Advanced options
Sand and cement render at about 2,000 kg/m3, applied as a scratch coat then a slightly weaker top coat. Use plastering (rendering) sand, add waterproofer to the scratch coat on exposed walls, and never render over damp or friable substrate without treatment. Monocouche and silicone renders are bought as bagged systems with their own coverage rates.
What The Render Calculator Works Out
- Render needed: wall area multiplied by total thickness at roughly 2,000 kg per m3. Traditional sand and cement render goes on at 10 to 20mm total across two coats, so a 20 m2 wall at 15mm needs about 600 kg of material.
- Plastering sand: the mass split by the mix: at 5:1 the sand is five sixths of the total, about 500 kg for that wall. Use plastering (rendering) sand, not building sand, and gauge by whole buckets so every batch matches.
- Cement: the remaining share in 25 kg bags, four bags for the example, plus plasticiser and waterproofer dosed per the bottle. 5:1 suits most masonry, 4:1 takes coastal and exposed elevations, and softer old brick wants 6:1 or a lime render entirely, since hard cement on soft substrate traps moisture and blows the face off the brick.
- Openings and wastage (advanced): deduct windows and doors from a gross elevation measurement, and add 5% to 10% waste for rough walls, droppings and the mix left in the tub at the end of the day.
- Scratch coat thickness (advanced): sets how the total splits between the stronger scratch coat, keyed with a scarifier for grip, and the thinner top coat ruled flat and floated. The coat table shows each coat's quantities, with the top coat one step weaker because a strong coat over a weak one delaminates.
- Sand and cement cost (advanced): whole bulk bags (about 850 kg) and cement bags at your prices. Typical UK prices are GBP 60 to GBP 85 per bulk bag of plastering sand and GBP 6 to GBP 9 per bag of cement, so materials for a 20 m2 wall come to well under GBP 150 before masonry paint and labour.
- Monocouche alternative (advanced): the number of 25 kg bags of through-coloured one-coat render for the same wall, at about 1.5 m2 per bag at 15mm, and their cost. Bags run GBP 11 to GBP 18 each, so monocouche costs several times more in materials but never needs painting; always check the datasheet coverage for your chosen product.
- Beads and mesh (advanced): a rough allowance for stop and corner beads based on the wall's likely perimeter, plus reinforcing mesh where materials meet. Beads set plumb first, renders stop above the damp course, and nothing goes on in frost or hot sun.