Technical articles 5 min read
Sloped screeds: how many cubic metres per hundred square metres?
The commonest ordering mistake is multiplying area by the minimum thickness. A fall is a wedge, not a slab of even depth.
Ordering short and ordering long both cost money: one stops the job mid-floor, the other leaves you with cubic metres and nowhere to put them. The arithmetic is two lines.
A fall is a wedge. Thickness starts at the low point and rises to the outlet or the edge, so volume has to be worked from the average: average equals minimum plus maximum, halved.
A worked example. 100 m² of floor, 12 m of run, a fall of 1.5%, and 5 cm of cover at the low point.
Rise over the run: 12 m × 1.5% = 18 cm. Maximum thickness is therefore 5 + 18 = 23 cm. Average thickness: (5 + 23) ÷ 2 = 14 cm, or 0.14 m. In-place volume: 100 × 0.14 = 14 m³.
Now the allowance. Three things sit between in-place volume and what you should order: compaction under float and vibration, settlement in corners and around upstands, and losses in transport and discharge. Add 8 to 12%. At 10%, the order comes to roughly 15.5 m³ per 100 m².
One detail that moves the invoice: bulk aggregate is sold by loose volume, not by compacted in-place volume. The figure above is in-place. Say which one you mean when you ask for a quote, or the two sides are discussing two different numbers.
Grading bears on the same sum. A 4–10 mm fraction is the usual choice for falls: coarse enough to stay light, fine enough to close under a float. If a thin floor finish goes over it, allow a 2 cm topping in 0–3 mm and add that to the volume separately — it is the two centimetres most often left out.
