So the first thing we do is work out what went wrong. Moisture from behind, movement in the structure, a bond failure from poor preparation, or salt attack. Each one needs a different fix, and only one of them is solved by putting new render on.
Drummy render and how we find it
Drummy means the render has lost its bond with the wall and there is now a void behind it. It still looks perfectly fine from a distance, which is why people are often surprised by how much of a wall has to come off.
We find it by sounding the wall, which means tapping systematically across the whole surface. Bonded render gives a solid dull note. Debonded render sounds hollow. It takes time and it is the only reliable way to map the extent.
Once we know how much is drummy, you get an honest number. Sometimes it is a two square metre patch. Sometimes it is sixty percent of the elevation and the right answer is to strip the whole wall and start again, because patching that much is throwing money at a wall that will keep failing.
Reading a crack
Not all cracks mean the same thing, and the pattern tells you most of what you need to know.
- Fine random crazing across the surface. Cosmetic, caused by the render drying too fast. Fills and coats over without concern
- Straight vertical cracks at regular spacing. Usually a missing or bridged control joint. Fixable by cutting a proper joint in
- A crack tracing every sheet joint in a grid. Missing mesh on a panel wall. Needs the system corrected, not patched
- Stair step cracking following the mortar joints, wider at one end. Structural movement. The wall is moving and the render is reporting it. Needs assessment before any repair
- Bubbling and flaking in a band along the bottom of the wall. Rising damp and salt attack. The moisture source has to be dealt with first
Matching an existing texture
The hard part of a render repair is usually not the render. It is making the patch disappear into a wall that has been weathering for fifteen years.
Texture grade has to match, and so does the application technique, because two renderers can use the same product and get a visibly different finish. We take a sample from the existing wall and match to it, and where a patch sits in the middle of a large plain elevation we will often recommend coating the full panel back to a corner or an opening rather than trying to blend into the middle of a wall. That way the eye has a natural stopping point.
Water damage and internal repairs
Internally, most render damage traces back to water. A failed shower membrane, a leaking pipe in a wall, a roof or gutter problem, or condensation in a poorly ventilated bathroom.
Replacing the render without fixing the water source is pointless and we will say so. Once the leak is dealt with and the wall has dried out properly, which can take weeks on a solid masonry wall, we will cut back to sound material and rebuild the render to match.
