuit is longer than the wall perimeter by roughly eight times the offset from the wall face. A 250 mm offset adds about 2 m to a 45 m foundation, which is the difference between 15 segments and 16.
- One rounding is applied to the whole circuit, which assumes the pipe runs as a single unbroken line. Rigid pipe is cut at every corner and at the tee to the sump, and each cut leaves an off-cut usually too short to start the next side. A 45 m perimeter taken as four 11.25 m sides needs four 3.05 m segments per side, so sixteen rather than the fifteen the single division returns.
- Nothing here is hydraulic. Pipe diameter is not an input, so the count is identical for a 75 mm and a 150 mm pipe, and no fall is asked for, so a run laid dead level returns the same number as one laid to grade. The length also stops at the circuit itself: a site with no gravity outlet needs a sump and pump, which this does not size.
- The segment length field is written for stock stick lengths and accepts nothing above 6 m (about 19.7 ft), so the 30.5 m (100 ft) coils of flexible perforated pipe commonly used on residential footing drains cannot be entered here, even though the field's help mentions coiled lengths. A coil is bought by the roll, and for a coiled product the quantity is the run length divided by the roll length, worked out separately.
- The pipe is one line item of a footing drain and the parts that make it work do not scale with the segment count. The washed stone envelope around the pipe is a volume, the filter fabric or pipe sock is an area, and cleanout risers are counted per fitting. Where the design calls for both an exterior footing drain and an interior line under the slab, this number covers one of them.
Fall, and the depth it forces on the far end
Nothing about a lawn drain's gradient comes from a plumbing code, and that is worth saying before anyone reaches for a slope calculator. What governs a subsurface drain's minimum grade is the velocity needed to stop it silting, which is why the NRCS Engineering Field Handbook chapter above sets minimum grades against pipe size and against the risk of fine sand entering the line rather than publishing one figure. A land drain that never runs fast enough to move what falls into it fills from the bottom up over a decade, and the first anyone knows is a wet patch reappearing over the lowest lateral.
The plumbing minimums are still a useful yardstick, and they are what the calculator below applies: a quarter of an inch per foot for pipes three inches and smaller, an eighth above that, which is how the Uniform Plumbing Code splits sanitary drainage. Converted, they are one in forty-eight and one in ninety-six — and that second number is worth keeping, because one in ninety-six is within a whisker of the one in a hundred a landscaper sets with a rotating laser and a staff. Note what the page will and will not take: a run length and a two-way pipe-size switch, not a gradient of your choosing, so it returns one of those two ratios over your measured run and nothing else. Do not treat it as an authority on what a field lateral must fall at.
The drop compounds, and it is spent at the wrong end. Fifteen metres of lateral at one in a hundred drops a hundred and fifty millimetres: start at four hundred and fifty deep and it arrives at six hundred. Eleven metres of main drops another hundred and ten, three metres to the outfall another thirty. The invert at the outfall is now around seven hundred and forty below the low corner — and that figure, not the depth at the top, has to clear the bed of the ditch, the soakaway inlet or the chamber you are connecting into. Prove it with a level before the first metre is cut.
Put your longest run in and read the drop, then add it to your starting depth and check the answer against the level of the thing you are draining into.
The horizontal length of the drain pipe run.
Larger drain pipes are allowed a gentler minimum slope.
Minimum required drop
4.875 in of drop (minimum)
This is the code minimum slope — always check your specific local plumbing code, which may set stricter requirements in some jurisdictions.
- Pipe run length
- 19.5 linear ft
They open the calculator with your figures already in it
Drain Pipe Slope Calculator: 4.88 in of drop (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 4.875 in of drop (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The size question is answered with two bands, so a 4 in branch and a 300 mm building sewer both land in the same band and get 1/8 in per ft. Real code tables are not that coarse: several add a gentler band for the largest sizes, and some jurisdictions require 1/4 in per ft at every size unless the reduction is specifically approved, so both the band boundary and the permitted figure are jurisdictional.
- Nothing here checks the drop you actually have against a maximum, and nothing checks that the fall will physically fit: a 20 m run of large pipe needs about 208 mm of drop at this slope, and that depth has to be available between the fixture connection and the invert of the sewer or septic tank at the far end.
- Slope is not capacity. Pipe size is set by the drainage fixture unit load on the run and the maximum permitted flow depth, and this page never asks how many fixtures discharge into it — the size band is something you tell the calculator, not something it works out for you.
- The length is treated as one straight horizontal run. No allowance is made for bends, offsets or developed length through fittings, and the page does not size or locate the vents, traps and cleanouts the drain also needs; a run at exactly the right slope will still siphon its traps if the venting is wrong.
- The figure is a total end-to-end drop and assumes the fall is delivered evenly along the run. It says nothing about hanger spacing or pipe bedding, and a single sag deep enough to hold standing water will collect solids even though the two ends of the pipe are the compliant distance apart.
The trench, and the ten tonnes coming out of a garden with one side gate
The machine decides the whole downstream takeoff. A walk-behind chain trencher cuts a slot a hundred to a hundred and fifty wide with near-vertical sides, throws spoil in a windrow you can shovel, and leaves a line the sward closes over in a season. A mini-digger with a three-hundred bucket doubles the width and so doubles the spoil, the stone, the fabric and the reinstatement, and leaves a scar the client points at for two years — plus ruts that become the next generation of pan. Where a trencher can physically reach the ground it is almost always the right tool; the exception is the main, which often wants width for a junction to be made up in.
Run the volume before quoting the muck away, because it is invariably underestimated. Fifty-nine metres of slot at a hundred and fifty by six hundred is five point three cubic metres in the ground. Excavated clay bulks by roughly a quarter as it loosens, so nearer six and a half in a heap, and wet clay runs close to one point nine tonnes per cubic metre — about ten tonnes to move out of a garden whose only route to the road is a side gate. Almost none goes back: the void is being filled with stone, and clay packed around a drain is the material the filtration exists to exclude. Check the skip's weight limit, because soil fills the allowance long before it fills the volume.
- Get services traced and marked before anything cuts — the gas service, an armoured supply to a shed, an irrigation main, and the clay field drains from when the estate was a field. HSE guidance HSG47 sets out the practice.
- Peg the outfall level first and work backwards up the main and then the laterals, so the deepest point is proved before the shallowest is cut.
- Stack turf separately if it is worth relaying, and keep topsoil spoil apart from subsoil spoil; the cap at the end of the job wants topsoil, not the grey material from the bottom.
- Cut the main first and leave it open only as long as bedding the pipe takes; an open trench in a wet garden softens its own formation.
- Trim the formation to a consistent fall by hand rather than accepting what the machine left, checking with a laser at three or four points along each run.
- Anything deeper than the threshold at which entry rules bite — 1.5 m in the United Kingdom, five feet under OSHA 29 CFR 1926 Subpart P — stops being a trench you lean into. That will not normally be a lateral, but it may well be the soakaway pit.
Its headline figure is loose backfill, which a stone-filled land drain does not use — take the excavation volume out of the breakdown instead, and treat it as material leaving rather than returning.
The total length of the trench.
The width of the trench.
The depth of the trench.
The outer diameter of the pipe being laid; 0 for a trench with no pipe.
Depth of bedding material under the pipe; 0 if the pipe sits on the trench bottom.
Depth of the same bedding material over the top of the pipe; 0 for none.
Extra loose material needed to achieve full compaction in the void.
Loose backfill material needed
50.9 yd³
Assumes the excavated soil itself isn't reused as backfill (e.g. importing clean granular fill) — if reusing native soil, account for its own swell factor separately.
- Excavation volume (the depth entered)
- 48.89 yd³
- Pipe volume (subtracted)
- 4.6 yd³
- Compacted backfill void
- 44.29 yd³
They open the calculator with your figures already in it
Trench Excavation & Backfill Volume Calculator: 50.93 yd³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
The pipe is drawn to scale inside the trench. Backfill is everything else in the section, which is why the pipe diameter changes the answer at all.
What this calculation does not cover
- The trench is modelled as a plain rectangular prism — vertical faces, one width and one depth over the whole run. Battered or benched sides, the extra width a trench box needs, over-break outside the drawn line, and a bottom that falls with the pipe's gradient are all outside it. A run cut back to a safe slope holds considerably more than this figure, and the shortfall rises with the square of the depth rather than in proportion to it.
- This is a quantity take-off, not an excavation safety assessment. Nothing here classifies the soil, checks the depth against sloping, benching or shoring requirements, or sizes a protective system — that comes from a competent person on site, and past the depths the rules set, from an engineer.
- Only the single pipe you enter is deducted. A second pipe or duct sharing the trench, cable bundles, manholes, chambers, valve boxes, thrust blocks and concrete surround all displace backfill and are not subtracted. No check is made that the pipe fits the trench you described either: where its volume exceeds the excavation, the answer is floored at zero rather than reported as impossible geometry.
- At most two materials: a bedding and surround zone when you enter a bed or a cover, and one backfill above it at one flat percentage. The bedding row is an in-place volume across the full trench width, capped at the trench depth, with no compaction or waste allowance, so add your own for a graded bedding that is compacted. Marker tape or protective tiles, and the sub-base, blacktop or topsoil at the surface are further materials in further thicknesses and are not split out. The percentage is a loose-volume allowance on the backfill and nothing else — it is not a density or Proctor specification, and it says nothing about lift thickness or how many passes the plant makes.
- Nothing is said about the spoil. The excavation row is a bank volume measured in place, not the loose volume that leaves in the truck, and the calculation does not judge whether the arisings can go back, how much of the void they would fill, or what has to be carted away. Rock, groundwater and dewatering, and over-excavation to remove unsuitable ground are all excluded.
What goes back into the slot, in order
The stone is the drain. The pipe only carries what the stone has already collected, which is why gradation is a specification rather than a preference: clean, washed, single-size, angular aggregate with the fines taken out. In a hundred-millimetre trencher slot a twenty-to-forty stone bridges across the gap and leaves voids nobody can see from the top, so ten to twenty is the usual choice for narrow work and twenty to forty for a wide dug main. A crushed product carrying dust binds with the first silt to reach it, and material that arrives with a light grey coating on the shovel is not what was ordered, however it was invoiced.
How high the stone comes is the one thing on the drawing that says what the line is for. Stone stopped a hundred to a hundred and fifty below finished level, capped with topsoil and turf, is a covered lateral draining the soil and nothing else. Stone brought to grade is a French drain that also takes surface water, at the price of a visible gravel strip and a line the mower is lifted over. Confusing the two — a capped lateral where the client expected puddles to vanish into it, or a gravel band through a lawn nobody agreed to — accounts for a good share of the arguments at handover.
A capped lateral in section
- Turf and topsoil cap — the last hundred millimetres, laid slightly proud because it is the only part of the trench that will settle and it settles in full view of the client Topsoil Calculator
- Geotextile separation — closed over the top and taken down both faces, so silt washing off a garden meets fabric rather than the voids the stone was bought for Landscape Fabric Calculator
- Clean single-size stone — the part that actually collects water across the full depth of the slot, which is why gradation and washing matter more here than the diameter of the pipe inside it Gravel Calculator
- Perforated lateral — a carrier rather than a collector, bedded low so it takes water at the bottom of the stone column instead of after the column has filled Perimeter Weeping Tile Pipe Segment Calculator
- Trimmed formation — hand-trimmed to a continuous fall rather than left as the machine cut it, because every hollow along this line is a place the pipe will hold water and silt Drain Pipe Slope Calculator
Run the total trench length by its width by the depth of stone to get the order in tonnes as well as volume, because stone is delivered by weight and then barrowed across the lawn you are trying to save.
Length of the area to cover.
Width of the area.
Depth of the gravel layer.
Extra to order because loose gravel compacts down.
Gravel required
6.59 US short tons
Weighs the loose load at a typical loose bulk density of 1600 kg/m³ (100 pcf) for crushed stone or gravel as delivered. Ask your supplier for the loose density of the specific grade — a dense basalt and a light limestone differ by 15%.
- Finished volume
- 4.07 yd³
- Loose volume to order
- 4.89 yd³
- Cubic yards
- 4.89 yd³
- US short tons
- 6.59 tons
They open the calculator with your figures already in it
Gravel Calculator: 6.59 US short tons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Length × width × depth is a single rectangle at a single thickness, so a drive that curves, splays at the entrance or opens out into a parking bay has to be measured as several rectangles and the answers added, and nothing is deducted for a manhole, a kerb line or a planted island sitting inside the area.
- That thickness is then spread evenly over the whole footprint, which assumes ground already graded true — ruts, hollows and the first load that disappears into a soft patch are material the product of three numbers cannot see, and there is no separate term for a levelling course under the stone.
- The compaction allowance is one flat percentage of the finished volume, applied to the whole job at once and stopping at 40%, rather than a figure derived from the grading or the moisture of what you are actually buying; it is also added to the finished volume rather than divided out of it, so 20% here means 1.20 times the finished figure, not the 1.25 that losing a fifth of a loose load to settlement would call for.
- The weight and the loose volume are the same load: the tonnage is the loose volume to order, compaction allowance included, at one fixed loose bulk density of 1600 kg/m³ (100 pcf), with no field to change it. That suits stone delivered loose and spread or lightly rolled; a dense-graded base (crusher run, DGA, Type 1) specified by its compacted thickness weighs about 2200 kg/m³ (137 pcf) in place and is better worked on the gravel base tonnage page, which takes a compacted density.
- Nothing in the arithmetic asks what will drive over the surface or what lies beneath it: any depth from 10 mm (0.39 in) to a metre is accepted without a check on wheel loads or subgrade strength, and a build that puts a coarse base course under a finer wearing course is two separate runs of this page rather than one.
- What comes back is an exact arithmetic tonnage, not an order quantity — it is not rounded up to whole bulk bags, to the minimum load a tipper will bring out, or to the increment your yard sells in, so the last step of rounding upward is yours once you know how the material is sold.
Fabric is a filter here, not a weed barrier
Two entirely different products share one word on a merchant's shelf. Weed membrane is specified on weed suppression, ultraviolet life and roll size. A drainage geotextile is specified on the things that decide whether a filter works: apparent opening size against the particle size of the soil it must retain, and permittivity or flow rate against the water it must pass. The filtration criteria are in AASHTO M 288, Standard Specification for Geosynthetic Specification for Highway Applications, and the required characteristics for drainage use in BS EN 13252. A woven weed barrier chosen on price will often have an opening size and a flow rate that suit weeds and not silt, and it will blind.
The quantity is the second trap and it is a large one. Fabric is not laid flat across the top of the trench; it goes down one face, around the stone and up the other, then laps over. On the slot above — a hundred and fifty wide with five hundred of stone, plus a hundred and fifty of overlap — that is half a metre up, a hundred and fifty across, half a metre down and the lap: one point three metres of developed width for every metre of trench. Fifty-nine metres therefore wants around seventy-seven square metres of fabric, not the eight and a bit you get by measuring the top of the slot.
Where it earns its place is soils with a real silt fraction; where it does harm is at the top of a line meant to take surface water. Capping a French drain with fabric under the gravel is among the commonest errors on this work: it turns a drain the client can see working into a sealed tube with decorative stone on it. In heavy plastic clay the value of a wrap is argued both ways, since a blinded wrap cannot be jetted clear while an open stone column can at least be re-dug. Choose the geotextile against the soil you actually have, from a datasheet, rather than wrapping out of habit.
Feed it the developed area — up, across, down and lapped, per metre of trench — rather than the plan area of the slot: seventy-seven square metres against eight and a bit, which is four rolls against one. It counts generic rolls, so check the width and the run length on the datasheet of the drainage geotextile you actually specified rather than on a weed membrane.
The total area to cover with fabric.
Fabric spent where strips lap each other so weeds cannot come up through the seam.
Landscape fabric rolls needed
2 rolls
- Area to cover (with overlap allowance)
- 473 sq ft
They open the calculator with your figures already in it
Landscape Fabric Calculator: 2 rolls — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The roll count assumes every roll covers 300 sq ft (about 27.9 m²), and there is no field for the roll you are actually buying. Landscape fabric is sold in several width and length combinations, and the answer moves in direct proportion: halve the coverage of the roll and you need twice as many. Take the "area to cover" figure from the breakdown and divide it by the coverage printed on your own roll before you order.
- Overlap is a percentage uplift on the area, not a lap calculation. Roll width, lap width and the number of strips a bed takes are not inputs, so the default 10 per cent only holds where the bed is wide enough that a narrow side lap costs roughly a tenth of the sheet. On a narrow border, on anything run across the roll rather than along it, or where end laps stack up on a long run, the lap fraction rather than the bed area decides the count, and the allowance has to be raised to match.
- Nothing is added for cutting waste. Fabric worked around established shrubs, tree trunks, posts, drain covers, curved bed edges, or any bed narrower than the roll leaves offcuts that will not be used anywhere else, and the allowance here is for overlap rather than for cutting. There is also no allowance for turning the sheet up at edges or tucking it under edging restraint.
- This counts fabric only. Pins or staples, seam tape, edging, the ground preparation underneath and the mulch or gravel that goes over the top are separate quantities, and none of them are in this number.
- The result is a quantity, not a specification or a judgement that fabric is the right answer. A light non-woven and a heavy woven of the same area return the same roll count despite very different permeability, weed resistance and service life, and under mulch the organic debris that collects on top of the sheet becomes its own seed bed whatever grade you buy. The area field also stops at 2,000 m² (about 21,500 sq ft); enter more and the figure is replaced with that limit, and a job at that scale is buying contractor-width rolls this retail-roll model does not describe.
The interceptor across the top of the fall
Where water is arriving rather than accumulating — off a neighbour's higher garden, off a drive, off a paddock at the top of the boundary — one cut-off across the top of the fall does more than a full field of laterals below it, because it stops the water entering rather than collecting it after the lawn has been soaked. It is the highest-value trench on most sloping sites and the one most often left out, because it does not look like a drainage scheme; it looks like one line across the top of the garden.
Build it wider and bring it to grade. Width matters here in a way it does not on a covered lateral: the stone at the surface is the aperture presented to water coming down the slope, and a three-hundred-millimetre band filled to finished level takes sheet flow a capped slot never sees. Eleven metres of that across the top of the example garden, three hundred wide by six hundred deep with a hundred-millimetre pipe, is a shade under two cubic metres of stone once the pipe's displacement is taken out. Give it its own rodding eye — an interceptor takes the dirtiest water on the site.
This one is a gravel volume net of the pipe inside it, so put the interceptor's own width and depth in rather than the lateral's — width is doing real work on a cut-off and it moves the order noticeably.
The total length of the drain trench.
The width of the trench, typically wide enough to work in comfortably.
The depth of the trench, below the frost line in cold climates if the drain must run year-round.
The outer diameter of the perforated drain pipe.
Gravel backfill needed
2.758 cubic yards
- Trench volume
- 2.67 yd³
- Pipe volume (subtracted)
- 0.16 yd³
They open the calculator with your figures already in it
French Drain Gravel Calculator: 2.76 cubic yards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
The pipe is drawn to scale inside the trench, because the gravel quantity above is the trench volume minus that circle. On a narrow trench with a big pipe the subtraction is a large share of the answer, and this is where you see that.
What this calculation does not cover
- The trench is modelled as a constant-depth rectangular prism, so a run laid to fall is only covered if the depth entered is the average depth over the whole length. A 15 m (49 ft) run at the usual 1 per cent fall finishes 150 mm (6 in) deeper than it starts, and a trench entered at its shallow upstream depth actually holds about 16 per cent more gravel than the figure returned.
- Gravel is taken as filling the trench to the full depth entered, from the trench floor to ground level. Any soil or turf cap placed over the stone has to come off the depth before it is entered — a 100 mm (4 in) topsoil cap on a 460 mm (18 in) trench is more than a fifth of the volume this page returns.
- The answer is a volume, not a tonnage, and the 10 per cent added covers ragged trench walls and settlement rather than compaction to a stated density. A quarry weighs stone out rather than measuring it, so converting needs the bulk density of the material actually supplied; clean drainage gravel is commonly quoted near 1.5 tonnes (1.7 tons) per cubic metre, which puts the default 2.81 yd³ order at roughly 3.2 tonnes (3.5 tons).
- No check is made that the pipe fits the trench. A 200 mm (8 in) pipe entered against a 100 mm (4 in) trench width is accepted and answered normally at 0.31 yd³, and where the pipe would displace more than the trench holds, the result is floored at zero rather than reported as impossible geometry.
- Pipe diameter enters this calculation only as displaced gravel, never as capacity. Whether the drain carries the water depends on the catchment area feeding it, rainfall intensity, the gradient and the infiltration rate of the surrounding soil — stepping from a 100 mm to a 150 mm pipe over 15 m moves this order by about 0.21 yd³ and settles none of those questions.
Where the water is allowed to go
The outfall is last in the direction of flow and first in the order of work, and three of its four destinations need somebody's permission. A new outfall into an ordinary watercourse is consented work in England and Wales under the Land Drainage Act 1991, administered by the lead local flood authority or, in its area, the internal drainage board — granted or refused before you build. A surface water sewer needs the undertaker's agreement, and Approved Document H, Drainage and Waste Disposal, sets out the hierarchy deciding whether you should be asking at all. A foul or combined sewer is not an option.
A soakaway is the fourth and the one that fails quietly. It works only if the subsoil accepts water at the depth its base will sit and only if that base is clear of the highest groundwater by a real margin — precisely the pair of things the bored hole and a BRE Digest 365 trial pit were for. In the garden this guide opened with, mottled from four hundred millimetres and standing at three-fifty in January, a soakaway is a hole that fills in November and stays full until March; draining the lawn into it moves the standing water eleven metres and buries it. That is the commonest way a competently built lateral scheme ends up doing nothing.
Then the boundary. Taking water that used to spread across a lawn, concentrating it in a pipe and delivering it over a boundary line is a different act in law from letting it drain naturally, and it creates a liability that outlives the invoice. Neither the highway drain nor the neighbour's ditch is yours to adopt because it is convenient.
And there is the honest no. A flat garden, a water table at three hundred and fifty millimetres and no legal outfall within reach of a gravity gradient is a site where a lateral scheme cannot work, and saying so is worth more to the client than a scheme that will not. What remains is real: raising the levels of the part that has to be usable, a pumped chamber where somebody will actually maintain a pump, generous surface fall taken to a swale rather than a pipe, and planting the wet corner with things that want to be there. Quoting a scheme into a site with nowhere to discharge is the one mistake on this page that cannot be corrected afterwards.
Putting the lawn back, and the line that shows for two springs
Stone does not settle; the cap does, and it settles in full view. Leave the topsoil cap proud — twenty to thirty millimetres on a narrow slot — and expect to top it the following autumn and probably the one after, as the fill consolidates and the turf knits across. The cap wants topsoil, not the grey subsoil from the bottom of the trench, which is why the spoil was stacked in two heaps on the day of the dig. BS 3882, Specification for Topsoil, is the document to quote at a supplier if material is imported: a bulk load of unspecified screened soil can arrive with anything in it.
Where sand slitting or coring has been done as well, the whole lawn takes a topdressing rather than just the trench lines, and the sums differ sharply. A light dressing over a hundred and seventy-six square metres at fifteen millimetres is well over two cubic metres — a delivery rather than a few bags — while the trench caps themselves come to less than one. Run both. The whole-lawn figure is the one that surprises people, and the one most often missing from a quote and absorbed by the contractor.
- Backfill the stone in layers rather than one drop, tamping lightly by hand — not with a plate, which crushes a corrugated lateral and drives fines into the voids.
- Close the geotextile over the stone before the cap goes on, lapped by at least the overlap you ordered for, and pinned so it cannot roll back as soil goes in.
- Cap to slightly proud, firm by heeling rather than by machine, and relay turf or seed to suit the season — turf knits over a trench line in one growing season, seed sown in November does not.
- Sketch the finished layout with rodding eye positions dimensioned off two fixed points and give it to the client, or the next fence post goes through a lateral.
- Walk the lawn during the first real storm after handover and see which lines run. A lateral dry while its neighbours flow is laid flat or already blinded, and it costs a fraction to find in the first winter rather than the third.
- Book the following autumn's topdressing at handover rather than leaving it to be remembered; the settled line across a lawn is what the client judges the whole job by.
Run it twice: once for the whole-lawn dressing at fifteen millimetres, which is the one that turns out to be a delivery, and once for the trench caps as a long narrow area — the length field stops at fifty metres, so a fifty-nine metre run goes in as two lengths and the two answers are added.
The length of the garden bed or area.
The width of the garden bed or area.
6-12 in (15-30 cm) is typical for a new garden bed; shallower for topping up an existing lawn.
Estimated topsoil needed
0.9877 cubic yards
- Area
- 40 ft²
They open the calculator with your figures already in it
Topsoil Calculator: 0.9877 cubic yards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The answer is a loose volume and nothing turns it into weight, because no bulk density is asked for anywhere in the form — topsoil is very often quoted, delivered and tipped by the ton or tonne, and how much a cubic yard of it weighs moves with how wet it is, how much organic matter it carries and how finely it has been screened.
- No allowance for consolidation is folded in: the number is a bare length x width x depth, so soil tipped to the depth you typed will read shallower once it has been rained on and walked over, and making the level good again is a second, smaller order.
- The footprint is one rectangle carried at a single depth, so a curved border reads as its bounding box, ground that falls away across the bed is not picked up, and the room already filled by shrub root balls, stepping stones or the thickness of edging boards is never deducted.
- Fill depth is capped at 24 in (0.61 m) and each side at 50 m (about 164 ft), which puts berms, deep planting pits and field-scale regrading outside the form, and inside that band the depth entered is simply taken as correct — nothing checks that it gives enough rooting depth for what you intend to grow.
- Only soil going in is counted, with nothing coming out, so where turf has to be stripped or tired ground dug down before filling, neither the spoil to be carted away nor the level already occupied by what is there is netted off the delivery.
Settle these before the trencher is booked
The workspace opens on the interceptor worked above — eleven metres across the top of the fall, three hundred millimetres wide, six hundred deep, with a hundred-millimetre perforated pipe in it. Swap the geometry for yours, then carry the same run lengths through the stone, excavation, pipe, gradient and reinstatement pages stacked beneath it.
- The fork: pan or table — A slot face read for colour and for a platey band, plus a cased hole watched through a wet week. Everything below is void until this is answered.
- Outfall invert level — The level of the ditch bed, chamber or soakaway inlet, proved with a laser before any trench is cut, and the consent that goes with it identified.
- Total pipe run and fittings — Laterals plus main plus outfall run as one length; junctions one per lateral, plus bends and a rodding eye at every change of direction.
- Stone, in tonnes as well as volume — Trench length by width by stone depth, single-size and washed, sized to the slot so it cannot bridge; delivered by weight and barrowed across turf.
- Geotextile, on developed width — Up one face, across, down the other and lapped, per metre of trench — 1.3 m of developed width against a 150 mm slot, so nearly nine times the plan area.
- Spoil leaving site — In-situ volume bulked by about a quarter, priced by weight rather than skip volume, with almost none of it going back in.
- Reinstatement, twice — Topsoil cap laid proud now, and a topdressing budgeted for the following autumn once the lines have settled.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.