Area times thickness
Render is a volume problem. Multiply the net wall area by the thickness and you have the cubic metres of wet render you need to put on the wall. Everything else, whether bags or sand and cement, follows from that number.
wet render (m³) = wall area (m²) × thickness (m) × (1 + waste)
A 50 m² wall at 10 mm is 0.5 m³ before waste. Render falls off the trowel, gets scraped off the floor and gets left in the barrow, so 10 percent is a fair allowance and it goes up on a rough or badly out-of-plumb wall where the thickness has to vary to get a flat finish.
Bags
Bagged render is dry sand, cement and additives blended in the factory. A 20 kg bag makes roughly 12 to 13 litres once mixed, so at 10 mm it covers about 1.2 square metres, and at 15 mm about 0.8. Brands vary, and some bags print a coverage figure at a stated thickness, which is the number to trust. The calculator lets you enter the yield from your bag.
| Wall | Area | Thickness | Wet render | 20 kg bags |
|---|---|---|---|---|
| Front fence 8 × 1.8 m, both sides | 28.8 m² | 10 mm | 0.32 m³ | 26 |
| Garage wall 9 × 2.7 m | 24.3 m² | 10 mm | 0.27 m³ | 22 |
| House 42 m × 2.7 m, openings 22 m² | 91.4 m² | 12 mm | 1.21 m³ | 97 |
| Same house, two-coat sand cement | 91.4 m² | 15 mm | 1.51 m³ | 1.8 m³ sand, 14 cement, 6 lime |
Figures include 10 percent waste and a 12.5 L yield per bag.
Sand and cement
Traditional render is mixed on site from render sand, cement and hydrated lime. Over clay brick a 1:1:6 mix by volume is common; over concrete block, which is stronger and less absorbent, renderers often go to 1:0.5:4.5. The lime is what makes the render stay workable on the trowel and what stops it cracking as it dries. Because sand bulks when it is damp and the cement fills the gaps between grains, you need more sand than the finished volume: allow about 1.2 cubic metres of sand per cubic metre of wet render, with nine bags of cement and four of lime for the 1:1:6 mix. The calculator does that arithmetic for you.
Coats and thickness
Bagged renders are designed to go on in one coat of 8 to 12 mm and be floated to a finish or left as a base for an acrylic texture. Sand-cement render is a two-coat job: a scratch coat of about 8 to 10 mm, scored while wet so the next coat keys into it, then a float coat of 5 to 8 mm rubbed up with a sponge or wood float. Going over about 20 mm in one coat is asking for slumping and cracking. If the wall needs more than that to come true, dub out the low spots first and let them set.
Curing and painting
Fresh render is alkaline and full of water. Sand-cement render wants about 28 days before it is painted, and it should be kept damp for the first few days in hot weather so it does not dry too fast and craze. Acrylic-modified bagged renders and texture systems usually allow coating sooner, often after seven days. The product data sheet is the authority.
Worked example
A rendered front fence 8 m long and 1.8 m high, rendered on both sides, is 28.8 m². At 10 mm with 10 percent waste that is 0.32 m³ of wet render, which is 26 bags at 12.5 L a bag. Mixed traditionally it would need about 0.38 m³ of render sand, three bags of cement and two of lime.
Common questions
How many m² does a 20 kg bag of render cover?
About 1.2 m² at 10 mm, or 0.8 m² at 15 mm. Check the figure on your bag.
How thick should render be?
8 to 12 mm for one coat of bagged render, about 15 mm total for two-coat sand and cement.
What is the sand and cement render mix?
1:1:6 cement:lime:sand over brick is common. 1:0.5:4.5 over block.
How long before painting render?
About 28 days for sand-cement. Often 7 days for acrylic-modified bagged renders. Follow the data sheet.
Quantities only. Mix design, substrate preparation and curing follow the product manufacturer's data sheet and AS 3700 for masonry.