How do I repair a cracked or damaged window sill?

An external sill has one job: throw water away from the wall below the window. When it cracks, spalls or splits, water starts going into the wall instead, and the damp appears inside as a patch below the window that looks like a failed seal. Repairing it is usually straightforward; the part people miss is the drip groove on the underside, without which even a perfect sill delivers water to the wall.

Safety

What you'll need

• A cold chisel and club hammer, or an angle grinder with dust control
• Stone or concrete repair mortar, or a proprietary repair system
• A stiff brush and a bucket of water
• uPVC solvent cement and filler for a plastic cill (optional)
• Masonry primer or a stone consolidant where specified (optional)
• Eye and respiratory protection

Step by step

1. Work out whether it matters.Fine surface crazing on a concrete sill is cosmetic. A crack that runs through the sill, a piece that has spalled off the front edge, or a sill with a back fall towards the wall is letting water in and matters. Look for staining and salting on the wall below and for damp inside.
2. Check the fall and the drip before repairing anything.A spirit level on the sill top: it should fall away from the window. Then feel or look at the underside for a groove or a projecting throat close to the front edge. A sill with no drip groove wets the wall regardless of its condition.
3. Cut out a stone or concrete repair properly.Rake and chisel back to sound material and undercut the edges slightly so the repair is keyed and cannot fall out. Square the edges — a feathered repair always fails at the feather. Brush out and damp the surface before applying anything.
4. Use a repair mortar no stronger than the sill.This is the rule that matters on old stone. A hard cement repair in a soft stone sill causes the surrounding stone to fail around it. Use a lime-based or a proprietary stone repair mortar chosen for the substrate, and match the colour with a sample before committing.
5. Build the repair in layers.Deep repairs go on in lifts, letting each set before the next, rather than in one thick application that will crack and shrink. Shape the final layer proud and cut it back once it is firm.
6. Reinstate the drip groove.Cut a groove in the underside near the front edge with a grinder and a diamond blade, or fit a proprietary plastic or aluminium drip. It must run the full length but stop short of each end, or water tracks out of the ends and down the reveal.
7. Repair a uPVC cill differently.Small cracks and holes in a uPVC cill are filled with a solvent-based uPVC filler that welds into the material, not with a decorators' filler. Clean, key lightly, fill proud, cut back and polish. Colour matching on a weathered white cill will be approximate.
8. Replace a damaged uPVC cill end cap.The end caps on a uPVC cill are the parts that break, and they are a cheap standard component. Clean the cill end, apply solvent cement, and fit a new cap. A missing end cap lets water directly into the cavity behind, so it is more important than it looks.
9. Check the joint between the sill and the frame.That joint is where a great deal of water gets in. Cut out any failed sealant completely, clean both faces, and reseal with a good exterior sealant in dry conditions. Do not seal over old sealant.
10. Deal with the pointing under the sill.Water that gets under a sill is running out through the bed joint below. Rake out and repoint that joint with a mortar appropriate to the wall, and leave a small gap at each end where a stop end is designed to drain.
11. Finish appropriately and leave it to weather.Repair mortars cure over days and colour-match better once they have. Do not judge the colour on day one, and do not paint a stone sill to hide a repair — paint traps moisture in the stone.

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