In short
Hairline cracks usually mean the render dried too fast, went on in the wrong weather, or the mix was too strong for the wall. They are cosmetic. Cracks you can fit a coin edge into let water in, freeze and push the render off, so they need cutting out and repairing. Diagonal cracks from window corners follow movement in the wall and need looking at first.
This is a job for our Rendering service.
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What the cracks are telling you
Hairline cracks that you cannot get a fingernail into are almost always shrinkage. The render dried too fast, went on in the wrong weather, or the mix was too strong for the wall behind it. We see this a lot on sand and cement render around Diss and Harleston that was put on in a hot week with no protection. It looks poor, but it is cosmetic.
Cracks you can fit a coin edge into are different. Water gets in, freezes and pushes the render off. Baumit describe exactly that cycle: hairline cracks open as the wall expands and contracts, water enters, freezes, and the damage grows each winter.
Cracks that run diagonally from a window or door corner, or step through the render like a staircase, usually follow movement in the wall itself. They need looking at before anyone renders over them.
Why render cracks in the first place
- Weather during application. K Rend say their render should go on at 5°C and rising, and not in direct sun, high wind or damp weather. Baumit give the same lower limit, with 30°C as the top end. Outside that window it dries unevenly and shrinks.
- Too strong a mix. A cement-rich sand and cement mix is harder than the brick or block behind it, so it cannot move with the wall.
- No mesh or beads. Corners, openings and the join between old and new work all need reinforcement.
- Movement in the wall. Lintels, settlement, or a new extension tied to an old house.
- Damp behind the render. Failed gutters or a bridged damp course keep the wall wet and the render never bonds properly.
What we do about it
We start by tapping the wall. Hollow-sounding render has lost its bond and has to come off, however small the crack looks on the surface. Sound render with hairline cracks can be filled and given a fresh finish; K Rend make a colour-matched filler for this, and we use it on our silicone jobs.
- Cut out anything hollow or blown back to solid, bonded render.
- Fix the cause: sort the gutter, the lintel or the damp before any new material goes on.
- Mesh the repair and the surrounding area so the patch does not show through.
- Re-coat with a system suited to the wall. On older solid walls that means something flexible and breathable rather than a hard cement skin. Matching the render to its background is what the external rendering standard, BS EN 13914-1, covers under design and background preparation.
Where a wall is cracked all over, patching rarely pays. We will tell you honestly if it is time to strip and re-render, and as K Rend approved applicators we can then back a silicone system with a 30-year warranty.