External Render & Plaster Coverage Calculator
Calculate render coverage for external walls — sand:cement, lime render, or monocouche — including materials needed.
External Render Guide
Render Type Selection
Sand:cement render: cheap, durable, traditional. Mix 4:1 to 6:1 sand:cement plus plasticiser. NOT breathable — traps moisture in solid-wall pre-1920 properties (rising damp, condensation, frost damage). Best for: modern cavity-wall properties. Lime render: breathable. Lime hydraulic (NHL 3.5 typical) for exterior. Slow setting. Mix 3:1 sand:lime. Allows moisture movement through walls — essential for pre-1920 solid-wall properties. Self-healing minor cracks. More expensive (£15-25/m² materials v
Sand:Cement Mix Ratios
Typical ratios by coat: scratch coat: 4:1 sand:cement + plasticiser. 8-12mm thick. Strong base layer. Top coat: 5:1 or 6:1 — weaker than scratch coat (important rule — never apply stronger over weaker). 8-10mm thick. Final smooth or textured finish. Why weaker over stronger: as render dries, it shrinks. If top coat is stronger than base, it cannot move with seasonal movement — cracks. Always reducing strength outward. Plasticiser (Feb-Mix or similar): replaces lime in traditional mixes. Improves
Common Render Failures
Cracking: too rich mix (high cement). Too thick coats. Drying too fast (sun, wind). Different rates of shrinkage between substrate and render. Blowing/spalling: water gets behind render, freezes, blows render off. Caused by: no expansion joints. Poor detailing around windows. Failed sills. Render reaching ground level (should stop 150mm above DPC). Hollow patches: poor adhesion to substrate. Dirty/dusty/painted surface. Painted walls must be scraped clean to bare substrate. Insufficient damping
Working Practices
Mix small quantities at a time — render becomes unworkable as it sets. 60-90 minutes max from mixing to application. Scratch coat: apply, score with trowel (creates key for top coat to grip). Leave to set 24-48 hours minimum. Damp before second coat to prevent suction. Top coat: apply quickly and evenly. Smooth with float for smooth finish. Texture with sponge or wooden float for textured finish. Drying time: never rush. 4-6 weeks before painting on sand:cement render. Lime render: 6-12 weeks. P
Recommended for this calculator