Cement render damages old buildings because it is rigid and near-impermeable on walls that were built to move and to breathe. It cracks, lets rainwater in through those cracks, then stops the wall drying back out. The trapped moisture causes internal damp, decayed stone and rotten timber — and the fix is a vapour-open lime render, not a 'better' waterproof coating.
How a solid wall manages moisture
Almost every British building constructed before about 1930 has solid walls — stone, brick or cob, with no cavity and usually no damp-proof course. These walls were never meant to be waterproof. They work like an overcoat rather than a raincoat: driving rain soaks a little way into the surface, some moisture rises from the ground, and in dry weather the whole lot evaporates back out through vapour-open lime renders, plasters and limewash. As long as evaporation keeps pace with absorption, the wall core stays dry, the interior stays healthy and embedded timbers survive.
Every traditional finish on these buildings — lime render outside, lime plaster inside, limewash on top — was chosen because it keeps that moisture cycle open. The system only fails when a modern material interrupts it.
What cement render does to that system
Strong cement renders became common on old buildings from the mid-twentieth century, usually applied with good intentions: to smarten a tired elevation or to 'cure' damp. They do the opposite, for three connected reasons.
- Cement is rigid. Old solid walls move with the seasons — shallow foundations, lime-mortar joints and timber lintels all flex. A hard cement coat cannot follow that movement, so it cracks, typically as a network of fine hairline cracks that are easy to miss from the ground.
- Cracks pump water in. Rain is drawn into hairline cracks by capillary action, and wind pressure drives it deeper. A cracked cement render often lets more water into a wall than no render at all.
- Cement traps what gets in. Because the render is largely vapour-closed, the moisture that enters through cracks — plus rising ground moisture and everyday interior humidity — cannot evaporate back out through the face of the wall. The core gets steadily wetter, year on year.
Cement also tends to be denser and stronger than the masonry beneath it. When moisture in the wall freezes or salts crystallise, it is the softer original stone or brick that sacrifices itself, not the render.
The symptoms you can see
- Persistent damp patches on internal walls, often worst at low level and in bad weather, that no injected damp course seems to cure
- Blown render — areas that sound hollow when tapped, where the cement has lost its bond and holds a sheet of water against the wall
- Spalling stone or brick, where faces of the original masonry are breaking away at the render edge or wherever the wall can partially dry
- Rotten embedded timber — lintels, joist ends, bressummers and wall plates decaying inside walls that can no longer dry
- Green staining, moss and algae on the render face, and peeling paint or salt crusts inside
The pattern that gives cement away is chronology: a building that stood dry for a century or more develops 'inexplicable' damp problems a decade or two after re-rendering. The wall did not change. Its ability to dry did.
Why 'waterproof' is the wrong goal
Most failed interventions on old buildings — cement render, plastic masonry paints, sealants, tanking — share one idea: keep water out and the wall will be dry. For a cavity wall that logic holds. For a solid wall it is backwards. You cannot make a solid wall perfectly waterproof; there will always be a crack, a junction or the ground itself. What you can control is the exit route. A wall that admits a little moisture and releases it freely stays dry in the only sense that matters. A wall that admits a little moisture and releases none of it becomes a reservoir. The goal is not waterproof; it is breathable, with the water shed by good details — roofs, gutters, drips and plinths — rather than by a sealed skin.
Can you put lime render over cement render?
On a solid wall, no — and it is worth being blunt about it. The cement layer is the problem, and covering it changes nothing underneath: the wall still cannot dry through a vapour-closed layer, however breathable the new coat on top is. Lime also bonds poorly to smooth, dense cement, so the new work is likely to fail mechanically as well. Where cement render is causing damp, the choice is between removing it and living with it — there is no useful halfway house of rendering over the top.
Removing cement render: how it is done, and when not to
Careful removal is hand work. The render is broken into small sections with hand tools or light mechanical assistance, and each piece is eased off the wall rather than hammered through it, working with the render's own cracks and hollow areas. Trial patches come first, in inconspicuous places, to find out how hard the render is holding. After removal, joints are raked out where needed and repointed in lime, the masonry is left to dry — often for a full season — and only then does new lime render go on.
Sometimes leaving the cement is the lesser evil. If the render was applied to soft brick, cob or friable stone and has keyed in hard, removal can pull the face of the wall away with it — the cure becomes worse than the disease. In those cases the honest options are targeted removal of loose and hollow areas only, scrupulous maintenance of everything that sheds water, and accepting a managed compromise. A specialist should make that call wall by wall, not from a ladder in ten minutes. On listed buildings, both removal and re-rendering are likely to need consent, and conservation officers generally support returning to lime.
The lime alternative
The replacement system is the one the building had originally: a lime render in two or three coats, using a mix matched to the exposure and the background — feebly or moderately hydraulic lime in most exposed situations, softer mixes on soft backgrounds — finished with limewash or a breathable mineral paint. Lime render accommodates movement, so it resists the hairline cracking that undoes cement; it absorbs driving rain and releases it in dry spells; and when it does eventually weather, it can be patch-repaired rather than stripped. Renewing the limewash every few years is genuine maintenance, but it is cheap, quick and keeps the coat sacrificial — the render protects the wall, and the wash protects the render.
What it costs
Every building is different, so no honest figure can be given sight unseen. What the price turns on: how hard the cement is holding, what the masonry beneath needs once it is exposed, scaffolding, and whether repointing or repairs are needed before the new lime system goes on. It costs more than a cement re-render would — and considerably less than a decade of chasing damp through a wall that cannot dry. We price after inspecting the wall, never before, and the quote itemises removal, repairs and the new render separately.
If you suspect cement on your walls
Tap the render: hollow-sounding areas mean the bond has failed. Look for hairline crack networks, damp inside that tracks the weather, and stone decaying at edges and openings. Then get an assessment from someone who works with lime, because the right answer varies from full removal to leaving well alone. We are based near Newport and assess and re-render solid-wall buildings in lime across South Wales and the West.