Four Courses Right and Sixty Millimetres Wrong
The wall wrapping the back terrace was set out at 450, formed at 450 and checked at 450 with a staff before anyone went home. It is a good wall. Nobody sits on it. The cap that went on the following week was 50 mm sawn stone on a 10 mm bed, which put the finished top at 510, and at 510 an average adult's feet leave the paving and the front arris cuts across the underside of the thigh. People lean on it instead, which is what a wall at 510 is for.
Nothing in that sequence was a mistake anyone could point at. The stem was right, the plumb was right, the cap was the cap that was specified. What went wrong is that the sitting height was treated as a property of the wall rather than as a sum, and the two terms nobody added came from a different supplier on a different order. Once the cap was bedded the only remedy left was to lift the paving in front of it by 60 mm, which is what happened, and the terrace falls have been fighting the door threshold ever since.
That is the whole difference between this job and the other walls on this site. A free-standing garden wall is checked for slenderness and for rotation in its trench; a retaining wall is checked for overturning and for sliding. A 450 mm seat wall passes all of that on inspection without a calculation being run. It fails on 60 mm of stone that was never in anyone's sum, and on a footing depth chosen from the wrong governing case. Both are settled before the trench line is marked, or they are not settled at all.
The Seat Height Is a Sum and the Cap Is Inside It
Write the sum out on the datum peg, downward from the finished paving: cap thickness, plus its bedding, plus the stem, plus whatever sits between the top of the footing and the paving surface. Read it the other way and you get the top-of-footing level, which is the one dimension on the whole job that cannot be corrected afterwards. Everything above the footing has some adjustment in it. The footing has none — it is a cast surface with a wall standing on it before anybody notices the number was wrong.
Take 450 mm as the target and a 50 mm sawn cap on a 10 mm bed. The stem has to come out at 390 mm above the paving, and if the top of the footing is set flush with the paving surface, that is the stem height you form. Set the footing 25 mm proud because the trench bottom was untidy and the finished seat is 475 mm, with nothing available to take it back except the bed joint, and a mortar bed has perhaps five millimetres of honest range in it before it either starves or squeezes out under the weight of the stone.
The target is not a matter of taste. Seat height wants to sit at or just below the underside of a shod adult's knee, which is why domestic chairs cluster between 430 and 460. The published statement anyone can look up is the 2010 ADA Standards for Accessible Design, Section 903, covering benches: a seat 17 to 19 inches — 430 to 485 mm — above the ground, 20 to 24 inches deep, at least 42 inches long, with back support, and proven under a 250 lbf load applied at any point. ICC A117.1 carries the same set. A landscape seat wall is usually not the bench that section governs and cannot give back support anyway, so where accessible seating is required it supplements rather than answers — but the range checks a number chosen for other reasons.
The build-up decides where the adjustment lives. A cast stem is continuous, so its height is a free variable and the cap can be whatever the quarry cuts. A stem built from units is modular, and the module wins: 190 mm block on 10 mm joints goes up in 200 mm steps, so cap thickness becomes the only lever left, and a 60 mm cap on two courses is 20 mm too high with nowhere to put it. That constraint arrives at the merchant rather than the drawing board, which is why it catches people who have only ever cast.
| Stem build-up | Stem | Bed | Cap | Finished top | Where the adjustment lives |
|---|---|---|---|---|---|
| Cast in place | 390 mm | 10 mm | 50 mm sawn stone | 450 mm | the formwork — set the stem to whatever the cap leaves |
| Two 190 mm units | 400 mm | 10 mm | 40 mm cast cap | 450 mm | the cap thickness; the block module is already fixed |
| Two 190 mm units | 400 mm | 10 mm | 60 mm stone | 470 mm | nowhere — only the paving in front can absorb it |
| Segmental units, 150 mm high | 300 mm, two courses | adhesive | 60 mm cap | 360 mm | nothing between this and 510 mm; the module has no half step |
What a seat wall is made of, and what each layer is bought by
- Cap — the seat, the weathering and the last 60 mm of the sitting height, ordered as a linear run rather than an area Parapet Coping Cap Linear Footage Calculator
- Cap bedding — the only layer with any adjustment left in it, and about five millimetres of honest range before it starves
- Facing veneer — bought by face area and its mortar by area times bed thickness, and never part of the stem thickness you type into a volume Manufactured Stone Veneer Mortar Coverage Calculator
- Concrete stem — the structural core, whose height is the seat height less the cap and its bed rather than the seat height itself Built-In Bench/Planter Seat Wall Concrete Volume Calculator
- Strip footing — usually the larger of the two pours on this job, and sized by frost depth rather than by the weight standing on it Continuous Footing / Grade Beam Volume Calculator
- Compacted formation — undisturbed ground or compacted stone, drained so there is no water available to form ice lenses under the footing Gravel Base Layer Tonnage Calculator
Depth Is Bought in Cubic Metres
Seat depth and wall thickness are the same decision seen from two ends. A 200 mm stem with 30 mm of veneer on each face finishes 260 mm wide; give the cap a 45 mm overhang each side and the seat is 350 mm front to back. Three hundred and fifty is a perch. People sit on the front 150 mm of it with their weight forward and their forearms on their knees, which is fine outside a pub and wrong in a garden where somebody is meant to stay for an hour.
The useful bands are narrow. Around 350 you get a perch; 400 to 460 is a seat one person uses properly; beyond about 500 two people sit back to back or one lies along it, and the ADA bench range starts at 510. The tempting way to buy that depth is to cantilever more cap, and it is the one route with no published rule behind it. A slab overhanging 95 mm each side with an adult's weight on the nose is a brittle plate in bending on an adhesive bed, and what decides it is that stone at that thickness: ASTM C880 measures the flexural strength, ASTM C1528 covers selecting stone for exterior use, ASTM C1242 covers the attachment. None yields a number that belongs in an article, and inventing one would put it under somebody sitting on it.
So buy the depth with concrete instead, and price it first. Taking the example wall from a 200 mm stem to a 300 mm stem across 9 m of 390 mm height moves the pour from 0.70 m³ to 1.05 m³, half as much again, and widens the trench with it. That is the exchange rate: about a third of a cubic metre and a wider dig per 100 mm of seat depth on a run this length. It rarely kills a scheme, and it is always cheaper than the alternative, because the wider stem is also what lets the cap overhang safely.
Three Numbers, and Only One of Them Is the Height You Sat On
The volume takeoff wants a length, a height and a thickness, and two of the three are routinely given the wrong value by people who have measured the finished wall accurately. Height means the stem — 390 in the worked example, not the 450 you sat on. Type 450 and the order comes out at 0.81 m³ against a form that holds 0.70, which is fifteen per cent of surplus concrete going stiff in a barrow while somebody decides what to do with it.
Thickness means the structural core, not the finished width. The veneer is a separate purchase measured by face area, its bedding mortar is area times average bed thickness, and neither belongs in a volume of poured concrete. Enter 260 for a 200 mm stem inside a 30 mm skin each side and you have over-ordered by thirty per cent — on this wall, 0.21 m³ of concrete bought for a space already occupied by stone somebody else is delivering.
Length means the centreline, and this is where L-shaped and U-shaped terraces catch people out. Run the tape along the front face of a U with two external corners and you lose one wall thickness at each turn — 400 mm of run on a 200 mm stem, about four per cent of this job. On the centreline the corners take care of themselves. The same discipline covers the shapes the calculation does not fit: it is a rectangular prism by construction, so a curve goes in as its centreline arc length and a stepped wall as one line per step, each with its own height, added afterwards.
What the figure leaves out is worth stating plainly, because two of the three omissions are trivial and one is not. Reinforcement displacement is negligible — four 12 mm bars running 9 m displace about 0.004 m³, well under one per cent, and rounding will swallow it. Formwork irregularity and barrow losses are real but small. The footing is neither: on this wall it is the bigger pour by a comfortable margin, and it has to be ordered as its own line rather than as an allowance on top of the stem.
Feed it the stem height rather than the sitting height, the core thickness rather than the veneered width, and the centreline rather than the front face. On the worked example — 9 m, 390 mm, 200 mm — that is 0.70 m³, or about 0.92 cubic yards, and every one of the three common input errors makes it bigger.
The overall length of the seat or planter wall.
The height of the wall above its base/footing.
The thickness of the concrete wall.
Concrete volume needed
0.6997 yd³
This is a simple rectangular volume estimate — curved or stepped seat walls, integral footings, and rebar/reinforcement volume are not included and should be accounted for separately in the full material takeoff.
They open the calculator with your figures already in it
Built-In Bench/Planter Seat Wall Concrete Volume Calculator: 0.6997 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.
What this calculation does not cover
- This is a solid section, and the thickness box will happily take a block dimension. A seat wall built from CMU takes grout only in the cores that are filled, not the full length by height by thickness — on standard block that lands near half of what this returns, so entering a nominal block thickness orders roughly twice the concrete the wall will swallow.
- The cap is not in it. A seat wall almost always finishes with a coping that overhangs the face on both sides, and that is a separate pour or a separate stone with its own thickness and its own quantity. It also moves the finished sitting height: pour the wall at a comfortable 450 mm (18 in) and then cap it, and people sit 50 to 75 mm (2 to 3 in) higher than intended.
- A landscape pour is usually smaller than what a ready-mix yard will sell. The default here is about half a cubic metre, and most suppliers apply a minimum load or a short-load surcharge well above that, so the number to price against is the yard's minimum rather than this volume — which is also why a small seat wall is often worth pouring alongside another element on the same day.
Nothing Up There Weighs Enough to Matter
Add the wall up per metre of run and the reason bearing never governs becomes obvious. The stem is 0.20 by 0.39, so 0.078 m² of normalweight concrete at about 24 kN/m³, or 1.87 kN/m. Veneer on both faces adds roughly 0.6, the cap 0.45, a 500 by 250 footing another 3.0 — call it 5.9 kN/m. Over a 500 mm wide footing that is about 12 kPa, and people shoulder to shoulder add perhaps 4 more. Against the least generous entry in the IRC's Table R401.4.1 of presumptive load-bearing values — 1,500 psf, near enough 72 kPa, for clay and silt — the wall uses a fifth of the worst soil that table will name.
Frost is the case instead. IRC Section R403.1.4 requires footings to extend below the frost line as established by the local jurisdiction, and that depth is a local number this site will not publish, because it varies by hundreds of millimetres between authorities an hour apart. What is portable is the mechanism: heave needs a frost-susceptible soil, water within reach of the freezing front, and the front itself. Name the soil with ASTM D2487 and the Unified Soil Classification System before arguing about it — silts and silty sands build ice lenses, clean sands and gravels largely do not, and that difference is the difference between a wall that moves every winter and one that does not.
The seat wall then carries a vulnerability a house does not, out of the same arithmetic that made bearing a non-event. Adfreeze — soil bonding to the sides of buried concrete and lifting it as it freezes — acts on buried surface area and is resisted by dead load, and 5.9 kN/m is very little dead load. That inverts the usual instinct: a row of slender deep piers has a great deal of side area and almost nothing standing on it, and is the worse detail against a wide shallow strip on drained ground. Three responses follow. Take the footing base below the local frost depth, the only straightforward answer for a rigid mortared wall. Insulate — ASCE 32, Design and Construction of Frost-Protected Shallow Foundations, governs — though its familiar tables borrow heat from a heated building, so the harder unheated case is the one that applies here. Or drain the ground, starving the mechanism at source, which is usually cheapest on a small terrace.
One line on the concrete order is skipped on garden work more than anywhere else, because the wall does not look like a road: exterior concrete in a freezing climate needs air entrainment. ACI 318 sets freezing-and-thawing exposure classes with air content attached, ACI 332 covers the residential case, and BS 8500-1 and BS EN 206 carry the XF classes in metric markets. Curing sits at the same level — ACI 308R is the guide, and a stem cast in summer and left uncovered because it is only a garden wall is a stem that crazes under its own cap.
- Fix the finished paving level and mark it on a datum peg clear of the trench; every height below is measured down from that mark.
- Subtract cap, bed and stem from the datum to get top of footing, and write that figure on the same peg.
- Dig to the depth frost sets, not the depth the footing section needs; the difference comes back as lean mix or compacted stone.
- Cast the footing to the marked level off a screed rail — the one surface with no adjustment left in it.
- Set the formwork off that surface and check the stem against the datum with the real cap and bed dimensions in hand.
- Cure the stem, then dry-lay the whole cap run to set its joints out before anything is bedded.
A seat wall's footing is the larger pour and the one people forget to order: 9 m at 500 by 250 is 1.13 m³ before waste against 0.70 m³ of stem. Enter the depth your frost requirement gives you, not the depth the section drawing shows, and take the two pours off one ticket if they happen on the same day.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The total linear length of the continuous footing or grade beam.
The cross-sectional width of the footing.
The cross-sectional depth (height) of the footing.
Extra concrete for spillage and formwork irregularities.
Concrete volume needed
6.844 yd³
- Base volume (no waste)
- 6.52 yd³
- Equivalent in cubic yards
- 6.84 yd³
They open the calculator with your figures already in it
Continuous Footing / Grade Beam Volume Calculator: 6.84 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.
What this calculation does not cover
- A GRADE BEAM AND A STRIP FOOTING ARE NOT THE SAME ELEMENT, and nothing here distinguishes them. A grade beam spans between piles or pads and is designed in bending, with steel top and bottom; a strip footing bears continuously and spreads load into the ground. They can share a rectangular cross-section and an identical concrete volume while having entirely different reinforcement, and the volume is the only thing this returns.
- A VOLUME, NOT A DESIGN. The width, depth and reinforcement of a footing come from the load it carries and the ground it sits on, and this takes all three as given. It answers what to order, not what to build.
- Excavation is not the same shape as concrete. Trench sides slump, over-dig happens at every corner, and soft spots get dug out and filled — which is why the volume placed routinely exceeds the volume calculated by more than the waste allowance covers, and why the allowance is worth setting from experience of the ground rather than from a default.
- Frost depth, the founding stratum and the water table decide how deep the footing goes before any of this arithmetic starts. A footing at the right size and the wrong depth is a heave failure waiting for a cold winter.
- Steps in a footing on sloping ground add concrete at every step and are easy to leave out of a straight-run take-off.
- Formwork, blinding, reinforcement, spacers and any waterproofing or damp-proof membrane are separate quantities that are not derived from the volume above.
The Wall That Is Allowed to Move
The other way to answer frost is to stop resisting it. A dry-laid seat wall of segmental units on a thick compacted base has no mortar to crack and no rigid element to break, and it rides seasonal movement the way a block retaining wall does — which is why the unit manufacturers' own details show compacted aggregate under the first course rather than concrete. It has to stay low, the units have to be heavy enough that a shove does not shift them, and the cap has to be adhered, but within those limits it is the more robust of the two systems on ground that moves.
Buy the base against the trench, not against the terrace. Nine metres of wall on a 600 mm wide base is 5.4 m², and at 200 mm compacted that is 1.08 m³, a little over 1.8 tonnes of crushed stone at a typical compacted density. The catch is that you buy loose and place compacted, so the delivered volume has to exceed the placed volume by whatever the material and the compactor lose between them — an allowance no standard publishes, and one the supplier is the only honest source for. What ASTM D698 and ASTM D1557 define is the far end of it: the reference dry density a stated compactive effort should reach, which is the figure a compaction specification is tested against rather than the figure on the delivery ticket. Order in whole tonnes and expect to barrow some of it, because a seat wall trench is rarely somewhere a machine reaches.
The failure mode of this system is mixing it with the other one. A rigid mortared stem on a floating aggregate base gets the worst of both: no footing to carry it below the frost front and no freedom to move without cracking. So does a continuous cast cap glued across a dry-laid wall, and so does the junction where a floating wall meets a rigid paving slab or a building. Pick one behaviour for the whole length, and detail a movement joint wherever the length is forced to change its mind.
The base under a seat wall is a trench rather than a slab, so enter the trench area — length times base width — and the compacted thickness the unit manufacturer asks for. The density field is where the honesty lives: use the figure for the material actually being delivered, not the default.
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.
Cover, Bars and the Crack You Cannot Joint Out
A cast seat wall carries continuous horizontal bars near the top and bottom of the stem and dowels lapping up out of the footing, and what constrains all of it is cover. ACI 318 specifies 3 inches — 75 mm — where concrete is cast against and permanently in contact with earth, which governs a footing poured against a trench, and 1½ inches, 38 mm, for No. 5 bars and smaller in formed surfaces exposed to weather, which governs both faces of the stem.
Do that arithmetic before choosing the thickness. A 200 mm stem less 38 mm each side leaves 124 mm of usable core, comfortable for a single central curtain and impossible for two. A 150 mm stem leaves 74 mm, at which point the bar is theoretically central and practically wherever the cage ended up when the concrete arrived. That is an independent argument for the 200 mm stem, reached from reinforcement rather than seat depth, and the two agreeing is a good sign.
Then accept that the wall will crack and decide where. A long, thin, restrained element cracks from drying shrinkage regardless of the mix, and ACI 224R, Control of Cracking in Concrete Structures, is the document that says so at length. Continuous horizontal steel does not prevent it — it redistributes one wide crack into several fine ones, which on a surface people look at from a metre away is the outcome worth paying for.
What does not work is importing the joint-spacing rule from taller masonry. The empirical concrete masonry method — NCMA TEK 10-2, and the simplified version this site carries for CMU walls — caps spacing at the lesser of about twice the wall height or 25 feet. At 400 mm high that is 800 mm, so the rule asks for eleven joints in a 9 m seat wall: not conservatism, but a rule used outside the geometry it was derived for, which assumed a panel tall relative to its length and restrained top and bottom. What governs here is joints at the changes — external corners, steps in the footing, the junction with a building or a rigid paving — with continuous steel between them, and TMS 402/602 where the wall is masonry rather than cast.
The Cap Does Four Jobs and Gets Ordered as a Length
It is the seat, it is the weathering that keeps water out of the top of the wall, it is the piece that finally sets the sitting height, and it is where people put a glass down. Only one of those four is a quantity, and it is a linear one: the run of the wall plus the waste corners and returns generate. Mitred external corners consume the most, dressed return ends are ordered as ends rather than cut on site from a through piece, and a U-shaped terrace wall has more of both than the drawing suggests.
The run is the easy part and the joints are where a cap fails. Water standing in an open cap joint freezes and works the stones apart, directly under where somebody sits. Give the cap a fall — 1:80 across a 350 mm cap is 4 mm, imperceptible to sit on and enough to shed a shower — and point it deliberately, away from the veneer face people brush past and towards a border, never into the back joint of a paving where it will stand.
Detail the two edges differently because they do different things. The outer arris wants a drip groove or a weathered edge so runoff releases instead of tracking back under the stone and drawing the vertical stains you see under every unweathered coping. The front arris people slide onto wants a small radius or a chamfer, because a sharp sawn edge chips at the point of highest contact and then catches clothing. Forty to 45 mm of overhang serves both without turning the cap into a cantilever problem.
Two site realities finish the cap off. Dark stone in full sun reaches a surface temperature nobody sits on — a real input in an exposed courtyard, irrelevant under a tree, and the usual explanation for a seat wall that goes unused at three in the afternoon. And a cap bedded on weak mortar is liftable: end pieces and corner units earn a pair of stainless dowels into the stem, which costs almost nothing during bedding and is unavailable afterwards.
Written for a parapet, but the quantity is identical: a linear run plus an allowance for the offcuts corners and returns create. Raise the waste percentage for a wall with several turns, and read the result as a number of pieces and joints rather than as a number of metres, because the joints are what the wall will be judged on.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The total length of the parapet wall to be capped.
Extra material to allow for cut waste, laps, and corner pieces.
Coping cap needed
204.7 ft
Coping is measured on the parapet's centreline length, but the quantity that matters is the number of joints, because a coping run fails at its joints and at its fixings rather than along its length.
They open the calculator with your figures already in it
Parapet Coping Cap Linear Footage Calculator: 205 ft — 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 corner and end units, splice plates and the continuous cleat or anchor chair the coping clips to.
- Does not account for thermal movement. A long run needs joints at intervals that suit the metal's expansion, and fixing a run rigidly at both ends guarantees distortion.
- The fall must be inward, toward the roof. Coping that falls outward stains the facade below and, at a joint, drives water into the wall head.
When It Also Holds Soil Back
Put soil behind one face and the wall changes species. At 400 mm of retained fill the lateral pressure is modest, but the governing failure mode has moved from cracking to rotation, and the checks belong to the retaining wall article rather than this one. What stays here are the two consequences that change the takeoff: the back of the wall is now permanently damp, and there is a volume to fill that is not the volume of the box you drew.
Treat the buried face as wet. Concrete against damp soil wicks, and on a veneered seat wall that surfaces on the front as efflorescence and dark patches at the base, months after everyone has left. Waterproof the back, drain the bottom of the planter somewhere that is not the paving people walk on, and resist weep holes through the front — on a retaining wall they discharge onto ground, on a seat wall onto a terrace at ankle height. Stop the veneer short of the paving with a reveal so the skin is not standing in water either.
Then measure the inside, not the outside. A planter 3.0 by 1.0 m externally with 260 mm finished walls has an internal footprint of about 2.48 by 0.48 m, or 1.19 m², and filled to 350 mm — soil below the cap, room for a drainage layer beneath — that is roughly 0.42 m³, about 420 litres. Bagged mixes settle after the first proper watering, so buy the volume and expect to return with a bag or two; filled to the brim on the day, it has a 40 mm gap round the inside of the cap by the following week.
Enter the internal dimensions and the fill depth rather than the external size and the wall height — on the worked planter that is 2.48 by 0.48 by 0.35 m. The bag count it returns is a product count, and it differs between markets because a 50 litre bag and a 1.5 cubic foot bag are not the same bag.
The length of the raised bed.
The width of the raised bed.
The depth of soil you're filling, which may be less than the full bed wall height.
Soil bags needed
11 x 1.5 cu ft bags
- Soil volume
- 15.67 ft³
They open the calculator with your figures already in it
Raised Garden Bed Soil Calculator: 11 x 1.5 cu ft bags — 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 count is locked to one bag size — 1.5 cu ft on the imperial view, 50 L on the metric one. Other stocked sizes (1 cu ft, 2 cu ft, 40 L, 60 L) and bulk-by-the-yard or by-the-tonne deliveries are outside it, and a printed bag volume is loose product that occupies less once it is wetted and settled in the bed.
- The figure is a straight length x width x depth with no allowance for settling, for the top-up most beds need after the first season, or for spillage and part-bags. The only slack in the number is the fraction of a bag rounded up at the end.
- It assumes a rectangular bed with vertical sides and one uniform fill depth. Tapered, circular, keyhole and L-shaped beds, beds stepped down a slope, and any bottom layer — drainage gravel, a hugelkultur wood core, landscape fabric or gopher wire — are not modelled; split those into layers or rectangular sections and add them up.
- This is a volume, not a growing-medium specification. It says nothing about what to fill with — a raised bed is normally filled with a blend rather than straight topsoil — and it does not check drainage, pH, nutrients, or whether the mix, the ground beneath it, or the bed material itself is suitable for food crops.
- A volume figure is not a load check. Filled and watered, a bed this size is heavy enough to matter on a balcony, roof terrace, timber deck or paved podium, and nothing here confirms that structure will carry it or that the bed walls can resist the outward pressure of the fill — the reason tall, thin sides bow and split.
Sitting on the Edge of a Drop
A seat wall along the open edge of a raised terrace raises a question with nothing to do with seating. IRC Section R312 requires a guard where the walking surface is more than 30 inches above the floor or grade below, measured within 36 inches horizontally, and limits its openings to those a 4-inch sphere cannot pass. A 450 mm seat wall is not a guard and does not become one by being solid. Where the drop needs a guard, the guard is a separate element and the seat wall has to be arranged so it does not undermine it.
The undermining runs the opposite way to instinct. Guard height is measured from the adjacent walking surface, and a seat wall in front of a required guard has created a new surface a person — most obviously a child — can stand on. Whether that reduces the guard's effective height on a given arrangement is a question for the authority having jurisdiction, and one to ask while the layout is still a drawing rather than after the stone is bedded. For scale: OSHA's construction guardrail provisions treat a wall of at least 21 inches, 533 mm, as removing the requirement for a midrail, and a seat is shorter than that.
Two boundary conditions belong in the same conversation. A seat wall returning into a flight of garden steps lands its cap somewhere on that flight, and unless it is set out with the risers it produces one step whose rise differs from its neighbours, which is a trip rather than a detail. And a wall along a public frontage may fall under highway or right-of-way rules rather than building ones, with sightline and projection requirements that say nothing about how comfortable it is to sit on.
Settle these before the trench line goes in
The order below is deliberate. The finished sitting height fixes the top of the footing, the top of the footing fixes the dig, and the dig is the only one of the three that cannot be revised once concrete is in the ground.
- Finished sitting height — The number a person actually feels. It belongs on a datum peg on site, not on a drawing that stays in the van.
- Cap thickness plus its bedding — Subtracted from the sitting height before the stem is set out. This is the 60 mm that turns a seat into a lean-on.
- Stem height, thickness and centreline length — Height is the concrete, not the seat; thickness is the core, not the veneered face; length runs down the middle, not along the front.
- Footing depth and width — Set by the local frost requirement and by drainage. The weight above is roughly a fifth of what the poorest presumptive bearing value allows.
- Cap run, corners, returns and joints — A linear order with dressed ends and mitres priced in, read as a count of joints rather than a count of metres.
- Retained soil, if it is also a planter — Internal dimensions and fill depth, a drainage layer under it, and the back face treated as permanently wet.
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.