Screed volume and materials

Depth to a datum — the levelling layer priced in volume, masses and bags

Thickness comes off the level survey, not the drawing. The nominal depth in the specification assumes a perfect slab; the real one sits under a grid of level readings, and the screed averages out every hollow the slab left behind. A survey that finds the slab low raises the true average depth — and the tonnage — before a mixer turns.

The thickness band the guard polices is structural, not stylistic. Laid too thin, a sand-cement screed dries out before it gains strength, curls at its edges and breaks up underfoot; past the top of the band it has stopped being a levelling layer and become a topping that deserves design. Bonded, unbonded and floating build-ups each live at their own depths inside that band.

The wet volume leads the answer because floors are priced and batched by the cube. It is the figure a forced-action mixer’s drum count, a retarded ready-mix delivery or a pump hire is quoted against — and it exists before any decision about who supplies the material.

The materials chain then converts that volume the way site batching actually happens: grossed to loose dry volume by the bulking factor, split by the ratio parts, and weighed through each material’s bulk density into kilograms and bags. A screed mix runs semi-dry — just damp enough to compact under the float — which is why the loose heaps outmeasure the finished layer.

What the figure excludes is the rest of the floor build-up: insulation and membranes below, reinforcement mesh or fibres within, underfloor-heating pipework threading through, and the curing and drying weeks before a moisture-sensitive finish may go down. Those are lines and lead-times of their own.

When this calculation is used

• Ordering cement and sand for a screeding package once the level survey has fixed the true average depth.
• Converting a floor area and specified depth into the wet cubes a ready-mix or pump quotation is priced against.
• Checking a screeder’s materials allowance by reproducing it from area, depth, ratio and a declared bulking factor.
• Re-costing a floor when the level survey finds the slab lower than the drawings promised.

Worked example

The pack’s declared vector: forty square metres of floor at fifty millimetres, a one-to-four mix, a bulking factor of one point three three, the usual loose densities and fifty-kilogram bags.

Four figures come back: the wet screed volume, the cement mass, the sand mass and the cement counted again in bags. Read the volume first — it is the figure every supply route shares — then the sand tonnage, which is the delivery that needs somewhere to be tipped.

Every figure in this example is produced by the certified engine when the calculator loads, checked against the signed pack’s declared test vectors; nothing on this page stores an answer. Add the few millimetres a low slab steals and watch all four figures rise together — depth is the multiplier the level survey exists to pin down.

What each input represents

Assumptions and limits

• The depth is an average over the area; a slab with severe local hollows deserves zoned take-offs, or a levelling compound doing what a screed should not.
• This is nominal volumetric proportioning of a site-batched semi-dry mix — it makes no claim about strength grade, and a specified design mix or a proprietary flowing screed follows its supplier’s figures instead.
• One bulking factor stands for the sand actually delivered at its own moisture; trial-gauging outranks the default.
• The result is the screed layer only: insulation, membranes, mesh or fibres, underfloor-heating pipework and the curing programme before floor finishes are separate lines.

What the guards protect against

• A thickness outside the band sand-cement screeds are built in is refused in both directions, and the pack ships the thin-side refusal as a declared test vector: below the band the layer curls and crumbles underfoot, above it the work has become a designed topping, and neither should be priced as a screed.
• A bulking factor outside the narrow range screed batching actually uses is refused, with its own declared vector on the high side — a factor beyond it usually means a guess has replaced trial gauging.
• The area, ratio parts, densities and bag mass must be strictly positive — the split divides by the ratio sum and the bag count divides by the bag, so a zero anywhere is refused before it becomes an order.

Provenance

Nominal volumetric mix proportioning for screeds (ACI 211.1 tradition; BS EN 998-2)

Wet screed volume as floor area times thickness; that volume grossed to loose dry volume by the bulking factor, split by the ratio parts, converted to masses through bulk densities, and the cement divided into bags, reported unrounded.

A batching convention to be verified against the project screed specification and level survey; build-up design and strength grade are never determined here. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.