A Metre and a Half of Fall, and a Price for Ninety Tonnes of Fill
The plot falls 1.4 m from the back boundary to the road and the house sits across the slope. Finished floor level has to be one level, so a ground-bearing slab means bringing the low corner up: roughly 90 tonnes of imported granular fill into a 96 square metre footprint, placed in layers a plate compactor can reach through, tested as it rises, and retained at the edge by a wall that then has to be designed for the push. Six or seven days of groundworks, a dozen lorries, and a compaction record somebody keeps.
The precast alternative closes the same gap with air. The substructure walls go up to floor level as they were going to anyway, the beams span between them, and the distance from the ground to the underside of the floor becomes a ventilated void instead of a purchase. Three people, a morning for the beams, no imported fill, and a deck the superstructure can work off once it has been closed and grouted. On shrinkable clay with mature trees it is also the only ground floor that does not have to be designed against heave, because there is nothing under it to be lifted.
What it saves in tonnage it spends in commitment. A slab tolerates a change of mind — move a partition, move an opening, the concrete does not care. A beam-and-block floor is a set of components cut to length in a factory and laid to a drawing, each chosen against a load that includes where your internal walls are going to stand, and the order goes in weeks ahead. Everything below is about the decisions that have to be made before it does.
The Layout Drawing Is a Groundworks Drawing
Order a floor and the supplier does not send beams first. They send a layout: every beam by reference, its length and position, which beams are doubled or trebled, where the infill runs, where openings are trimmed, and — critically — every line of support the design has assumed. That drawing is the document the groundworker needs, because two of the lines on it may be internal footings nobody has dug.
This is the sequencing error that costs self-builders a fortnight. Foundations get set out from the architect's plan, which shows external walls and the internal walls load-bearing for the roof. The precast designer has meanwhile looked at the clear spans and decided a 5.8 m room needs an intermediate support — a sleeper wall part-way across, or a deeper beam that will not fit the void you set out. If that drawing lands after the trenches are backfilled, you are breaking out to put a footing in.
So the order that works is: internal layout frozen, loads agreed, layout drawing issued and checked against the foundation setting-out, then excavation. On a piled site it bites harder, because the floor beams land on ground beams positioned by the pile cap layout and moving a support line means moving a pile. Eurocode 7 governs the geotechnical design and Eurocode 6 the masonry carrying the bearings; neither will tell you a line is missing from your dig.
The concrete under those lines is the same shape whether it is a strip footing on soil or a ground beam spanning between piles, so it prices the same way. Take the perimeter run and each internal support line separately — the internal ones are the lines that get forgotten, and the ones with awkward access once the external walls are up.
Run the external perimeter first, then run each internal support line the layout drawing shows as a second and third pass — adding the lengths together in your head hides exactly the item this section is about, which is the footing nobody dug.
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.
One Beam, Three Different Spans
A prestressed inverted-T beam has no single maximum span, and a number quoted over the phone is meaningless without three others attached to it. The same beam reaches further at single-block spacing than at double, because halving the number of beams doubles what each carries. It reaches further under a lightly loaded floor than under 65 mm of screed and 100 mm of insulation. And it reaches further with nothing on it than with a 100 mm dense blockwork wall along its length. The load/span tables published by the precast flooring manufacturers — Milbank, Longley Concrete and Forterra's Bison Precast among them — are read the same way: pick the spacing, pick the total dead plus imposed load, read the span.
The imposed side is the easy half. Domestic floors fall in the residential category of BS EN 1991-1-1 Eurocode 1: Actions on Structures — General Actions, and the value to design to comes from the UK National Annex rather than the base document, so take it from there or from the engineer rather than from memory. The dead side is where self-builders under-declare: grout, insulation, membrane, screed and tile bed all count, and a floor specified against a 50 mm screed and finished with 75 mm has quietly moved up a load band.
Partitions turn a table lookup into a drawing. A stud wall running across the beams spreads over several and is usually absorbed. A blockwork partition running parallel to them stands on one, and that beam has to be doubled or trebled — two or three laid side by side with no infill between — which is why the internal layout is settled before the floor is ordered rather than after. Openings behave the same way: a stairwell, a chimney base or a soil stack needs the layout redrawn around it, not a beam cut short on site.
| Input | What moving it does | Where the value comes from |
|---|---|---|
| Beam spacing | Single-block, double-block and treble-block layouts put progressively more deck on each beam | The manufacturer's table has a separate column per spacing — read the right one |
| Beam depth | The depth available is limited by your void depth and your floor level, not by what spans furthest | Manufacturer's range, checked against the substructure heights already built |
| Imposed load | Sets the design load band before any of your own build-up is counted | The residential category of BS EN 1991-1-1 with its UK National Annex |
| Dead load of the build-up | Grout, insulation, membrane, screed and finish all sit on top and all count | Your own specification, weighed rather than assumed — screed depth is the big term |
| Line loads from partitions | A blockwork wall parallel to the beams lands on one beam and needs doubling or trebling | The frozen internal layout, marked on the precast supplier's drawing |
| Clear span versus room dimension | The table is read on span, and the bearing at each end is not part of the room | The layout drawing's dimension, not a tape across the finished room |
Which Beam, Which Way Up, and Never a Cut One
The pretensioned wires in an inverted-T beam sit in the bottom flange close to the soffit, because that is where a simply supported member is in tension. The section is asymmetric for that reason: wide flange low, narrow web up. A beam lying flange-up is not one that sags a little more — it is one with its steel in the compression zone, and BS EN 15037-1, the part of the beam-and-block standard covering the beams themselves, assumes the thing is installed as designed. The deeper and more nearly rectangular sections are the ones that actually get turned over on site.
Beams also arrive marked, and on most jobs the mark matters more than the orientation. A load carries three or four references of different length and sometimes different depth, banded together, and the only thing separating a 3.6 m from a 3.9 m at ten paces is a stencilled reference on the end. Check them against the layout drawing as the load comes off rather than as it goes down: a beam in the wrong bay is a lift-and-relay, and a beam in the wrong bay under a stack of infill blocks is a day.
Nothing gets cut, notched, drilled or trimmed. Cutting a pretensioned member releases the prestress from the cut end and there is no site remedy; the same goes for chasing a service into the soffit or drilling a flange to hang something. A beam that turns up damaged goes back on the lorry. Handling is a lifting operation in its own right, and the Lifting Operations and Lifting Equipment Regulations 1998 and BS 7121-4, the lorry-loader part of the crane code, apply on a self-build plot as they do anywhere else.
Then there is camber. A pretensioned beam leaves the casting bed with a slight upward bow, and beams from different beds bow differently, so a long span reads as a gentle ripple rather than a plane. Nothing is wrong. But the deck is therefore not a datum: set insulation and screed depths down from a laser at finished floor level, not up off the blocks, or midspan comes out short.
- Check references off the layout drawing as each bundle lands, and set the bundles down in the order the bays will be filled.
- Store on level, firm bearers close to the ends, battens in a stack vertically above one another — a beam bearing on a mid-span batten is loaded backwards.
- Confirm the bearing surface is at level, true and clean, with the damp-proof course already laid beneath it rather than threaded in afterwards.
- Land each beam onto its full designed bearing at both ends; one short at an end does not get packed up, it gets moved.
- Set the first beam off the drawing's dimension from the wall face, then gauge every spacing off the blocks rather than off the previous beam.
- Walk the deck against the drawing before any infill goes in, while a misplaced beam is still a two-person lift.
The Lorry Has a Reach Chart and Your Site Has Mud
A floor of beams is one or two articulated deliveries with a lorry-mounted crane, and the crane is the constraint nobody prices. Its capacity falls away with radius, so a beam it lifts easily at four metres may be beyond it at nine. The question is not whether the lorry can get onto the plot but whether it can stand somewhere that puts the whole footprint inside its chart — which on a set-back plot usually means a temporary hardstanding along one flank, positioned by drawing the reach radii on the site plan before anybody books a slot.
Whatever it stands on takes outrigger loads, arriving as point loads through spreader pads onto ground that has just been dug and backfilled around. Clean crushed stone over a geotextile is the usual answer, and it is a real purchase: an area, a compacted depth and a tonnage, ordered like any other sub-base to BS EN 13242. It also gets reused by the brick deliveries, the trusses and eventually the scaffold, which is the argument for building it once and properly.
Take the platform area off the reach radii you drew on the site plan rather than off the width of the lorry, and enter the compacted depth you intend to achieve — this is a temporary works item that gets specified by eye and then found to be 40 mm of stone floating on wet clay.
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 Void Is a Dimension, Not a Gap
The space under the beams is designed, and two independent requirements set its depth. Ventilation is the first: air has to move through it, and it will not move through 40 mm of clearance blocked by a mortar snot. Heave is the second: on clay where trees have been felled, the ground swells for years as it recovers moisture, and the void has to accept that movement without pushing the floor up.
Approved Document C publishes a ventilated air space of at least 150 mm to the underside of a suspended timber ground floor and 75 mm to the underside of the wall plate. Those figures are written for a timber floor and this is not one, so treat them as a floor rather than a specification: the clear void beneath a precast deck comes from NHBC Standards Part 5, which covers substructure and ground floors, and from the beam manufacturer's own installation literature.
The heave figure is not a rule of thumb. It comes out of the soil investigation and the tree survey read against BRE Digest 240, 241 and 242, Low-Rise Buildings on Shrinkable Clay Soils — and the compressible void former that holds the void open, its two separate load figures, and why a slip membrane does not substitute for it are all set out on the near-trees guide, which is where anyone building on clay should start. The consequence for this page is narrow and worth stating on its own: on such a site the ground beneath the floor cannot be covered with rigid concrete either, which decides the next section for you.
Covering the Ground Before You Lose Access to It
Approved Document C requires the ground under a suspended floor to be covered so that vegetation cannot grow and moisture is controlled, and it accepts more than one way of doing it: an unreinforced concrete cover over the stripped ground, or a membrane laid over blinded hardcore where the design suits it. On a heave site the membrane-over-hardcore route is the one that stays available, because a rigid oversite in a void that is going to close up is a slab waiting to bear on something. On an ordinary site the concrete is the honest choice, because it survives a winter of rain and a hardcore blanket walked on by every following trade does not.
The thing to grasp is that this pour happens inside a substructure and finishes before the deck goes on, and after that there is no access. Nobody is going back in to level a hollow or fish out the length of ducting that turned out to be needed. So over-prepare: strip the vegetable matter, set the level with a laser rather than by eye so the void depth is uniform across the footprint, get the drainage runs and duct sleeves in and proven, and sweep before the truck arrives. Barrow access is through door openings that may not be formed yet, so agree the route before it is booked.
Measure the internal footprint between the substructure walls rather than the external plan size, and put in the thickness the specification actually names — an oversite is a thin pour and the calculator will flag it as thin, which is correct here rather than a warning to override.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The length of the slab or footing.
The width of the slab or footing.
How deep the concrete pour is.
Extra concrete for spillage, uneven subgrade, and forming imprecision.
Estimated concrete needed
1.358 cubic yards
- Volume (no waste)
- 1.23 yd³
- Volume with waste factor
- 1.36 yd³
- Cubic feet
- 36.67 ft³
- 80 lb bags needed
- 62 bags
They open the calculator with your figures already in it
Concrete Calculator: 1.36 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
- Geometry is one rectangular prism: length x width x a single uniform thickness. Thickened edges, integral footings, haunches, steps, curbs and any non-rectangular outline are not in the figure, and nothing is subtracted for block-outs or openings. Take those off as separate volumes and add them.
- It assumes a flat, compacted subgrade sitting at exactly the depth you entered. Ruts, soft spots, over-excavation and a base that dishes in the middle all take concrete the geometry never sees, and a flat waste percentage is not a measurement of that. On a rough base, check depth across the whole pour rather than trusting the allowance.
- This is a volume take-off, not a structural decision. It accepts whatever thickness you type without sizing it, and says nothing about mix strength, aggregate size, air entrainment, fibre, or rebar and mesh. Slabs carrying vehicles, footings, and anything supporting a structure are a code and engineering question.
- The bag count assumes an 80 lb (36 kg) bag yields about 0.6 cubic feet (17 litres) of mixed concrete, and rounds up to whole bags. Real yield shifts with the product and with how much water goes in, and no other bag size is converted for you.
- The volume is not an order quantity. Ready-mix is sold in fixed increments with a minimum load and its own short-load charges, and concrete left in the drum, the chute or the pump line is not counted. The waste factor covers spillage and forming slop, not the plant's ordering rules.
Code thresholds this tool can check
Code thresholds this tool can check
Checked for United States. Each check below names the body that published the limit it uses. Switching market re-runs them. This is not a code review and has no official standing.
These checks cover only the specific numeric limits listed below. They are not a complete code review: fire separation, egress, structural capacity and accessibility provisions are outside their scope, and only the handful of local amendments offered in the selector are modelled — your municipality may have others. Passing every check here does not make a design compliant. Final approval rests with your local building authority.
WITHIN LIMIT — Concrete floor slabs on ground: minimum 3.5 in (89 mm) thick.
Slab thickness 4.00 in meets the 3.5 in IRC floor-slab minimum. Expansive soils are handled separately under IRC R403.1.8, and any slab carrying vehicles or point loads should be designed rather than taken from the code minimum.
ICC · IRC R506.1
Air In One Wall and Out the Opposite One
Both halves of the phrase are load-bearing. Openings on a single elevation give a void with a pressure gradient and no path; what is wanted is a free route across it, which means vents on opposing external walls and nothing solid between them — so every internal support wall standing in the void is honeycombed, built with gaps in the bed so air passes through.
How free area works — why an airbrick's published figure is a fraction of the hole it sits in, and why a buried one stops counting — is set out on the suspended timber floor guide and reads across here unchanged. What does not read across is the number you design to. Approved Document C's figures, the greater of 1500 mm² per metre run of external wall or 500 mm² per square metre of floor area, are written for a suspended timber ground floor, and this is not one: treat them as a floor and take the requirement from NHBC Standards Part 5 and the floor system's own literature. Then divide by the free area of the telescopic vent rather than the airbrick, because on a cavity wall the vent is almost always the narrower of the two.
The detail that goes wrong is the crossing itself. The airbrick sits low, below the damp-proof course, and the cavity it has to cross is a drainage space; a plain sleeve laid level lets cavity water run inward into the void, and a sleeve laid to the wrong fall does the same. A telescopic vent exists to negotiate the change in level between the outside face and the void while shedding water outward, and it is built in as the wall goes up rather than knocked through afterwards. Grilles and rodent mesh are part of the assembly, not an accessory.
The advantage of designing a void rather than surveying one is that you get to choose the height. Set the vents high enough above finished external level that the void survives somebody else's drive, patio or mulched border, mark them on the as-built drawing, and tell the owner what they are — because from outside they look like decoration.
| Where the air is | What stops it | What to check on site |
|---|---|---|
| At the external face | The airbrick's free area, which is much less than its face size | The manufacturer's published free area for that specific unit, in writing |
| Crossing the cavity | The sleeve or telescopic vent, usually the narrowest point in the chain | That it is built in as the wall rises, and falls outward rather than inward |
| Entering the void | Mortar droppings on the cavity base and on the inner leg opening | A clean cavity below DPC, checked before the deck closes over it |
| Crossing the void | Solid internal support walls, and sleeper walls built without gaps | Honeycombed coursing through every wall standing in the void |
| Reaching a dead corner | A bay projection, a porch return or the inside leg of an L-plan | Its own opposing pair of vents, or a duct run through to it |
| Years later | A drive, a raised patio, a render coat or a mulched border | Vent height above finished external level, recorded and handed over |
Closing the Deck, and Then Brushing It Solid
Infill blocks are a specified component, not whatever is left on the pallet. They are aggregate concrete masonry units to BS EN 771-3 of a stated strength and format, and BS EN 15037 carries a part of its own for the blocks used in these systems; a lightweight block substituted for a dense one changes the deck's weight and its behaviour, and a trench block is not an infill block in any sense.
A deck of loose blocks resting on flanges is not yet a floor. It is a set of independent pieces, each able to rock under a point load, with nothing making adjacent beams share what lands between them. Grouting turns it into one element: a sand-cement slurry brushed into every joint, block to block and block to beam flange, so the blocks key to the beams and to each other. Take the mix from the system manufacturer's installation literature — a sand-rich cement grout gauged wet enough to flow, to which a bricklaying ratio does not read across.
It is also the operation that gets skipped, because the deck looks finished before it happens and the trades behind want to be on it. Do it as soon as the deck is closed and before anything is stacked. Sweep first so the joints are open rather than packed with dust, damp the deck in warm weather so the blocks do not pull the water out of the grout before it fills, brush in two passes at right angles, and go back along the beam-to-block line specifically, because that is the one the brush skates over.
Quantity is a genuine unknown, and worth treating as one. No coverage rate in litres per square metre would be safe to repeat here, because it turns on joint widths, which turn on your beam spacing and block tolerances. Measure it instead: take a square metre of your own deck, add up the running length of open joint, multiply by the joint width and by the depth the grout will actually reach before it chokes. That scales. The converter below accepts 200 litres at a time — roughly a large room — so work room by room and add the answers.
A suspended precast floor, from the ground up
- Floor screed — gauged down from a laser set to finished floor level rather than up off the deck, because a cambered beam means the blocks are not a plane Heated Screed Volume Calculator
- Perimeter edge strip — the one linear item in a floor otherwise made of areas, keeping the screed off the masonry and interrupting the path from warm floor to cold edge Under-Slab Perimeter Rigid Foam Board Calculator
- Rigid floor insulation — sized against Approved Document L rather than against the depth left over once the deck came out where it came out Foam Board Insulation Calculator
- Damp-proof membrane — laid over the structure rather than beneath it, so the beams and blocks sit outside the waterproof envelope and stay dry by ventilation instead Vapor Barrier Calculator
- Grouted infill blocks — loose units resting on the beam flanges until slurry is brushed into every joint, which is the operation that makes the deck one element Grout Weight Consumption Calculator
- Prestressed inverted-T beams — wide flange down with the pretensioned wires near the soffit, chosen off a load and span table and never cut, notched or drilled on site
- Bearing walls and ventilated void — a designed clear depth with air crossing it from vents on opposing walls, not the space left over once the beams landed
- Cover over the ground — concrete or a membrane over blinded hardcore, finished before the deck closes because nobody is getting back in to correct it Concrete Calculator
Put in the joint volume you measured off your own square metre scaled to the room, and the density from the product or mix you are gauging — the default sits in the range of a sanded cement grout, which is a fair starting point for a sand-cement floor slurry but is not a substitute for the supplier's figure.
The total grout volume needed, from a joint volume calculation.
The specific grout product's density.
Grout mass needed
53.4 lb
Density varies by grout type — sanded cement grout is commonly ~1600 kg/m³ (100 pcf), unsanded ~1500 kg/m³ (94 pcf), and epoxy grout can be denser — check your specific product's technical data sheet.
They open the calculator with your figures already in it
Grout Weight Consumption Calculator: 53.41 lb — 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
- Nothing is added for waste. The mass returned is the joint volume multiplied by the density and nothing else, so it makes no allowance for grout left in the mixing tub and on the float, material washed back out with the sponge, joints struck slightly proud, or a batch that stiffens before it is worked in. Order above this figure rather than to it.
- One density is applied to one volume with no water term anywhere in the arithmetic, so the result is the mass of grout occupying the joints, not the weight of dry powder to buy. Bulk cement grout is sold as a dry blend that picks up mixing water on site; enter a dry bulk density if you are ordering powder by weight, and a mixed density only if the in-place mass is what you want.
- The volume figure is taken entirely on trust. Joint width, joint depth, tile format and how completely the joints are actually filled are never seen here — the calculation multiplies whatever number is typed in, so any optimism in the upstream joint-volume estimate is carried straight through into kilograms.
- The answer is a continuous mass figure with an empty breakdown behind it: no sack or pail count, no pack size, and no rounding up to a whole purchasable unit, so the figure you get is not a figure you can buy from a product that ships in fixed sacks. The volume field also stops at 200 litres (53 gal), roughly 320 kg (700 lb) at the default density, so a genuinely bulk order has to be split across several runs and added up by hand.
- The density field accepts 1200 to 1900 kg/m³ (75 to 119 pcf) and refuses anything outside it, so a product heavier than that ceiling cannot be entered even though the note beneath the answer allows for denser epoxy grouts. Within the range the value is treated as fixed for the whole batch, so the same product mixed wetter or stiffer than the basis its data sheet quotes will not weigh what this returns.
Here the Membrane Sits on Top of the Structure
This is the detail that catches people who have only built ground-bearing floors. Under a slab the membrane goes beneath the concrete and everything above it is inside the dry envelope. On a suspended floor it goes over the grouted deck, and the beams, blocks and grout are deliberately left outside it. They are precast concrete and meant to be out there; what keeps them dry is the ventilated void, not a sheet. A membrane under the deck achieves nothing except sealing the void you spent two sections ventilating.
The sheet is doing a different job up here. It stops a wet screed draining away through the joints before it has gained anything from the water, it keeps moisture out of the insulation and the finish, and it links the floor to the walls' damp-proof course so the two read as one line. That last connection is the one to draw before it is built: on a beam-and-block floor the wall course usually sits right at bearing level, which makes the lap easy to form and just as easy to leave out.
Where the site carries a ground gas requirement the sheet stops being a damp-proof membrane and becomes something specified, tested and verified, with welded seams, preformed penetration units and a named verifier — a discipline of its own, covered on the gas membrane guide rather than restated here. Order the plain case by covered surface: deck area, plus the perimeter turn-up, plus laps.
Enter the deck area including the perimeter turn-up rather than the room's plan size, then take the sheet area out of the result and re-divide it by the roll your supplier lists — the roll count assumes a standard poly roll, and a floor membrane is rarely that.
The total crawlspace floor or basement wall area to cover.
Sheet spent where seams overlap before they are taped.
Vapor barrier rolls needed
2 rolls
- Area to cover (with overlap allowance)
- 1,188 sq ft
They open the calculator with your figures already in it
Vapor Barrier 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 is fixed at a 10 ft x 100 ft (3 m x 30 m), 1,000 sq ft (93 m²) sheet. The calculator never asks what you are buying, so a 12, 16 or 20 ft wide roll, a 50 ft roll, or a reinforced 10-20 mil barrier will not divide into this count. Take the area figure from the breakdown and divide it by your own product's stated coverage.
- The overlap allowance is for flat-plane seams only. Nothing is added for turning the sheet up the foundation wall, wrapping piers and columns, sealing around penetrations, or the off-cuts an irregular crawlspace footprint produces. Measure and add those separately.
- Only the sheet is counted. Seam tape, mastic, mechanical fasteners and termination bar are not in this estimate.
- This is a quantity take-off, not a vapour-control design. It says nothing about the permeance, thickness or puncture class the barrier has to meet, or which face of the insulation it belongs on. In a cold store or an unvented crawlspace the wrong side traps moisture inside the assembly no matter how many rolls you order.
- It is not a radon or ground-gas membrane specification. Those are designed, jointed and verified systems with their own material, welding and testing requirements, and a 6-mil poly roll count does not substitute for one.
A Warm Floor Over Deliberately Cold Air
Every other floor on this site sits on ground that is, at worst, at soil temperature. This one sits on outside air, moving by design at whatever the February wind is doing. The insulation under the screed is therefore the only thing between the room and ambient rather than sharing the work with the ground, and the floor edge stops being a mild junction and becomes a genuinely cold one. Approved Document L sets the U-value, and the calculation takes the exposed perimeter to area ratio into account, which is why a long narrow footprint is harder to satisfy than a square one of the same size.
The perimeter strip carries that edge, and its mechanical job — isolating a shrinking topping from the masonry, measured off the internal perimeter with every reveal and alcove return counted — is set out on the failed-slab guide and is the same here. What is not the same is the second job. On a ground-bearing floor the strip interrupts a path into a wall standing in soil. On this one it interrupts a path into a wall standing in outside air, moving, on the coldest night of the year, and that makes it the difference between a warm skirting line and a permanent cold strip around every room.
Measure the internal perimeter of every room with the returns counted in, and set the board length to what your supplier actually sells rather than leaving the default — this is a linear item on a floor whose every other line is an area or a volume, which is precisely why it drops off orders.
The total length of the foundation perimeter to be insulated.
The length of a single rigid foam board as sold.
Foam boards needed
17 boards
They open the calculator with your figures already in it
Under-Slab Perimeter Rigid Foam Board Calculator: 17 boards — 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 divides perimeter by board length and nothing else. There is no upstand depth and no board width input, so it assumes one board covers one board length of run at the full depth required: a 2400 x 1200 mm (47 in) sheet ripped into four 300 mm (12 in) slab-edge strips actually covers 9.6 m (31 ft) of run, while a skirt deeper than the board width needs a second course and doubles the figure.
- There is no waste allowance and no corner allowance in this number. It assumes every offcut is carried onto the next run, which a plan with re-entrant corners, steps and door thresholds does not allow; the related area-based foam board calculator adds 10 per cent for exactly this reason.
- Where boards sit under a thickened slab edge they are a load-bearing layer, and this is a piece count that says nothing about compressive strength grade or long-term creep under sustained load. A board specified for a vertical face is not necessarily graded to bear beneath an edge beam.
- Nothing here covers protecting the foam once it is in. Exterior perimeter foam is a concealed route for termites, and jurisdictions differ on whether an inspection gap or termite shield is required and whether exterior below-grade foam is permitted at all; the above-grade portion also needs render or a protection board against UV and impact, which is a separate material line.
The Deck Is Not a Scaffold Yet
A finished beam-and-block floor is the best working platform on the site and will be treated as one the moment the last block goes in. That is fine once the grout has some strength in it. It is not fine before, and the sequence that goes wrong is always the same: the deck closes on the Friday, a load of blocks and a mixer land on it Monday, and the grouting gets done around the stacks. Point loads on ungrouted infill are precisely what the deck has no capacity for, and a cracked block mid-room is not something a screed hides.
Services deserve a decision at the same stage. Anything rising through the floor — soil stack, rainwater connection, incoming water, gas, electricity, ventilation ducting — passes through a gap the layout drawing left, and gaps do not get made afterwards. Below the deck, drainage in the void stays accessible in a way it never is under a slab: use that, support it off the ground rather than off the beams, and record where it runs.
Keep a short file, specific to this floor. The layout drawing as built, marked up with anything that moved. The tickets showing which beam references arrived. A photograph of each elevation with the vents visible and a tape against the height above finished ground level. A photograph of the grouted deck before the membrane covered it. And the load and span data for the beams actually installed — because the question asked five years later, when somebody wants to take out a wall or lay a stone floor over the screed, is what this floor was designed to carry, and only the person holding that sheet can answer it.
What a suspended floor is bought as, in the order it is committed
The tonnage a beam-and-block floor saves in imported fill comes back as components ordered off a drawing weeks in advance. These are the quantities that have to be right before the lorry is booked, and the two — the internal footings and the grout — that are missed most often.
- Footings and ground beams, perimeter and internal separately — The internal support lines come off the precast supplier's layout drawing, so that drawing has to be in hand before the trenches are dug rather than after.
- Delivery hardstanding, by area and compacted depth — Sized from the crane's reach radii drawn on the site plan, not from the width of the lorry — and it will serve the brick, truss and scaffold deliveries afterwards.
- Ground cover under the void, before the deck closes — Concrete on an ordinary site, membrane over blinded hardcore where heave rules out a rigid oversite; either way there is no access to it once the beams are on.
- Beams by reference, from a frozen internal layout — Doubled and trebled runs under blockwork partitions and trimmed openings are all decided by the layout drawing, and none of them can be improvised on site.
- Infill blocks and the grout that ties them together — Grout volume measured off a square metre of your own set-out rather than taken from a published coverage rate, then converted to a weight of sand and cement.
- Ventilation, counted in free area rather than in openings — The telescopic vent's published free area usually governs the whole chain; opposing walls, honeycombed sleeper walls and a route into every dead corner.
- Membrane over the deck, plus the perimeter strip — Covered surface including the turn-up, re-divided by your merchant's roll; the strip taken off the internal perimeter with every reveal and alcove return counted.
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.