External Render & Plaster Coverage Calculator
Calculate render coverage for external walls — sand:cement, lime render, or monocouche — including materials needed.
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External Render Guide
What do I need to know about 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, at £15-25/m² for materials versus cheaper standard sand:cement renders, but the breathability and self-healing properties are essential on older, solid-wall buildings.
What do I need to know about 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 reduce strength outward, with a weaker top coat over a stronger base coat, so any cracking that occurs happens in the more easily repaired outer layer. Plasticiser, such as Feb-Mix, replaces lime in traditional mixes and improves workability without the slaking time lime requires.
What should I know about 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 down of the substrate before application causes the render to dry and crack too quickly, since the wall draws water out of the fresh render faster than it can properly cure.
What should I know about 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, considerably longer than sand:cement, since lime cures through slow carbonation rather than a simple chemical set.