Two Courses of Brick, and a Threshold That Wants to Be Flush
Stand at the back door of a typical brick house and the damp-proof course is visible as a thin dark seam two bed joints above the soil. Two courses of standard brickwork is 150 mm — a 65 mm brick and a 10 mm bed joint, twice — which is roughly six inches, and which is not a coincidence. That is the clearance the guidance asks for, and the house was built to it.
The door threshold sits at or a little above internal floor level, which is at or a little above the damp course. So a patio laid flush with the back door is a patio laid about 150 mm higher than the ground it replaces, tight against brickwork that was never meant to have anything piled against it. The homeowner is not asking for something unreasonable; they are asking for a level threshold, which is a perfectly normal thing to want and, in new dwellings, something Approved Document M expects at the principal entrance. They are just asking for it in a way that, done carelessly, deletes the one detail keeping the wall dry.
There are three honest ways out and one dishonest one. You can keep the paving down and take a step at the door. You can bring the paving up and put a drained slot between it and the wall, which is the detail the accessible-threshold guidance actually shows. You can coat the buried face of the brickwork, which helps and settles nothing on its own. Or you can butt the flags to the wall and hope, which is the option that generates the callback. The rest of this is the arithmetic each of the first three forces on the order sheet.
Where the paving meets the wall
- Paving units — laid to fall away from the building, and every unit along the wall becomes a cut once the channel takes the first 100 mm Paver Calculator
- Threshold channel — the slot at the door that takes water the fall cannot move, and the reason the paving is allowed to sit high
- Laying course — screeded sharp sand left uncompacted, or a mortar bed — the layer that most often ends up touching the brickwork Paver Base & Sand Calculator
- Compacted sub-base — granular fill ordered by weight rather than volume, and stopped clear of the wall so the channel surround has room Gravel Base Layer Tonnage Calculator
- Formation — the cut face the whole build-up sits on, set by working down from the finished level rather than up from the old ground Slope & Grade Calculator
- Coated band of brickwork — a waterproof coating on the face between formation and the damp course — a supplement to the levels, never a replacement Basement Wall Waterproofing Coating Coverage Calculator
- Damp-proof course — the seam two courses up, and the datum every other vertical dimension on this job is measured down from
- House wall — brickwork whose footing may be shallower than the excavation about to be dug alongside it
Bridging Is a Route, Not a Height
It is worth being precise about the failure, because the 150 mm gets quoted as if the number itself were doing the work. It is not. A damp-proof course fails when liquid water finds a continuous path from wet ground to masonry above the barrier, and height is only one of the things that closes that path. Approved Document C deals with this in its section on walls, and BS 8215 is the code of practice behind the detail; both are about the route, and the dimension is a proxy for keeping the route open.
The routes are boringly consistent. A full mortar bed run tight to the brickwork carries water up the face and across the seam in one move. Sub-base tipped against the wall does the same more slowly, because compacted granular fill in contact with brick is a wick with a large surface area. Render or a haunching fillet taken down past the barrier hands water a rigid bypass that no amount of sealant later removes. On a cavity wall, mortar droppings sitting on the cavity tray turn the cavity itself into the bridge, and nothing visible from outside shows it.
Then there is splash, which is what the 150 mm is really for. Rain hitting a hard surface bounces, and on smooth flags it bounces higher than on gravel or grass. Water landing on brickwork above the damp course soaks in, moves down, and meets the barrier from above with nowhere to go. This is why a patio that technically clears the seam by 40 mm still produces a tide mark: the level passed a tape measure and failed the physics.
The presentation is slow and easy to misdiagnose: a salt band along the internal wall a hand's width above skirting level, plaster blowing in a horizontal line, skirtings soft at the back while looking sound at the front, and none of it necessarily in the first winter. If the ground floor is suspended timber there is a second failure hiding underneath, where paving that buries or half-buries an airbrick starves the void and rots joist ends without any damp appearing on the wall at all. Approved Document C sets the ventilation opening area a suspended timber ground floor needs, per metre run of wall and per square metre of floor; a periscope vent restores the level but not automatically the free area, because the duct throttles it. Count the airbricks before anything is dug.
Three Ways the Level Gets Resolved
The choice is usually made by the door rather than by the paving. If nobody in the house needs level access, dropping the paving is the cheapest and most durable answer and it needs no maintenance for the life of the surface. If a wheelchair, a pushchair or a bad hip is in the picture, the drained threshold is the detail that exists for exactly that case, and it arrives with an outfall to build and a channel somebody has to keep clear.
The third column is the one worth reading carefully. Coating the buried brickwork is a real and useful measure, and it is not a level. Nothing you brush on the outside face changes where the water table of a saturated sub-base sits relative to a seam of polythene, and a vapour-tight coating on solid masonry can push moisture inward rather than remove it. Treat it as protection for the band of brick that will be permanently damp, applied alongside a correct level, not as the reason you were allowed to skip one.
| Route | What it buys | What it costs | Where it goes wrong |
|---|---|---|---|
| Drop the paving, step at the door | Full clearance to the damp course with no ongoing maintenance | One step of roughly 150 mm at the threshold, and spoil to cart away | The step gets built at an awkward height, or the far edge of the fall ends up below the lawn it drains onto |
| Level threshold with a drained channel | Flush access, which is the point of the exercise | A channel, a concrete surround, an outfall and a grating somebody has to lift and clean | The channel is laid without a working outlet, or a grating rated for feet is put where a wheel will cross it |
| Coat the buried brickwork | Protection for the band of brick that will sit permanently damp | Small material quantity, real surface preparation, and honesty about what it is | It is sold to the client as the fix, and the paving goes on at the wrong level behind it |
| Butt the flags to the brick and haunch it | Nothing | Nothing today | A salt band above the skirting in the second winter, and a floor to lift to find out why |
Fall Away From the Wall, Fixed Before the Dig
Set the levels first, on paper, because the fall decides the dig and the dig decides the muck-away. Two numbers govern: how far the paving may sit below the damp course at the wall, and how much the surface drops between there and the far edge. The second one is a gradient and it is not optional — a patio laid dead level against a house is a patio that ponds against the house.
In North America the number is written down: the International Residential Code's drainage provision, Section R401.3, requires impervious surfaces within ten feet of the foundation to slope a minimum of two percent away from the building, which is one in fifty, or 20 mm per metre. In UK practice the working envelope is conventionally one in eighty at the flattest for smooth units, one in sixty as the usual target, and one in forty where the geometry allows — the BS 7533 series and the paving manufacturer's own literature govern the specific figure, and the flatter you set it the less tolerance you have for a single slab laid proud.
Run the arithmetic both ways before committing. A 3.6 m deep patio at one in sixty drops 60 mm across its width; the same patio at one in forty drops 90 mm. Start at the wall with the paving 150 mm below the damp course and the far edge finishes 210 to 240 mm below it, which is fine if the garden falls away and a problem if it does not — at which point the far edge needs a channel of its own, a soakaway, or a step down into the lawn. Start at the wall with a level threshold instead and the far edge is only 75 to 105 mm below the door, which is much easier to land but puts the whole burden of protecting the wall on the channel at the top.
- Mark the damp-proof course on the brickwork with a pencil line and measure everything from it, not from the existing soil.
- Establish the finished paving level at the wall: 150 mm below the line for a dropped patio, or just under the threshold for the drained detail.
- Set a peg at the far edge, drop it by the fall you have chosen over the measured run, and check what it lands on — lawn, boundary wall, or a neighbour's ground.
- Subtract the paving thickness, the laying course and the sub-base from both pegs to get formation level at each end.
- Walk the wall with a torch and note every airbrick, gully, soil pipe and meter box against the new finished level.
- Only then work out spoil volume, because that is the number that decides whether this is a skip job or a grab-lorry job.
It follows these steps in order — the finished level from the damp-proof course, the drop over the run you measured, the build-up taken off both ends — and returns the dig at the wall and at the far edge, and the spoil, before a peg goes in.
How far the patio runs along the house wall.
How far the patio runs out from the wall — the direction it falls in.
How high the damp-proof course sits above the soil at the wall today.
How far below the damp-proof course the paving finishes at the wall.
How steeply the paving falls away from the house: 1 unit of fall in N units of run — enter N.
The thickness of the slab, flag, sett or paver.
The mortar bed or sand bed the paving is laid on.
The compacted crushed-stone layer that carries the paving.
How much higher or lower the soil is at the far edge than at the wall.
Dig depth at the wall
7.25 in
Worked from the top down: the damp-proof course fixes the finished level at the wall, the fall fixes it at the far edge, and each dig depth is whatever the paving, laying course and sub-base leave below the existing ground. Two brick courses below a damp-proof course is the usual clearance; finishing higher is only safe as a level threshold with a drained channel against the wall.
- Finished paving at the wall, above (+) or below (−) the existing ground
- 0 in
- Fall across the patio
- 2.88 in
- Fall, as a gradient
- 2 %
- Build-up: paving, laying course and sub-base
- 7.25 in
- Dig depth at the far edge
- 10.13 in
- Spoil to take away, as dug
- 5.63 yd³
- Compacted sub-base
- 2.59 yd³
- Laying course
- 0.81 yd³
- Patio area
- 210 ft²
They open the calculator with your figures already in it
Patio Levels and Dig Depth Calculator: 7.25 in — 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 — 7.25 in — 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
- One rectangle falling one way, over ground that changes evenly between the wall and the far edge. A patio that turns a corner of the house, falls two ways, or sits on ground with a hump or hollow in it is levelled piece by piece.
- It sets levels and volumes, not drainage. Where the far edge finishes below the lawn or the boundary, or the fall runs towards a neighbour, the water needs somewhere to go — a channel, a soakaway or a step down — and a level threshold needs its drained channel against the wall.
- Dig a hand hole against the wall before any machine arrives. Older houses often sit on shallow footings, and a dig alongside a wall can expose or undermine them; where the footing is shallower than the formation, the dig stops short of it and the difference is made up in lean concrete.
- The spoil is the in-place volume. Dug soil bulks up by a good share once it is loose, so the skip or lorry has to hold appreciably more than this, and the sub-base volume is compacted — order the stone by weight at its compacted density.
The Excavation Is Governed From the Top Down
This is where jobs go wrong quietly. The instinct is to dig a spade's depth off the existing ground and build up from there, which sets the finished level by accident. The finished level is the constraint; formation is what you solve for. Work down: finished paving, minus the unit thickness, minus the laying course, minus the compacted sub-base, and whatever depth that lands on is where the machine stops.
Take a plain example on a pedestrian patio. A 50 mm flag, a 30 mm laying course and a 100 mm compacted sub-base is 180 mm of build-up. If the paving must finish level with the existing ground — the dropped-patio route — the machine takes 180 mm off across the whole area, and on a 5.4 by 3.6 m patio that is a little under 3.5 cubic metres of spoil before any bulking allowance. Take the level-threshold route instead and the finished surface rises about 150 mm, so the same build-up needs only 30 mm of dig. The second job looks cheaper, and it is, right up until you notice that the reduction in muck-away is exactly the 150 mm you have removed from the wall's protection and now owe back in drainage.
Sub-base depth is not a free variable. The convention the BS 7533 series and ICPI guidance support is 100 mm compacted for pedestrian paving, rising to 150 mm for a car and 200 mm for anything heavier; if there is any chance of a vehicle on part of this surface — a bin lorry crossing a corner, a car parked on the end of it — the deeper number applies to that part and the build-up grows with it. Getting this wrong does not show as damp; it shows as rutting, and it shows late.
One thing to establish before the first bucket. Older houses frequently sit on shallow footings — sometimes two or three courses of corbelled brick below ground — and a 180 mm trench alongside a wall can expose or undermine them. Dig a hand hole against the wall before the machine arrives; if the footing is shallower than formation, the excavation stops short of it and the margin is made up in lean concrete instead. Neither the paving nor the damp course matters if the wall moves.
This one carries the area, the sub-base depth for the traffic you expect and the bedding in a single pass, which is the combination that gives you formation depth — the number the dig is actually priced against.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
Length of the paved area.
Width of the paved area.
What will use the surface.
Screeded sand layer under the units.
Allowance for cuts and breakage.
Paving area
279 ft²
Sub-base includes a 25% compaction allowance. Depths follow common guidance and should give way to a specific specification where one exists.
- Net paved area
- 253.5 ft²
- Sub-base depth
- 3.94 in
- Sub-base to order (loose)
- 3.85 yd³
- Sub-base weight
- 10,384.18 lb
- Bedding sand
- 0.98 yd³
- Area in ft²
- 278.85 ft²
They open the calculator with your figures already in it
Hardscape Area & Base Calculator: 279 ft² — 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
- Assumes a competent subgrade. Soft or clay ground needs excavation to a firm formation and usually a geotextile separator, neither of which is calculated here.
- No allowance for edge restraint, which paving needs on every free edge or it spreads.
Sub-base, and the Strip Beside the Wall It Must Not Reach
Granular sub-base is bought by weight and specified by grading. In the UK the default is the Type 1 unbound mixture described in the Specification for Highway Works, Series 800; in North America the equivalent is a dense-graded aggregate base to ASTM D2940 or the local highway department's own gradation. Whichever it is, the merchant sells it in tonnes and the design is in millimetres of compacted thickness, so somewhere between the two there is a density and a compaction allowance that has to be right or the last load arrives on a Saturday.
A compacted dense-graded sub-base such as Type 1 weighs around 2,100 to 2,200 kg per cubic metre in place (about 131 to 137 lb per cubic foot — the 1.85 US tons per cubic yard highway specifications pay on), and loose material placed at that thickness will lose roughly a fifth to a quarter of its depth under a plate compactor. Order against the compacted design thickness with that allowance added, place in layers no thicker than the plate will genuinely compact — 75 to 100 mm at a time on a wacker, not the whole 150 mm design in one go on a driveway — and check the level with a straightedge off the pegs rather than by eye.
The detail specific to this job is negative: keep the stone away from the wall. Leave a clean margin along the brickwork wide enough for whatever is going in there — the channel and its concrete surround, or a shingle-filled gap on a dropped patio — and do not let loose stone spill into it and get compacted against the face. A shingle margin wants a liner at the back so fines cannot migrate into contact with the brick, a top surface below the damp course, and an annual clear of leaves. Compaction discipline matters here too: running a plate hard against an old wall transmits vibration into brickwork that may already be lime-mortared and tired.
Convert the compacted design thickness over the measured area into tonnes at a realistic density, because the design is written in millimetres and the delivery note is written in weight.
The total area to be covered with gravel base.
The target compacted thickness of the base layer.
The in-place density of the base once it is compacted.
Gravel base needed
73.8 tons
Actual density varies by material gradation and compaction — confirm with your supplier's specific product density for a precise order quantity.
- Volume
- 39.81 yd³
- Equivalent in US (short) tons
- 73.82 tons
They open the calculator with your figures already in it
Gravel Base Layer Tonnage Calculator: 73.82 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
- This is a take-off, not a pavement design. It multiplies out whatever compacted thickness you enter; nothing here derives that depth from traffic loading, subgrade strength, drainage or frost depth, which come from a pavement design or your local road authority's standard.
- Geometry is a flat plan area at one uniform depth. Crown and cross-fall, a formation that steps between thicknesses, a dig that deepens where the subgrade was soft, and edge thickening or haunching at the perimeter all fall outside area x thickness.
- No waste, spillage or subgrade-loss allowance is applied. The figure is the exact in-place mass, so stone lost into a soft or uneven formation, over-excavation, haul and spread losses, and the tail end of a part-load all sit on top of it.
- The density field is an in-place compacted density. A supplier's loose bulk density and a weighbridge ticket carrying free moisture are different quantities, and substituting either moves the tonnage: the wetter the delivered material, the less dry stone a given delivered weight puts on the ground.
- It covers one layer of one material. A base and sub-base of different gradations, a bedding or blinding course, and the geotextile or separation membrane between stone and subgrade are not counted here.
The Laying Course Stops Short of the Brickwork
Two traditions meet on a domestic patio and they behave differently at the wall. The screeded-sand method — 25 to 40 mm of sharp sand, struck off to level and deliberately left uncompacted until the units are down, which is what ICPI Tech Spec 2 describes — is a loose layer that cannot itself bridge anything. The full wet mortar bed, standard practice for riven flags and natural stone in the UK, is a continuous rigid layer that absolutely can, and does, the moment it is run tight to the brick.
So the mortar bed stops. Hold it back from the wall face, and if the paving genuinely has to finish against the brickwork, close the last gap with a compressible filler and a flexible sealant rather than mortar. A mortar fillet or a haunching triangle worked into the angle between flag and wall is the single most common bridge on this kind of job, and it is the one that looks like careful workmanship in the photograph.
Bedding depth is a design input, not a fudge factor. It is tempting to use the laying course to absorb a level error discovered late, but a thick bed consolidates unevenly under load and produces exactly the dips it was employed to prevent. If the level is out, the sub-base is where you fix it. That is also why the bedding depth belongs in the calculation that set formation: change it after the dig and you have changed the finished level.
Jointing follows the same logic. Whatever fills the joints — kiln-dried sand, a polymeric jointing compound, a wet mortar joint on flags — it stops at the wall, and the perimeter joint against the brickwork stays a movement joint. On the sand-set method the perimeter also needs a restraint on every free edge, because unrestrained unit paving spreads sideways and the first thing that opens is the joint you were relying on.
Lay from the wall outward on this job rather than from the open edge in. The cut course against the channel is the one that has to be right and the falls are set from that line, so finishing the awkward cuts at the far edge, where a planted bed will hide them, is more forgiving than finishing them at the threshold where everyone stands.
The gravel base and the bedding sand come off the same footprint but not the same depth, and it is worth having both volumes separately before you decide whether this is a bulk-bag job or a tipper job.
The length of the paved area.
The width of the paved area.
4-6 in (10-15 cm) is standard for a patio; use the higher end for driveways or heavier loads.
1 in (2.5 cm) is standard for the leveling sand layer directly under the pavers.
Estimated paver base & sand needed
1.605 cubic yards (gravel base)
- Patio area
- 130 ft²
- Bedding sand needed
- 0.4 cubic yards
They open the calculator with your figures already in it
Paver Base & Sand Calculator: 1.6 cubic yards (gravel base) — 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 volume covers the paver footprint only — length × width at the depth you enter. A compacted base is normally carried past the paver edge on every side so the edge restraint has something to sit on, and that perimeter band is not counted here.
- Both figures are finished, in-place depths. Crushed base and bedding sand are delivered loose and lose volume as they are compacted and screeded, so the quantity to order is higher than the number shown. No waste, over-dig or shrinkage allowance is applied.
- Only the two layers you enter are counted. Geotextile separation fabric, an open-graded drainage course, edge restraint and the jointing sand that goes between the pavers all sit outside the figure — as does the paver thickness, so this is not your excavation depth.
- The calculation applies whatever depth you type; it does not test whether that depth suits the subgrade, the frost depth or the load. Soft clay, organic or filled ground, poor drainage and anything carrying vehicles are pavement design decisions made from the soil and the traffic, not from these dimensions.
- The area is treated as one rectangle at a single uniform depth. Curved, L-shaped or multi-level layouts have to be split into rectangles and run separately, and any local thickening — a deeper base under a step, a soft spot dug out and backfilled — is not in the total.
What the Channel Has to Do, and Where It Goes
The drained threshold is a documented detail, not a workaround. The DETR guidance Accessible thresholds in new housing shows it and Approved Document M is what makes it necessary in new dwellings. The arrangement: paving finishing just below the threshold, a linear channel immediately outside the door in the gap against the wall, and the surface falling away from that channel across the rest of the patio. The channel intercepts water at the point where the level protection has been given up, which is why the whole detail stands or falls on it.
Specify the channel by load class, not by looks. BS EN 1433 classifies drainage channels for vehicular and pedestrian areas, and the classes run from A15 for areas used only by pedestrians and cyclists, through B125 for footways and car parks, upward from there. A domestic patio outside a back door is A15 territory; a channel that a car will ever cross, even a wheel clipping the end of it while turning, is not. The grating is the part that gets specified carelessly and the part that fails visibly.
The outfall is where these jobs stall. A channel is a collector and it has to discharge somewhere legitimate — a trapped gully connected to the surface water drain, or a soakaway. Approved Document H, Section H3 governs rainwater drainage and sets a minimum separation between a soakaway and a building; check the current edition for the figure rather than the one you remember, because putting a soakaway close to the wall you are trying to keep dry is a self-inflicted wound. Connecting surface water into a foul drain is not an option to reach for casually, and in many places it is simply not permitted.
Install it to the manufacturer's written instructions, which for almost every domestic channel means a concrete surround of a stated thickness beneath and to both sides, formed before the paving goes down. Set the grating a few millimetres below the finished paving so water enters rather than skating across, and leave the client a grating they can actually lift — a channel nobody can clean silts up within two autumns, and then the threshold ponds.
Every Unit Along the Wall Is a Cut
The channel changes the paving order in a way the area calculation does not see. A 100 mm nominal channel plus its surround takes 150 to 200 mm out of the first course along the wall, so every unit in that course gets cut, and on a patio that also has a fall in two directions there will be tapered cuts as well as straight ones. Set out from the wall so the cut course lands at a sensible width; a 40 mm sliver against a channel breaks during compaction and looks like a mistake for the next twenty years.
Add waste accordingly. Ten percent covers a rectangle laid square with clean edges; a cut course along a whole side, a diagonal pattern or a curved far edge takes it to fifteen and beyond. Order that course from the same batch as the field — stone and concrete units both drift in colour between batches, and the course that draws the eye is the one at the door.
Count the units off the paved area and the unit size with the waste factor set for a job that has a cut course along its entire back edge, not for a clean rectangle.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The length of the area to be paved.
The width of the area to be paved.
Any part of the rectangle that is not paved — a pool, a planting bed, a tree pit. Leave it at 0 for a plain rectangle.
The length of a single paver.
The width of a single paver.
Extra pavers for edge cuts, curves, and breakage.
Estimated paver needed
143 pavers
The pavers are the visible tenth of the job. What determines whether they stay flat is the base beneath them and the edge restraint around them, and both are larger quantities than the surface.
- Patio area
- 130 sq ft
- Coverage per paver
- 1 sq ft
They open the calculator with your figures already in it
Paver Calculator: 143 pavers — 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.
Paver setting-out grid — drawn from your figures
A 3.96 x 3.05 m patio set out in 0.3 x 0.3 m pavers, stack bond.
A DXF plan in metres. Opens in AutoCAD, LibreCAD, QCAD and most jobsite viewers.
What this drawing does not show (3)
- The bond. This is drawn as a stack bond because the calculator counts by AREA and assumes no pattern — running bond, herringbone and basketweave all cover the same area and change only the cuts.
- Joint width and edge restraint.
- Where the cuts fall, which depends on which corner you start from.
It is a setting-out aid drawn from the figures you entered, not a construction drawing, and nobody has checked it against a design.
What this calculation does not cover
- Excludes the sub-base, bedding layer and jointing sand, each of which has its own depth and its own compaction allowance.
- Excludes the edge restraint, without which the field spreads laterally under traffic and the joints open.
- Does not model cutting waste on a curved or angled layout, which is far higher than on a rectangular field.
Coating the Wall Face, and What It Cannot Buy You
Whichever route is chosen, a band of brickwork ends up permanently below the finished surface and permanently damp. Coating it is worth doing, and it is a small quantity — the band runs from formation level up to just short of the damp course, so on 5.4 m of wall with a 250 mm band it is around 1.35 square metres. Small enough that the tin size and the number of coats matter more than the area does.
Coverage is a manufacturer figure and it is a rate per coat, on a stated substrate. Brickwork is porous and a first coat on dry brick can absorb well beyond the data sheet, so the arithmetic is area divided by the stated coverage, multiplied by the coats specified, and then judged against the smallest container the product comes in. Most cementitious and bituminous coatings for this use need at least two coats to give a continuous film, and the second coat is the one that closes pinholes.
Be clear with yourself and the client about the standard this sits under. BS 8102 covers protection of below ground structures against water ingress and is the reference for tanking a basement or a retaining structure; a brick wall whose base happens to be below a patio is not that, and nothing in BS 8102 makes an external coating a substitute for a correct damp-proof course level. On a solid masonry wall there is a further caution: an external coating that is effectively vapour-tight reduces the wall's ability to dry outward, and moisture that used to leave through that face has to go somewhere else.
Two absolutes. The coating stops below the damp-proof course and never laps over it — a continuous film carried across the seam is a bridge built deliberately, out of the product bought to prevent one. And it goes on a prepared face: soft joints raked and repointed, brushed clean, dry enough to key. A coating applied over friable mortar comes off with the mortar.
The band of wall is small, so put the real coverage rate from the data sheet and the specified number of coats against it before deciding what size tin to buy.
The total basement wall area to be coated.
The area one unit of coating covers in a single coat, per the manufacturer's data sheet.
The total number of coats specified for the application.
Waterproofing coating needed
32.5 gal
A coating on a basement wall is one part of a system whose other parts — drainage, a drainage board, and somewhere for water to go — do most of the work. Applied alone against real hydrostatic pressure, a coating is being asked to do something it cannot.
They open the calculator with your figures already in it
Basement Wall Waterproofing Coating Coverage Calculator: 32.5 gal — 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
- Excludes the drainage board, protection layer, perimeter drain and the sump or outfall the drain discharges to.
- Coverage rates assume a sound, prepared substrate. A rough or porous wall consumes considerably more, and voids must be filled before coating rather than by it.
- Positive-side and negative-side applications are different products for different situations; a coating designed to be applied externally will not necessarily work applied internally.
What to Photograph, and What the Second Winter Will Tell You
The evidence on this job disappears under the paving in a single afternoon, so record it while it is visible. Photograph the wall face with the damp course marked and a tape held against it, the open channel before its surround is poured, the margin against the wall before the sub-base goes in, and every airbrick with its new finished level shown. If there is ever an argument about whether the patio caused a damp problem, those four photographs settle it in either direction, and they cost nothing.
Then check the things that only show later. Run a hose on the finished surface and watch where the water goes — into the channel, or across the threshold, or ponding at the wall in a spot the level looked fine at. Lift the grating and confirm the outlet actually runs. Look for splash marks on the brickwork after the first heavy rain, because splash height is the one thing you cannot design out with a tape measure, and a wall showing wet marks 200 mm up is telling you the surface is too smooth or the fall is too flat.
One last piece of honesty for the client. Salt bands and blown plaster arrive slowly, often a year or two after the work, by which time the patio is established and looks blameless. Saying beforehand that the level and the channel are what prevent that, and that the coating is a supporting measure rather than the fix, is what makes it possible to specify the drained threshold properly instead of quietly laying the flags 40 mm below the damp course and calling it close enough.
Ordering from the damp course down
Every quantity on this job hangs off one level. Fix the finished paving level against the damp-proof course first, then let the build-up, the dig and the deliveries fall out of it.
- Paved area and sub-base depth, set by traffic — Pedestrian paving takes a shallower sub-base than anything a car will cross; the depth chosen here is what sets formation, so it is decided before the dig, not during it.
- Fall from the wall to the far edge — Expressed as a percentage or a ratio, checked against what the far edge lands on. Two percent within ten feet of the foundation is the IRC minimum for an impervious surface; UK practice runs 1:80 to 1:40.
- Sub-base by weight, with a compaction allowance — Ordered in tonnes against a compacted design thickness. Keep the margin along the wall clear of it for the channel surround or the shingle gap.
- Laying course, separate from the base — 25 to 40 mm screeded and uncompacted for sand-set, or a mortar bed held back from the brickwork. Changing this depth after the dig changes the finished level.
- Units, with a cut course along the whole back edge — The channel takes 150 to 200 mm out of the first course, so waste runs above the rectangular-patio figure and the cut course should come from the field batch.
- Coating for the buried band of brickwork — Formation level up to just below the damp course, at the data sheet coverage and the specified coat count. It stops short of the seam and never laps it.
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.