Foam concrete screed volume
Screed quantity is a volume calculation, not a weight calculation — density only enters once the volume is fixed and materials are being ordered. This page works through unit-area volumes, the two allowances that actually apply on a roof deck, and a full worked example carried to bags of cement and litres of foaming agent.
Volume, not weight
A screed quantity take-off is a volume problem. The floor or roof area and the specified thickness fix a volume in cubic metres or cubic feet; density only enters afterwards, when that volume is converted into a batch of cement, water and foaming agent for a chosen density class. Ordering by mass first and working backwards to a volume is the wrong direction, and a common source of under-order on foam concrete jobs, because the material's low mass per cubic metre makes a given tonnage cover far more area than the same tonnage of a normal-weight screed would.
The sequence that avoids that mistake is: measure the area, fix the nominal thickness, apply the two allowances that actually apply to a foam concrete pour, and only then multiply the ordered volume by the mix design to get quantities of material.
This is also why a foam concrete quote cannot be sanity-checked against a conventional screed quote on a per-tonne basis. A supplier pricing per cubic metre of finished volume is comparing like with like; a supplier pricing per tonne is comparing figures that move independently of each other every time the specified density class changes, which makes the tonne an unreliable unit for this material outside a fixed, already-agreed mix design.
Volume per unit area by thickness
Volume per unit area is simply the thickness expressed in the matching length unit — one square metre of screed at 80 mm nominal thickness is 0.080 m3, because the base area is unity. The same logic gives cubic feet per square foot directly from the thickness expressed in feet.
| Thickness | Thickness | Volume per m2 | Volume per 100 sq ft |
|---|---|---|---|
| 50 mm | 1.97 in | 0.050 m3 | 16.4 ft3 |
| 75 mm | 2.95 in | 0.075 m3 | 24.6 ft3 |
| 100 mm | 3.94 in | 0.100 m3 | 32.8 ft3 |
| 150 mm | 5.91 in | 0.150 m3 | 49.2 ft3 |
Multiply the relevant row by the plan area to get a nominal volume, then apply the two allowances below. Neither allowance depends on which of these thicknesses is chosen; both are percentages of the nominal volume.
The falls and substrate-tolerance allowance
On a roof deck laid to falls, the specified thickness is a minimum, not an average. A deck falling from 50 mm at the outlet to, say, 120 mm at the ridge has an average thickness well above its nominal figure, and an uneven structural substrate adds further variation that a flat take-off does not capture. A falls and substrate-tolerance allowance of 10 to 20 % over the nominal volume is normal for roof work; flat, well-prepared floor substrates sit at the low end of that range, or need none at all.
There is no substitute for checking the actual fall geometry and substrate survey where they exist. Where they do not, 12 % is a reasonable planning figure for a typical warm-roof deck — used in the worked example below — but it is a placeholder, not a specification.
The pump-priming and placing allowance
Every pump line has to be primed before it delivers usable material, and a working margin is needed to cover minor over-placement, hose flushing and the material left in the pump at the end of a pour. A placing allowance of 5 to 8 % on top of the falls-adjusted volume covers this on a typical line run; long lines and multi-storey lifts sit toward the top of the range.
Why there is no compaction allowance
Do not add a compaction factor
A granular sub-base or fill material is ordered loose and compacts down under load, so its take-off includes a bulking or compaction allowance. Foam concrete is self-compacting: it is poured or pumped, it self-levels, and it is never vibrated or tamped. Its placed volume is its final volume. Carrying over a compaction allowance from granular-fill practice — a mistake seen regularly where the same estimator prices both — over-orders the pour by whatever factor was applied, typically 10 to 15 %, for no benefit.
From volume to materials
Once the ordered volume is fixed, converting it to cement, water and foaming agent uses the absolute volume method set out on the mix design page: fix the dry density class appropriate to the application — an insulating roof screed generally sits at 200 to 400 kg/m3, see density classes — read off the binder and water content per cubic metre for that mix, and multiply by the ordered volume. Foaming agent consumption follows the same multiplication, using the concentrate figure per cubic metre established for that mix.
This is a simple multiplication once the per-cubic-metre figures exist, which is why fixing the mix design before the take-off, not after, is worth the extra hour. It also means the same volume take-off serves any density class without being redone: only the per-cubic-metre figures in the final multiplication change, not the falls, tolerance and placing allowances that fixed the ordered volume in the first place.
Worked example: 250 m2 at 80 mm
An insulating roof screed, 250 m2 at 80 mm nominal thickness, targeting the 400 kg/m3 mix design used throughout this site — see the roof-screed applications this class is suited to.
- Nominal volume = 250 × 0.080 = 20.0 m3
- Falls and substrate-tolerance allowance, 12 %: 20.0 × 1.12 = 22.4 m3
- Placing and pump-priming allowance, 6 %: 22.4 × 1.06 = 23.7 m3, round up to order 24 m3 (31.4 yd3)
Converting the ordered 24 m3 to materials, using the 400 kg/m3 screed mix from the mix design page (335 kg/m3 CEM I, 184 kg/m3 water, 1.14 L of foaming agent concentrate per cubic metre, 0.708 m3 foam per cubic metre):
| Material | Per m3 | × 24 m3 |
|---|---|---|
| CEM I cement | 335 kg | 8040 kg · ~161 bags at 50 kg |
| Water | 184 L | 4416 L · 4.4 m3 |
| Foaming agent concentrate | 1.14 L | 27.4 L |
| Foam generated | 0.708 m3 | 17.0 m3 |
That is the full chain from a plan area to an order for cement bags and drums of concentrate. The calculator below runs the same volume step for any area, thickness and density class.
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