Render and skim coverage

Per square metre, per millimetre — the datasheet rate that prices a thin coat

The coverage rate belongs to the product, not to rendering in general. Two bags from rival ranges laid at identical thickness consume different masses, because one is lightened with fillers and the other is not — which is why the rate is a required input read from the datasheet rather than a default this page could honestly supply.

Thickness multiplies the demand linearly, and thin work punishes small errors in it. On a coat of two or three millimetres, half a millimetre of extra build across the whole wall is a large fraction more material — so the thickness entered should be what the trowel will really leave, not the brochure’s minimum.

The ten-millimetre ceiling is the boundary of what a finish coat is. A skim is a skin that follows the surface beneath it; anything needing more build than that is a backing-coat system with its own products and its own rates, and the engine refuses to price one as if it were the other.

Wastage on thin-coat work is mostly time’s doing: mixed material stiffening in the bucket past its working life, the film sanded back off after feathering, what the hawk drops. The default twentieth suits steady work on a sound background; a first attempt or a hot day earns more.

Bags are the answer because bags are the unit of purchase and of mixing. The engine reports the material mass with wastage applied and the unrounded count of bags it fills — the fraction is the honest part, the difference between one more bag and a firmer wastage judgement.

When this calculation is used

• Ordering bagged skim or render once the area is measured and a product’s datasheet names its coverage rate.
• Re-pricing the same walls when the product changes and drags its rate — and its bag size — along with it.
• Checking a renderer’s materials allowance by reproducing it from area, thickness, the stated rate and a declared wastage.
• Splitting a job between products — a base-coat zone and a skimmed zone — each run at its own datasheet rate.

Worked example

The pack’s declared vector: forty square metres coated at three millimetres, a datasheet rate of one point two kilograms per square metre per millimetre, twenty-five-kilogram bags, and the default twentieth of wastage.

Two figures come back: the material mass once wastage is applied, and the unrounded bags that mass fills. Read the fraction on the bag count before ordering — on thin coats it is the thickness judgement, more than the area, that decides whether it rounds up.

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. Halve the thickness and watch the order halve with it — on a skim, the millimetre is the price.

What each input represents

Assumptions and limits

• One product at one datasheet rate covers the whole area; zones on different products, or a base coat under a finish coat, are separate take-offs at their own rates.
• The rate already embodies the product’s density and formulation — the model multiplies the manufacturer’s figure and never derives consumption from first principles.
• Wastage is a single flat percentage for bucket losses, feathering and sanding-back; it is a judgement, not a property of the product.
• The result is coating material only: primers, beads, scrim over board joints and the labour of keying or suction control are their own lines, and product suitability for the background is the datasheet’s ruling.

What the guards protect against

• An applied thickness beyond ten millimetres is refused, and the pack ships the refusal as a declared test vector: that is no longer a skim or finish coat but a backing-coat system, and pricing it at a finish coat’s rate would understate the job wholesale.
• The wastage allowance is held to the range coating take-offs actually use, with its own declared refusal vector — beyond three tenths, the background needs preparing or the area re-measuring, and the engine will not launder that problem into extra bags.
• The area, rate and bag mass must be strictly positive — the bag count divides by one and multiplies by the others, so a zero anywhere is refused before it becomes a nonsense order.

Provenance

Manufacturer coverage-rate estimating convention (Chudley and Greeno, Building Construction Handbook)

Material mass as area times applied thickness times the manufacturer’s stated rate, grossed up by the wastage percentage; the bag count as that mass divided by the bag size, reported unrounded.

Quantity reference to be verified against the datasheet of the product actually bought; product selection and background preparation are never determined here. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.