Render Calculator
Calculate sand, cement and bags for sand and cement render, monocouche or thin coat systems. Door and window deductions included. 2026 UK pricing.
Enter dimensions above to see net area.
Total wall area after deducting doors and windows.
Reference
Render Coverage Rates
Application Guide
How to Apply Render
Preparation
Clean the substrate thoroughly — remove all loose material, dust, grease and existing failing render. Rake out mortar joints on brickwork to provide a mechanical key. On dense substrates (engineering brick, smooth concrete) apply two coats of PVA diluted 1:5 — allow first coat to dry, apply second, then render while the second coat is still tacky. Pre-wet porous substrates with clean water an hour before rendering.
Fix stainless steel stop bead and movement bead at all external corners and junctions with different materials (e.g. where render meets timber window frame). Embed alkaline-resistant fibreglass mesh at corners and junctions to prevent cracking.
Scratch coat
Mix 3:1 sharp sand to cement with a small amount of plasticiser. Apply 10–12mm thickness using a hawk and trowel, pressing firmly into the surface. While still green (firm but not set), scratch the surface horizontally with a devil float or notched scratcher to leave horizontal grooves for the top coat to key into. Allow to cure for minimum 3–7 days before applying the top coat. Keep damp for the first 24 hours to prevent rapid drying and cracking.
Top coat and finishing
Mix 4:1 (or softer) sand and cement with plasticiser. Apply 6mm thickness evenly over the scratch coat. Work from bottom to top, keeping a wet edge. Finish to suit specification: smooth with a steel trowel (prone to crazing — not recommended for external), sponge float for a matt texture, or wooden float for a sand texture. Do not overwork.
Avoid rendering in direct sunlight, frost, or strong wind. Ideal conditions: 5–25°C air temperature, overcast. Protect fresh render from rain for 24 hours and from frost for 72 hours.
FAQ
Common Questions
Use sharp sand (grit sand) for scratch coats and strong base coats. Sharp sand has angular particles that produce a stronger, more open mix that bonds well. Use soft building sand for top coats and fine finishes where workability matters more than strength. Some renderers use a blend (50/50) for the top coat. Never use unwashed beach sand.
Minimum 3 days in warm dry conditions, 7 days in cool or damp weather. The scratch coat must be firm enough to take the weight of the top coat without distorting. Test with thumb pressure — it should not indent. For monocouche, follow manufacturer's instructions; most require a minimum 28 days for full carbonation before painting.
Only if the existing render is sound — no hollow spots, no cracks, firmly bonded to the wall. Tap the surface systematically with a hammer; hollow sections must be cut out and patched before re-rendering over. If more than 30% of the existing render is failed, full removal is more reliable. Applying new render over failed render just masks the problem and will fail again.
Adding NHL (natural hydraulic lime) improves flexibility and reduces cracking, particularly on older buildings with softer masonry. A typical lime render mix is 1:1:6 (cement:lime:sand). For solid wall or heritage buildings, pure lime render (1:2.5–3 lime:sand) with no cement is often specified to allow movement and moisture management. This calculator covers standard sand:cement and pre-bagged systems — for lime render, consult the product manufacturer's guidance.