How the render calculator works
The calculator finds the area to render (wall area minus about 1.5m² per window or door), multiplies by the thickness and adds 10% for wastage. For traditional render, the mix volume is converted to dry materials at about 2,000kg per m³ and split by your sand : cement ratio. Proprietary renders use typical usage rates in kg per m² per mm.
Typical coverage
| System | Thickness | Coverage |
|---|---|---|
| Sand & cement, 2 coats | ~20mm | ~2.9m² per 25kg cement (4:1) |
| Monocouche | 15mm | ~1.2m² per 25kg bag |
| Silicone thin coat | 1.5mm | ~9–10m² per 25kg tub |
Rendering tips
- Don't render in frost, rain or hot sun. Mist dry backgrounds so the wall doesn't suck water out of the render.
- Scratch the first coat and let it cure (usually at least 24 hours, longer in cold weather) before the top coat.
- Use a waterproofer or plasticiser in the scratch coat as the manufacturer directs, and never render over the DPC.
- Large walls need movement joints; follow the render manufacturer's spacing.
Worked example: an 8m × 5m gable wall
The wall is 40m². Four windows and doors take off about 6m², leaving 34m² to render.
- Sand and cement, 20mm in two coats, 4:1: about 1.5 tonnes of dry mix, which is 12 bags of cement and 1.2 tonnes of sand (2 bulk bags).
- Monocouche at 15mm: about 760kg, so 31 × 25kg bags. That's more expensive in materials, but there's no painting and only one main coat.
- Silicone thin coat at 1.5mm: about 95kg, so 4 tubs, plus around 7 litres of primer. Remember the base coat underneath is extra.
Either way, you'll need about 37m of beads: two corners, the four openings and a bell-cast along the base. That's 14 lengths of 3m, plus a mesh patch at each corner of every opening to stop cracks.
Which render system?
- Traditional sand and cement is the cheapest in materials and very durable, but needs painting with masonry paint and is prone to hairline cracking if mixed or cured badly.
- Monocouche is coloured all the way through and scraped to a textured finish. It needs no painting and is popular on new builds. It's best applied by someone who has used the specific product before.
- Silicone thin-coat finishes are flexible, water-repellent and self-cleaning to a degree. They go over a base coat or an insulation system, and cost the most per m².
Before you start
Check the wall is sound, clean and free of paint or loose material. Never bridge the damp-proof course: stop the render at a bell-cast bead about 150mm above ground level. In conservation areas, on listed buildings, or where an Article 4 direction applies, changing the look of a front wall can need consent, so check with your council. Adding insulation under the render (external wall insulation) brings in Building Regulations.
Common rendering mistakes
- Too strong a mix on soft brick or old walls. Hard cement render traps moisture in old solid walls. Lime-based renders are usually better on older buildings.
- Rendering in poor weather. Frost, rain and hot sun all cause cracking or poor bonding.
- Skipping the beads. Corners and edges without beads chip easily and look untidy.
Building a new wall to render? The block calculator covers the blockwork, and the mortar calculator the mortar. More in bricks, blocks and mortar.