Which type of render should I use?

Every render system is a stack of coats on a background, and one rule governs the whole thing: each coat must be no stronger than the one behind it. A render stronger than what it sits on cannot move with it, so it shears off, taking the surface of the wall with it. That single rule is why a wall's construction, and not the look you want, decides the material. The section shows how the coats build up and where the details at the top and bottom of a render have to be right.

Safety

Step by step

1. Establish what the wall is, first and above all.Solid or cavity, and what mortar it is built in. Measure the thickness at a window reveal, look at the bond for headers, and scrape a joint to see whether the mortar is lime or cement. Everything else follows from this.
2. Apply the strength rule.Each coat no stronger than the one behind it, and the whole render no stronger than the background. Backgrounds go from strongest at the wall to weakest at the finish coat. Getting this the wrong way round is what makes render debond in sheets.
3. Choose lime on a pre-1919 solid wall.It is permeable, soft and flexible, so the wall can still evaporate through it and small movements do not shear it off. On a solid wall a cement render is a raincoat that leaks and then will not let the wall dry.
4. Choose cement or monocouche on sound modern blockwork.A modern cavity wall keeps water out at the cavity, so a strong render on a stable background is doing a different job and is entirely appropriate.
5. Choose a thin-coat system where movement is expected.Over insulation, over a mixed background, over a wall with a history of cracking, or where a flexible finish is wanted. The embedded mesh is what handles movement, and it is why these systems crack far less.
6. Consider the background's key.Smooth dense blockwork, engineering brick, painted surfaces and existing render all need a mechanical key or a bonding coat before render will hold. A spatterdash coat, a proprietary primer or a mesh fixed to the wall are the usual answers depending on the system.
7. Get the details right at top and bottom.A bell cast bead at the bottom edge to throw water clear, the render stopping above the damp proof course rather than crossing it, stop beads and sealed joints at frames, and sills that project with a drip. These details cause more render-related damp than the material choice does.
8. Think about what happens to the wall's drying.Render changes how a wall handles moisture. Before covering an old wall, consider whether it is currently dry, whether it has salts in it, and whether the ground level, the gutters and the pointing are sound. Render over a wet wall traps the water in.
9. Check whether consent is needed.Rendering can change a building's external appearance materially. On a listed building it needs listed building consent, or the equivalent under the separate Scottish and Northern Irish regimes; in a conservation area it may need permission; and in England an Article 4 direction can remove the right. Check with the local planning authority before ordering anything.
10. Follow one manufacturer's system throughout.For monocouche and thin-coat systems, the beads, the primer, the base coat, the mesh and the topcoat are designed together and warranted together. Mixing products from different systems voids the guarantee and often the performance.

Alternatives

• Traditional sand and cement: Two or three coats of cement, sand and usually lime, scratched to key between coats, painted afterwards. Cheap, strong, widely understood, and appropriate on modern blockwork and sound cavity construction. Rigid and largely vapour-closed, so it is the wrong material on a soft solid wall and it cracks where a background moves.
• Lime render: Natural hydraulic lime or lime putty based, applied in coats with a haired base coat on some backgrounds. Soft, permeable, forgiving of movement, and the correct material on pre-1919 solid walls. Slower to apply, needs long protection while it cures, and is a warm-season job.
• Monocouche: A factory-blended, through-coloured one-product render applied in a single operation, usually in two passes, and scraped back to a texture. No painting, and no repainting cycle. Strong and rigid, so the background has to be sound and stable, and colour matching a repair years later is difficult.
• Thin-coat silicone or polymer systems: A polymer-modified base coat with a glass fibre reinforcing mesh embedded in it, then a thin through-coloured topcoat. Flexible, crack-resistant, breathable to varying degrees depending on the product, and the standard finish over external wall insulation. More expensive and system-specific — you follow one manufacturer's specification throughout.
• Insulated render (external wall insulation): Insulation boards fixed to the wall with a rendered finish over them. A thermal upgrade rather than a repair, with implications for eaves, sills, openings, ground level and planning. A separate decision with its own design requirements.

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