In short
- Thin-coat systems use a 1–2 mm finish over a meshed base coat.
- Thick-coat renders include monocouche, sand and cement and lime, at 15–20 mm or more.
- Thin-coat suits EWI, boards and previously rendered walls; thick-coat suits sound masonry.
- Thin-coat finishes are crisper; thick-coat can level uneven walls.
Comparison
| Thin-coat | Thick-coat | |
|---|---|---|
| Examples | Silicone, acrylic | Monocouche, sand and cement, lime |
| Total thickness | Around 8 mm with base | 15–20 mm or more |
| Substrates | Block, board, EWI, old render | Brick, block, masonry |
| Levelling | Limited | Good |
| Weight | Light | Heavier |
Which to choose
Use thin-coat over external wall insulation and boards. Use thick-coat on uneven masonry.
How each type handles an uneven wall
Thick-coat renders are applied at 15 to 20 mm or more, so they can absorb moderate unevenness in brick and blockwork within their own depth. Thin-coat finishes cannot: the 1 to 2 mm topcoat follows whatever is beneath it. That is why thin-coat systems depend on a properly levelled base coat. On a wall with bulges, old patches or pebbledash, the base coat does the levelling and the topcoat provides the finish.
Weight and substrate load
Thin-coat systems are much lighter per square metre. That matters on render-carrier boards over timber frame, on insulation boards in external wall insulation, and on older walls where adding weight is undesirable. Thick-coat cement renders are best kept to sound, well-bonded masonry.
Movement
A thick cement render is rigid. If the wall behind moves, it cracks unless movement joints and mesh are designed in. A thin-coat system with a fully meshed base is more tolerant of small movements, which is one reason it is preferred on mixed substrates such as an extension joined to an older house.
Finish quality
Thin-coat silicone and acrylic finishes give crisp, even textures and sharp arrises, which suit contemporary designs. Scraped monocouche has a softer, slightly open surface. Lime has an irregular, hand-made character that belongs on historic buildings. See render finishes explained.
Cost over time
Thin-coat silicone usually costs more to install per m² than monocouche, but on walls that need levelling anyway, the difference narrows. Neither needs painting. Sand and cement is the cheapest to install but brings repainting every five to ten years.
Questions people ask
Is thin-coat render weaker?
No. It relies on its meshed base coat for strength.
Can thick-coat render go on insulation?
Generally not; EWI systems use thin-coat finishes.
Which is better for an old rendered wall?
Usually a thin-coat system with a levelling base coat, once the existing render has been tested and any hollow areas removed.