Estimating render has caught out more than a few people I know. A friend re-rendered the side elevation of a Victorian terrace and ran out of material with about a quarter of the wall left to finish. He had ordered for the gross wall area but forgotten to account for the fact that the product he was using had a much lower coverage rate than the generic figures he had used to plan.
The finished wall had a visible band where he had to patch with a different batch - slightly different colour, never quite matching. It is not catastrophic but it is there, every day, every time he looks at it.
Getting the quantity right for render is about understanding two things: coverage rate and net area. Here is how to use both correctly.
Coverage rates by render type
Coverage is typically expressed as kg per m2 per mm of coat thickness. To get bags per m2, multiply by your coat thickness and divide by the bag size.
| Render type | Coverage (kg/m2/mm) | At 15mm total (kg/m2) | Bags per m2 (25kg, inc 15% wastage) |
|---|---|---|---|
| Sand:cement external | 1.9 | 28.5 kg | ~1.3 bags |
| Sand:cement internal | 1.75 | 26.3 kg | ~1.2 bags |
| Monocouche | 1.5 | 22.5 kg | ~1.0 bags |
| Pre-mixed bag render | 1.7 | 25.5 kg | ~1.2 bags |
These are estimates. Your product's data sheet will give a specific coverage figure - use that number rather than a generic rate, especially for pre-mixed and branded products.
The net area calculation
The most common mistake in render estimating is not deducting openings. Every window and door opening is an area you do not render. On a typical house front elevation, windows and doors might account for 25-35% of the gross wall area.
Net area = gross wall area - all opening areas
For each opening, measure the width and height and multiply to get area. Add up all openings and subtract from the total gross area.
The Rendering Calculator does this automatically - you add each wall and each opening separately and the deduction is applied for you.
Worked example - house rear elevation
A typical UK semi-detached rear elevation:
- Main wall: 7m wide x 5m high = 35m2
- Two windows: 1.2m x 1.0m x 2 = 2.4m2
- French doors: 1.8m x 2.1m = 3.78m2
- Net area = 35 - 2.4 - 3.78 = 28.82m2
Sand:cement external render at 15mm total:
- Material needed: 28.82 x 28.5 = 821 kg
-
- 15% wastage = 944 kg
- Bags (25kg): 944 / 25 = 37.8
- Order: 38 x 25kg bags
Or if using monocouche at 12mm:
- Material needed: 28.82 x 1.5 x 12 = 519 kg
-
- 15% wastage = 597 kg
- Bags: 597 / 25 = 23.9
- Order: 24 x 25kg bags
The difference is significant - 38 bags vs 24. The product choice affects your material cost substantially, independent of the labour cost.
Two-coat vs single-coat
Traditional external sand:cement render is typically two-coat: a scratch coat of 10-12mm followed by a finish coat of 5-8mm, giving 15-20mm total. You order material for the total thickness.
Monocouche is a single-coat product applied at 12-15mm in one pass. The bag coverage figure already assumes single application. Do not apply monocouche in two separate passes - it is not designed for that.
Pre-mixed bagged render products vary. Some are single-coat, some are two-coat systems with different products for each. Read the system specification before ordering.
What to do with the leftover
On most render jobs you will have partial bags remaining. Sand:cement render mixed to the right consistency and unused within 30 minutes should be discarded - do not re-temper it. Pre-mixed products have a similar working time.
Plan your batches to minimise waste. On a large job, work out roughly how many complete bags you can apply per working session and mix accordingly.
My tips on render quantities
Add more wastage than you think you need. 15% is a minimum. On a complex elevation with lots of reveals, changes of plane, ornamental features or areas that require multiple coats to build up to level, use 20%. The cost of an extra bag is trivial compared to the cost of running short mid-job.
Order from the same batch. Render products - especially monocouche - can vary in colour between batches. Order all your material at once from the same supplier and check the batch codes are identical. Mix from multiple bags as you go to average out any minor variation within a batch.
Weigh your waste. On any job over 30-40 bags it is worth weighing the material that falls off the wall into the trough or board. Adjust your future wastage calculation accordingly - experienced renderers know exactly how much they lose and can dial their orders in precisely.
Factor in the reveals. Window and door reveals (the sides of the opening) are not included in the main wall area but they do need rendering. Add them as separate wall sections in your calculation - typically 100-300mm depth x the perimeter of the opening.
Use the Rendering Calculator to work out net area and bag count for your project before ordering.