Render & plaster

Render is coats, not a thickness

"Twenty millimetres of render" is two coats. A scratch coat around 12mm to key and level the wall, and a top coat around 8mm to finish it.

That matters for two reasons. The quantities are the sum of the coats, and they are often different mixes at different thicknesses. And a 20mm coat applied in one go slumps off the wall before it has any chance to set.

So this asks for the coats separately rather than asking for a thickness and letting everybody mean whatever they meant.

The rule that matters more than any quantity

Every coat must be the same strength or weaker than the one beneath it.

Put a strong coat over a weaker one and the strong layer shrinks as it cures against something that cannot restrain it. It cracks, water gets behind it, and it comes away in sheets: taking the coat beneath with it.

It feels completely backwards. The outside is the face that takes the weather, so instinct says make it the strong one. That instinct is why so much render fails on walls that were otherwise perfectly sound.

So the tool checks each coat against the one under it, marks the offending row, and refuses to produce a quantity for it. It is the only stop on the page, and it is the right thing to stop on: the quantities would be correct and the wall would still fail.

Why render fails when the quantities were right

Suction. It is not a number and it does not appear on any calculator, and it decides whether the material you ordered was worth ordering.

A dry concrete block is extremely thirsty. Put render on it in summer and the block pulls the water out of the mix before the cement has hydrated. The render never gains strength: it goes off looking fine and then dusts off under a thumbnail six months later.

Damp the background down, or use a bonding agent, or both. Which background you have changes the answer completely:

• Concrete block: very thirsty. The classic failure.
• Brick: depends entirely on the brick. Engineering brick is nearly non-absorbent and needs a key; a soft stock drinks like a block.
• Metal lath: no suction at all. The first coat is about pushing through and forming a key behind the lath, not sticking to it.
• Existing render: only as good as what is under it. Tap the whole wall first. Anything hollow comes off, and rendering over it buys a year.

Twenty per cent waste, and that is not generous

Render waste is worse than mortar waste and much worse than brick waste. What misses the wall is gone. What falls off the hawk is gone. What stiffens in the bucket while you are cutting in round a window is gone.

Twenty per cent is the working figure and plenty of people use more on a cut-up elevation.

Where the numbers come from

Sand and cement are worked by the same engine as the mortar calculator, so a job costed on both pages cannot disagree with itself. Cement is taken to fill the voids between the sand grains rather than adding to the volume, which is the conventional assumption and is stated rather than hidden.

One thing this is not: a monocouche or insulated render calculator. Those are systems, with the basecoat, mesh, beads and topcoat specified together, and the manufacturer's coverage figures are the ones that apply. Pick a board background on the tool and it will say so rather than giving you a confident wrong answer.