Render Mix Ratios: Sand to Cement by Coat and Background
Quick Answer
Scratch coat around 4:1 to 5:1, float coat 5:1 to 6:1 (sand to cement): each coat weaker than the one below it
Pick a pair, not two numbers from each range at random: a 4:1 scratch coat should be followed by a 5:1 or 6:1 float coat, and a 5:1 scratch coat needs a 6:1 float coat, not another 5:1. The float coat must always end up weaker than the scratch coat beneath it. These are the same mix-ratio options used by default in our rendering calculator; our mortar mix ratios reference gives the equivalent gauged cement:lime:sand figures for the same coats. The right rendering mix ratio also depends on what you’re rendering onto: standard brick and dense block take the ratios above, low-suction or smooth backgrounds need a bonding coat first, lightweight block needs a weaker mix, and old stone or solid pre-1919 walls are usually rendered in lime rather than cement. See the background guidance below.
Render Mix Ratios by Coat
| Coat | Sand : Cement | With lime (Cement:Lime:Sand) | Typical thickness | Note |
|---|---|---|---|---|
| Scratch coat (1st, backing coat) | 4:1 to 5:1 | 1 : 0.5 : 4-4.5 | 8-12mm | Stronger mix; keys mechanically into the brick or block and is scored to key the next coat |
| Float coat (2nd, top coat on 2-coat work) | 5:1 to 6:1 | 1 : 1 : 5-6 | 6-8mm | Weaker than the scratch coat by design, so shrinkage cracking stays here rather than at the base |
| Finish coat (3rd, heritage/3-coat work only) | 5:1 to 6:1 | 1 : 1 : 5-6 | 3-6mm | Same weaker ratio as the float coat; not used on standard domestic 2-coat rendering |
Sand:cement ratio options match the defaults in our rendering calculator. Coat thicknesses and gauged cement:lime:sand figures match the rendering row of our mortar mix ratios reference.
Worked Example: What the Ratio Does to the Cement Order
Take 1 tonne of sharp sand and compare the two standard ratios our rendering calculator offers, using the same density assumptions the calculator uses (1600 kg/m³ sand, BS EN 13139; 1440 kg/m³ loose cement):
- Sand volume: 1 tonne ÷ 1600 kg/m³ = 0.625m³
- At 1:5 (standard): cement volume = 0.625 ÷ 5 = 0.125m³; cement mass = 0.125 × 1440 = 180kg; bags = 180 ÷ 25 = 7.2, rounds to 7 bags
- At 1:4 (stronger): cement volume = 0.625 ÷ 4 = 0.15625m³; cement mass = 0.15625 × 1440 = 225kg; bags = 225 ÷ 25 = 9 bags
- Difference: 9 − 7 = 2 extra bags of cement for the same tonne of sand, about 29% more
That is the practical cost of a stronger mix: going from the standard 1:5 to the stronger 1:4 scratch coat adds nearly a third more cement to the order, for exactly the same amount of sand.
Why the Top Coat Must Be Weaker
Render is always built stronger at the bottom and weaker on top, never the other way round. The scratch coat needs the most cement to key firmly into the brick or block behind it, but the float or finish coat on top should be a weaker mix, more sand relative to cement. If the top coat is stronger than the scratch coat beneath it, the two layers shrink at different rates as they dry and cure, and the stronger layer pulls away from the weaker one underneath: cracking and delamination follow, often within the first year. Our own mortar mix ratios reference puts the same rule for mortar generally: render should always be weaker than the substrate it is applied to, and each coat weaker than the one below it.
Ratios by Background: Brick, Block and Stone
The ratios above are the starting point for a typical UK job, but what you are rendering onto changes the mix:
- Standard brick or dense concrete block: the most common domestic background. Use the standard ratios in the table above, 4:1 to 5:1 for the scratch coat and 5:1 to 6:1 for the top coat, with a good pre-wetting of the wall before applying.
- Low-suction or smooth backgrounds (engineering brick, dense smooth-faced block, previously painted masonry): there is little for the render to grip mechanically, so apply a bonding agent first rather than relying on the mix ratio alone to hold the render on.
- Lightweight or aircrete block:use a weaker mix than the standard ratios above, towards the 6:1 end or a lime-gauged mix. Our rendering calculator’s ratio options run from 1:4 to 1:5 cement:sand (the same 4:1 to 5:1 sand:cement mixes in this page’s notation); for a 6:1 mix, use the 1:5 option as the closest calculator match, or measure a 6:1 mix by hand. Concrete block manufacturers’ guidance for these blocks is consistent on this point: a mix that is too strong cannot accommodate the block’s greater movement and cracks, where a weaker mix flexes with it instead.
- Old stone or solid pre-1919 masonry: skip cement render altogether and use a lime render. SPAB (the Society for the Protection of Ancient Buildings) advises against cement render on buildings from before about 1919, because it traps moisture that older, more permeable walls need to evaporate out through, causing decay rather than preventing it.
- Exposed or coastal locations, regardless of background: move to the stronger end of the range, a 1:4 scratch coat, as set out in our step-by-step rendering guide.
Plasticiser or Lime, Not Both at Full Strength
Both plasticiser and lime do the same basic job, making the wet mix more workable so it goes on smoothly without excessive water, but they are not the same thing and are not normally combined at full dosage. A liquid plasticiser entrains small air bubbles into a straight sand:cement mix (the ratios in the table above) and is the faster, cheaper option for standard brick and block work. Lime, used in a gauged cement:lime:sand mix, does more than improve workability: it also makes the render more flexible and more breathable, which is why it is the preferred choice on older or solid-wall backgrounds. Never substitute washing-up liquid for a proper plasticiser: it entrains uncontrolled air and can weaken the mix significantly, a point our mortar mix ratios reference also makes for mortar generally.
Sand:cement figures match the ratio options used by default in our rendering calculator; coat thicknesses and gauged cement:lime:sand figures match our mortar mix ratios reference table. Guidance on coat design and background suitability follows BS EN 13914-1 (Design, preparation and application of external rendering) and NHBC Standards Chapter 6.11. Lime-render advice for stone and older solid-wall backgrounds follows the SPAB (Society for the Protection of Ancient Buildings) position against cement render on pre-1919 buildings. Last updated: July 2026.