Eleven Millimetres of Skirting, and a Door That Only Binds in February
The house is a 1958 semi with a solid ground floor throughout. In the back room the skirting has lifted clear of the plaster along two walls and stands about eleven millimetres proud at the middle of the run, tapering to nothing at the corners. The lounge door catches on its own threshold and has been eased twice. Nothing outside has moved: the render is uncracked, the bay is plumb, the drains have been surveyed and are sound. A laser in the hallway reads the middle of each downstairs room higher than its perimeter, and the two worst rooms are the two with the deepest made ground beneath them.
That pattern — a floor rising in the middle of the room while the walls stay where they are — is not subsidence and it is not damp. Subsidence takes walls down and cracks them; this is a floor going up, and it is going up because something underneath it is occupying more space this year than it did last year. The material doing that is almost never the concrete. It is whatever was tipped in as hardcore before the slab was cast, in a decade when the cheapest available fill on a housing site was frequently whatever the nearest industry had a spoil heap of.
Some Fill Is Inert and Some Is Still Reacting
Clean crushed limestone under a slab does nothing for a hundred years. Several of the materials used in its place in the 1950s and 60s do a great deal, and they take decades to finish. BRE Digest 276, Hardcore, catalogues them, and it exists because so much of what went under post-war floors was chosen on haulage distance rather than chemistry.
Three mechanisms account for most of what is seen. Unburnt colliery shale contains pyrite; wet it and expose it to air over years and the pyrite oxidises to sulfuric acid, which attacks any carbonate present and precipitates gypsum, and gypsum takes more room than what it replaced. Some steel-making slags carry free lime and free magnesia that hydrate slowly and swell as they do. And demolition rubble containing gypsum plaster delivers a large soluble sulfate load straight to the underside of the slab.
Once soluble sulfate is in the ground water around cement paste, the reaction moves into the concrete. Sulfate reacts with the hydrated aluminate phases to form ettringite, which is bulkier than what it forms from, so the affected concrete expands, softens and loses its arrises. In cool, persistently wet conditions where carbonate is also available, the thaumasite form can develop instead, and it is worse because it consumes the calcium silicate hydrate that gives the paste its strength rather than only disrupting it. The Thaumasite Expert Group report to DETR, The thaumasite form of sulfate attack: risks, diagnosis, remedial works and guidance on new construction, sets out that distinction and the signs that separate the two.
Which of these is running decides how much has to come out. Expansion inside the fill continues under anything you put on top of it; expansion inside the slab stops once concrete and sulfate source are separated. On a 1950s floor both are usually present and both driven by the same fill, so the floor answer is the same either way — but the answer for the wall foundations, which stand in that ground and are not coming out, depends on knowing which.
Three Samples and a Level Survey Beat Any Amount of Opinion
Open a trial hole through the slab in the worst room before pricing anything. A core or a cut square 300 mm across gives the slab thickness, whether there is a membrane and where it sits, what the fill is, how deep it goes and whether it is wet. Sample the fill through its full depth rather than the top 100 mm: tipped material is often layered, and the reactive part is frequently at the bottom where it has stayed wettest.
Send it for water-soluble sulfate and pH. The test methods are in BS 1377-3, Methods of test for soils for civil engineering purposes: Chemical and electro-chemical testing; the interpretation is in BRE Special Digest 1, Concrete in aggressive ground, which turns a sulfate result and a pH into a design sulfate class and an aggressive chemical environment class. Those classes are what the concrete specification is written against later, and without them any mix you order is a guess dressed as a decision. Ask for total potential sulfate too where the fill looks like colliery shale — that is the figure that says how much more sulfate is still to come out as the material weathers.
Run the levels properly at the same time. A laser and a staff across a grid of a metre or so gives a contour of the lift, and its shape is diagnostic: a dome centred in the room points at the fill, a ridge along a wall line points at something else. Repeat after six months if the programme allows, because a rate of movement is worth more than a magnitude. Record damage to the fabric above against the categories in BRE Digest 251, Assessment of damage in low-rise buildings, so what you write down means the same thing to the next reader.
| Observation | Recorded as | What it decides |
|---|---|---|
| Depth of made ground below the slab soffit | A dimension from the underside of the slab to natural ground | Whether a ground-bearing replacement is permitted at all |
| What the fill is made of | A description: colliery shale, slag, brick and plaster rubble, ash, clean stone | Whether the material is still reacting or is merely poor |
| Water-soluble sulfate and pH, tested to BS 1377-3 | A design sulfate class and ACEC class read off BRE Special Digest 1 | The concrete specification for everything cast into that ground |
| Presence and position of any existing membrane | Above slab, below slab, or absent | Whether the floor has been wet for sixty years or only recently |
| Slab condition at the underside | Sound, softened, or crumbling with visible white deposit | Whether the attack has reached the concrete or is still in the fill |
| Contour of the lift across the room | A levels grid, dated, repeated where time allows | The rate of movement, and whether the source is under the room or under a wall |
Ground-Bearing Again, or Spanned Clear of It
There are two honest replacements and a third that is neither. The first digs out the reactive fill, replaces it with inert material and casts a new ground-bearing slab. The second leaves the ground where it is, or takes out only what working room needs, and spans a suspended floor wall to wall so nothing the ground does afterwards is transmitted upward. The third — recasting on the same hardcore — buys perhaps a decade, and is worth considering only where testing shows the fill is inert and the slab was attacked from a source now removed.
Depth of fill usually settles it, before anybody's preference gets a vote. Guidance for new housing sets a depth of made ground beyond which a ground-bearing floor slab is not accepted and a suspended floor is required instead; BS 8103-1, Structural design of low-rise buildings, and the NHBC Standards both draw that line, and where a job sits relative to it is the most useful thing to establish on the first visit. Deep fill is also where digging out stops being a floor job and becomes an excavation beside the wall foundations, bringing temporary works and, on a semi or a terrace, the excavation provisions of the Party Wall etc. Act 1996 into scope.
The suspended option costs more in material and less in disposal, and changes the shape of the risk rather than removing it. A beam-and-block floor to BS EN 15037, Precast concrete products — Beam-and-block floor systems, bears on the existing walls, so those walls have to take it and the bearing has to be formed rather than chopped into whatever brickwork survives the breakout. It also needs a through-ventilated void, meaning sleeved openings out to air on opposing walls — straightforward in an extension, awkward mid-terrace with a lounge one side and a kitchen the other. Against that: the reactive fill stays put, arisings drop to the slab alone, and the void absorbs whatever the ground does next.
| Compared on | New ground-bearing slab on clean fill | Suspended floor spanning the room |
|---|---|---|
| Suits | Shallow made ground, all of it removable, sound wall foundations | Deep made ground, fill that cannot be fully removed, or ground still moving |
| Depends on | Getting every last band of reactive material out under the whole footprint | The existing walls being able to carry a bearing and the void being ventilable |
| Arisings | Old slab plus the entire fill volume, all of it barrowed out | Old slab only, and whatever is dug for the bearing |
| Material in | Clean granular fill, sand, membrane, insulation, concrete | Beams, infill blocks, membrane, insulation, screed or deck |
| Residual risk | Any reactive material left behind keeps expanding under the new slab | The floor is isolated, but the walls still stand in the same ground |
| Governed by | BRE Special Digest 1 for the concrete, Approved Document C for the floor | BS EN 15037 and BS 8103-1 for the floor, Approved Document C for the void |
What Goes Back In, Bottom to Top
Assume the ground-bearing answer for the rest of this page, because it is the one with the takeoff in it. The replacement is six or seven distinct purchases sitting one on another, each bought in a different unit, and floors like this get mispriced because the estimate covers the concrete and the skip and treats everything between them as sundries. It is not sundries: on a 20 m² room the fill outweighs the concrete.
Read the stack below as a bill of separate orders rather than as a construction detail. The accented layer is the one this job exists to change — everything else would be identical on a new-build floor, and the fill is the reason anybody is breaking up a sixty-year-old slab at all.
The replacement floor, as six purchases and a formation
- New slab — specified against the sulfate class the testing produced, not against the mix that happened to suit the last floor the plant supplied Concrete Calculator
- Perimeter edge strip — keeps the slab from bearing on or bonding to the wall, so drying shrinkage pulls the concrete inward instead of loading the masonry Under-Slab Perimeter Rigid Foam Board Calculator
- Rigid floor insulation — a renovated ground floor is a thermal element, so the board thickness answers to Approved Document L rather than to what fits the level you inherited Foam Board Insulation Calculator
- Damp-proof membrane — laid as a tray with the edges turned up and lapped into the wall's own damp-proof course, because a flat sheet stopped at the skirting drains into the wall Vapor Barrier Calculator
- Sand blinding — a smooth bed so the sharp arrises of the fill cannot work through the membrane under the weight of the wet concrete above
- Clean granular fill — the entire point of the job: inert graded stone with a sulfate result on the ticket, placed in shallow layers and compacted as it rises Gravel Base Layer Tonnage Calculator
- Compacted formation — natural ground exposed once the reactive material is gone, checked for softness while a barrow of stone is still the cheapest available repair Standard/Modified Proctor Compaction Percentage Calculator
The Floor Is a Weight Before It Is a Volume
Two containers and a grab lorry get booked on a job like this, and what fills them is a weight, not a volume. A 100 mm slab across 20 m² is 2 m³ of concrete, which is close to five tonnes before a hammer touches it; the fill beneath it, at 350 mm across the same room, is another 7 m³ and another twelve tonnes or more. Order a container by its cubic capacity and it will be refused at the kerb long before it looks full.
There is a trap in converting it. Loose-density figures for rubble — around one and a half short tons per cubic yard for broken concrete — describe a heap with the voids that breaking creates, not the intact slab. Put the slab's own 2 m³ into a rubble conversion and the answer comes back light, because the material has not been bulked. Breaking adds something in the order of half again to the volume, so bulk the measured slab first and then convert. Divide the result by the permitted weight of the container the yard offers and book that many. Separating arisings at the point of breakout — clean broken concrete one way, mixed fill the other — costs nothing; separating at the tip costs a gate fee.
Two obligations bind before any of it moves. A refurbishment and demolition survey under the Control of Asbestos Regulations 2012 comes first, because thermoplastic floor tiles and the black bitumen adhesive beneath them were routinely asbestos-containing in exactly this period and sit precisely where the breaker is about to go. And the dust from breaking concrete is respirable crystalline silica, controlled under the Control of Substances Hazardous to Health Regulations 2002 — in a sealed room that means on-tool water suppression or extraction, not an open window. How long anyone holds the breaker is governed by the Control of Vibration at Work Regulations 2005.
Enter the bulked heap volume rather than the intact slab volume — the density behind this conversion is for broken material with the voids that breaking creates, and the tonnage it returns is what decides how many containers get booked.
The volume of concrete rubble in cubic yards.
Approximate weight
15 short tons
About 1.5 short tons per cubic yard for broken concrete; solid concrete is denser, but rubble carries large voids. A planning figure, not a specification. Check the figure against the container's or vehicle's weight limit before booking.
- Conversion factor applied
- 1.5 short tons per cu yd
They open the calculator with your figures already in it
Concrete Rubble Cubic Yards to Tons Calculator: 15 short tons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- What leaves site is more than the element on the drawing. A ground-bearing slab lifts with blinding, hardcore or screed still attached and a grubbed-up foundation comes away with soil on it, while a suspended slab, a wall or a column carries none of that. Those extras add to the load, but they are not broken concrete and do not convert at the density used here, so work them out separately instead of folding them into this volume.
- This factor is for broken material, not for the structure. Breaking a slab out bulks it up, so the heap takes appreciably more room than the element did, and a volume measured off an intact slab or wall converts at a solid-concrete density nearer two short tons per cubic yard rather than the 1.5 used here.
- The tonnage has to stand somewhere before it is collected. A few cubic yards of arisings is a modest-looking heap and several tons pressing on whatever is beneath it — a suspended slab, a basement lid, a drain run, a paved surface. The conversion gives you that load; it says nothing about whether the standing will carry it.
Demolition arisings are the material most likely to catch out a skip booking, because broken concrete is heavy and containers are usually weight-limited long before they are full. The density here is for broken material with the voids that breaking creates — solid concrete is considerably denser per cubic yard, which is why an intact slab converts differently from the rubble it becomes. Estimating the volume from the structure being removed and converting to weight is the way to find out whether a container will be refused, and it is a far cheaper discovery than making it on collection day.
No Machine Gets Past the Front Door
Everything crosses the same threshold twice: several tonnes out through a hallway, and several tonnes of stone, sand and concrete back in along the identical route. That route is the programme. A mini digger that cannot get through a 760 mm door or turn in a hall is not on site, so the honest way to price the work is by the barrow, in both directions, with a real figure for how far each one travels. It is also the route the household uses, which is where the Construction (Design and Management) Regulations 2015 stop being paperwork and start being a question about who walks past an open floor at seven in the morning.
Convert the arisings and the incoming material to loads separately, then treat the count as a floor rather than a forecast. The calculator works from a practical load of about five cubic feet, the usable fill of a standard six-cubic-foot barrow — fair for mulch or dry sand, and a serious overestimate for anything heavy: a barrow filled to that volume with wet concrete carries something over three hundred kilograms, and with broken concrete around two hundred. Nobody pushes that up a ramp. A heavy barrow gets loaded to a third or half of that, so double or treble the count for concrete and rubble and leave it alone for stone and sand.
- Board and sheet the whole route before the first blow — floor protection, corner protection on the architraves, and a taped dust screen at each doorway.
- Check what the barrow route crosses: a suspended timber floor in the hall will not take repeated point loads from a laden barrow without a spreader board.
- Set the ramp out of the door at a gradient somebody can push a loaded barrow up, not the steepest one that fits.
- Run arisings out and material in on the same protected route, and never leave a full container overnight where a laden barrow has to reach past it.
- Convert both volumes to loads before quoting, then apply the weight correction for concrete and rubble.
Run the arisings and the incoming fill through separately — the load count is the labour line on this job, and it is the one thing about a floor replacement that a merchant's quotation will never contain.
The total material volume to move.
Wheelbarrow loads needed
27 wheelbarrow loads
This assumes a standard 6 cu ft (170 litre) wheelbarrow loaded to a practical, non-spilling level — a smaller or larger wheelbarrow changes the load count proportionally.
- Total volume
- 135 ft³
They open the calculator with your figures already in it
Wheelbarrow Loads Calculator: 27 wheelbarrow loads — 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 assumes one barrow size - about 5 cu ft of usable volume in a standard 6 cu ft builder's barrow. There is no input for the barrow you actually own, and the trip count scales in direct proportion to its capacity, so a small garden barrow or a large contractor barrow will not match the figure shown.
- It counts volume, not weight. Density is never read, so a load of wet concrete, saturated gravel or wet clay counts the same as a load of bark mulch even though it can be several times heavier. Dense material normally has to be moved in part-loads, which pushes the real trip count above this figure.
- Nothing about the route is modelled: distance, gradient, ramps, steps, gates, ground condition, or how many people are barrowing. This is a trip count, not a time or labour estimate, and on a concrete pour it says nothing about whether the loads can be moved and placed before the mix stiffens.
- The volume you enter is taken at face value. Swell in excavated soil, compaction of delivered material, spillage between loads and material left clinging in the barrow are not added, so a figure lifted from an in-place or compacted volume understates the loose material actually shifted.
- This is not a manual-handling assessment. It does not judge whether a full load is safe for one person to lift, push or tip, how often it can be repeated, or over what distance - that judgement sits with the workplace handling rules applying on your site.
Clean Means Something Specific on the Ticket
The replacement fill is the only reason this floor is being dug up, so it is the item to specify hardest and to check on delivery. What is wanted is inert, well-graded, angular material of known low sulfate content: crushed limestone or granite sub-base to BS EN 13242, Aggregates for unbound and hydraulically bound materials for use in civil engineering work and road construction, or the Type 1 granular sub-base described in Series 800 of the Specification for Highway Works. Ask for the sulfate result with the delivery, not as a favour afterwards.
Recycled aggregate is where this goes wrong. A crushed demolition product can be a perfectly good sub-base and can also be the same category of material being removed from this floor, and the only thing separating the two is a test certificate. Where it is permitted at all it should be permitted against a stated sulfate limit — and on a job whose entire premise is sulfate heave, paying the difference for virgin stone is a defensible line.
Placing it is shallow layers and patience. Fill tipped to full depth and rolled from the top does not densify through its thickness whatever the plate does to the surface, so each layer goes in at a depth the compactor can reach through and is compacted before the next arrives. In a room with no machine access that means a vibrating plate or a rammer working around obstructions — slower than it sounds, and the second labour line an estimate usually misses. Take the tonnage off the compacted thickness and let the merchant state the bulking allowance for loose delivery.
Enter the room area and the compacted depth you actually intend to achieve — and if the depth you need exceeds what this will accept, that is a signal to go back to the choice between bearing on the ground and spanning clear of it.
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.
One Sheet, or It Is Not a Membrane
A damp-proof membrane fails at its discontinuities and nowhere else. The sheet is effectively impermeable; what lets water and ground gas through is an unsealed lap, a tear off the corner of a fill stone, or a pipe threaded through and left. Approved Document C, Site preparation and resistance to contaminants and moisture, requires the floor to resist moisture from the ground; the specification names the material and the position, and neither is a site decision.
Position is worth settling on paper. Below the slab, the membrane also keeps ground sulfate off the concrete's underside, which on this job is a second reason to put it there. Above the slab and under the screed it protects the finish but leaves the slab in contact with the ground. Below the insulation and above it are both used and differ in where condensation can form. Whichever is chosen, the sheet is laid on a blinded surface rather than straight onto stone: sharp arrises under wet concrete perforate polythene reliably and invisibly.
Quantity is straightforward and the trap is the roll size. The calculator below works to a nominal thousand square feet per roll with an overlap allowance built in — a common poly roll, and not what a UK merchant hands you for a floor DPM, which is typically 4 m wide on a 25 m roll in a heavier gauge than the six-mil material that figure assumes. Take the sheet area from the result and re-divide it by the roll your supplier stocks. Material classes for under-slab vapour retarders are in ASTM E1745 and their installation in ASTM E1643 — the documents to quote when someone proposes a lighter sheet.
Take the sheet area for the room, then re-divide it by your merchant's roll width and length — the coverage assumed here is a standard poly roll, and a floor DPM is usually sold heavier and in a different size.
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.
Where the Membrane Has to Find the Wall's Own Course
The junction between the floor membrane and the wall's damp-proof course is where retrofit floors leak, and a new-build slab never has to solve it this way round. Here the wall's course was laid in 1958 at whatever height the bricklayer worked to, and the new build-up has to reach it, lap it in the correct direction, and be dressed up behind the plaster line without being cut by a skirting nail six months later. Find the existing course before the levels are fixed, not after the fill is in.
Sealing makes the sheet a barrier, and it comes in three parts counted separately: the run-to-run seams where sheets lie side by side down the room, the turn-ups at the perimeter and the seal into the wall, and a taped collar at every pipe, duct and drain coming through. The calculator answers only the first — room width against sheet width gives the number of parallel seams, each run the length of the room. Perimeter and penetrations are added on top, and on a small room with a soil pipe, a gas entry, a water main and a heating flow and return, the penetrations can be the larger number.
This counts the seams between sheets laid side by side and runs each the length of the room; add the perimeter turn-up and a collar for every penetration separately, because on a small floor those two together often exceed the run-to-run total.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The overall width of the slab area being covered by the vapor barrier.
The width of a single vapor barrier roll or sheet.
The overall length of the slab area, running the direction of each seam.
Extra tape to allow for overlaps, patches, and cut waste.
Seam tape needed
154.3 ft
- Number of seams
- 3
They open the calculator with your figures already in it
Under-Slab Vapor Barrier Seam Tape Calculator: 154 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
- The sheet count divides the slab width by the full roll width, so nothing is deducted for the side lap where adjacent sheets overlap each other. Laps are typically specified somewhere between 150 mm (6 in) and 300 mm (12 in) depending on the barrier manufacturer and the project specification, and once that is taken off the effective coverage of each sheet a wide slab can need one more sheet, and one more seam, than this calculation reports. Enter the lapped coverage width rather than the printed roll width if you want the lap counted.
- Only the longitudinal seams between sheets laid side by side are counted, and each is taken as running the full slab length in one unbroken run. If the roll is shorter than the slab, every run also carries an end lap partway along it, and there is no roll-length input here, so that transverse tape is absent from the total.
- The figure is seam tape between sheets and nothing else. Sealing the barrier to the footing, the foundation wall or the slab edge, and taping around pipes, conduit, column bases and other penetrations are all excluded and have to be estimated separately. On the default 20 m by 15 m slab the perimeter alone is 70 m against the 47.25 m of seam tape returned, so the excluded work can be the larger quantity.
- Which dimension you enter as the width decides the answer, and the calculator does not compare the two lay directions for you. Running 6.1 m sheets across the 20 m side of a 20 m by 15 m slab gives three seams and 47.25 m of tape, while running them across the 15 m side gives two seams and 42 m. The geometry also assumes one plain rectangle, so an L-shaped or stepped slab has to be broken into rectangles and the results added.
The Slab Must Not Touch the House
A ground-bearing slab shrinks as it dries, and a slab cast hard against the surrounding masonry has nowhere to shrink to. Across a room 4.5 m wide that movement is small in absolute terms and quite large enough to crack a plaster line, lift a skirting, or push load into a wall never asked to carry any. The compressible perimeter strip keeps the two independent, and on a retrofit it does a second job: it interrupts the thermal path from slab edge into external wall, which is where a solid floor loses most of what it loses.
It is bought by the metre of perimeter, not the square metre of floor, which is why an estimate written from an area takeoff loses it. Count the full internal perimeter of every room, including the returns into a chimney breast, an alcove and a door reveal — those returns are where the strip runs short, and a strip stopping at the last easy corner leaves the slab bearing on masonry exactly where a crack will show. Rigid boards for this use are made to BS EN 13163 for expanded polystyrene and BS EN 13164 for extruded polystyrene, and the product literature states the compressive behaviour that decides whether a board suits going under a slab edge rather than beside it.
Height matters as much as length. The strip runs from the top of the insulation to finished floor level so the slab surface is isolated too, and it is trimmed after the pour and before the skirting goes back — a strip cut down to slab level during the pour leaves the topping bridged to the wall and undoes the detail.
Measure the full internal perimeter including every alcove and reveal return, then let this turn it into board lengths — this is a linear item on a job whose every other line is an area or a volume, which is exactly why it goes missing.
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.
Pouring Into a Room
The concrete specification comes from the testing, not from habit. Where the ground around the new slab is aggressive, BS 8500-1 and BS 8500-2, the complementary British Standards to BS EN 206, provide designated concretes selected against the design chemical class that BRE Special Digest 1 produced from the sulfate and pH results. Ordering a general-purpose mix into ground that has already destroyed one slab is the most expensive available way to save an afternoon.
Volume comes off the excavation you finished, not the slab you drew. Once the fill is compacted and blinded, that surface sits at a real level, and the difference between it and the finished floor is the thickness you are actually paying for. Gauge it on a grid rather than at the corners: a fill layer left 20 mm low across a 20 m² room is 0.4 m³ of extra concrete, bought at full rate and buried forever. Add a modest allowance on top of the measured figure, because a short load in a room with one access point is far worse than a barrow of surplus.
Getting it in is the constraint nobody costs properly. A pump line through a window, a conveyor to a rear opening, or a truck on the pavement feeding a chain of barrows are three different day rates, and which is available depends on the frontage, the parking suspension and what the neighbours tolerate. The pour rate is then set by the slowest link in that chain rather than the plant's discharge rate — which argues for part loads or a second truck over one large delivery stiffening in the drum.
Everything after that behaves like any other slab, and that common ground lives elsewhere on this site. What is specific here is a closed, unheated, often unventilated room, which stretches the whole timetable.
Measure the finished excavation rather than the drawing: length, width and the thickness you can actually verify off the blinding, plus an allowance, because a short load into a room with one access point is not a problem that gets fixed with a wheelbarrow.
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
The Slab Is Not Ready When It Is Hard
A new ground-bearing slab holds a great deal of mix water and gives it up slowly, through one face only, into a room that has been sealed for weeks. Laying vinyl, a resilient covering, an engineered board or a resin over it too early is the commonest way this job produces a second complaint: trapped moisture raises the pH at the interface and destroys the adhesive. Drying is measured, not assumed. BS 8203, Code of practice for installation of resilient floor coverings, sets out hygrometer testing; ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes, is the equivalent method elsewhere; ACI 302.2R, Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials, is worth reading before promising a date. The threshold itself comes from the covering manufacturer.
Two things get decided in this window and both are easy to miss. If the property is in a radon affected area, replacing a ground floor is the moment protective measures go in — BRE Report BR 211, Radon: guidance on protective measures for new buildings, with Approved Document C, says what they are, and retrofitting them into a finished floor is not practical. And because a renovated ground floor is a thermal element, the insulation answers to Approved Document L rather than being an upgrade to trim when levels get tight.
Keep the file. Trial hole photographs, the sulfate and pH certificates, the fill tickets with their own sulfate result, the concrete tickets, the dated levels survey and a photograph of the membrane before it was covered are the only evidence that will exist if the floor is questioned. That matters more here than usual, because the question asked two years later is rarely whether the work was done well — it is whether all of the reactive material came out, and nobody can answer that from a finished floor.
The six orders this floor is actually bought as
A floor replacement priced as concrete plus a skip will be wrong by a wide margin. Quantify it as what leaves the house, what comes back in, and the two labour lines — barrowing and compacting — that neither the merchant nor the ready-mix plant will ever quote you for.
- Arisings, converted to weight — Slab and fill volumes bulked for breaking before they are converted, then divided by the permitted weight of the container the yard offers rather than by its cubic capacity.
- Barrow loads, both directions — Out and back in are separate counts on the same protected route; double or treble the figure for concrete and rubble, which are loaded well below a barrow's struck volume.
- Clean granular fill, at the compacted depth — Taken off the depth you intend to achieve after compaction, with a sulfate result asked for on the delivery ticket and the bulking allowance confirmed by the merchant.
- Membrane, tape and collars — Sheet area re-divided by the roll your supplier actually stocks, run-to-run seams counted separately from the perimeter turn-up and from a collar at every penetration.
- Perimeter strip, by the metre — Full internal perimeter including alcove and reveal returns — the one linear item on a job otherwise made entirely of areas and volumes.
- Concrete, off the finished excavation — Gauged on a grid from the blinded surface, specified against the design chemical class the sulfate testing produced, and split into part loads if the pour is fed by barrow.
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.