Why is there damp under my window sill?

The drip groove that stops water running back under a sill, the three other ways a sill leaks, and the repairs in order of cost.

Difficulty: beginner
Time: 1 hr
Read: 5 min

Short answer

Nine times out of ten it is the drip groove. A sill has a channel routed or cast into its underside so water dripping off the front cannot run back along the soffit and into the wall. Fill that groove with paint or lose it to erosion and every shower sends water straight into the brickwork under the window.

A window sill is a small piece of engineering doing a specific job: throwing water clear of the wall below it. It has to project past the face of the wall, it has to fall outwards, it has to be sealed where it meets the frame and the reveals, and it has to have a drip groove underneath. Four requirements, and a damp patch under a window means one of them has stopped being met.

What you'll need

Torch and a mirror to see the underside of the sill
Multi-tool or a router with a small cutter (optional)
Proprietary drip bead (optional)
Exterior sealant (optional)

Step by step

Look at the underside of the sill.Use a mirror or a phone camera pointed upwards. You are looking for a continuous groove a few millimetres wide running the full length, back from the front edge. A painted-over, filled or missing groove is your answer.
Check that the sill actually projects.It should stand proud of the wall face below by enough to throw water clear — typically around 25 to 45mm. A sill flush with the wall, or a replacement window fitted onto the old sill without one, delivers water straight down the wall.
Check the fall.The top surface must slope outwards. A level or back-falling sill puddles, and the water finds the joint with the frame. A spirit level tells you in seconds.
Check the joint between the sill and the frame.On a replacement window this is a sealant joint and sealant has a finite life. Split, shrunken or missing sealant along that line lets water behind the sill entirely.
Check the ends of the sill.A sill should run past the reveals into the wall — 'stooled' ends on a stone or concrete sill, end caps on a uPVC one. A sill cut flush with the reveal channels water into the wall at both ends, which is why the damp is often worst at the corners.
Look for cracks across the sill itself.Concrete and stone sills crack, often over the centre of the opening. A crack all the way through takes water in and holds it, and in frost it opens further.
Reinstate the drip groove.Rake out a filled groove with a hooked blade, or where there is none, glue on a proprietary plastic or aluminium drip bead under the front edge. Both are cheap and both are effective.
Reseal the frame joint properly.Cut out all the old sealant, clean and dry both faces, and apply a low-modulus exterior sealant in one continuous bead. A bead over old sealant will not stick.
Deal with the wall below once the source is fixed.Let it dry through a season. Repair pointing or render only after the water has stopped arriving, or you will do it twice.

Tips

Painters routinely fill drip grooves with successive coats of paint. If a house has been painted many times and the damp appeared gradually, that is a likely explanation.
A uPVC sill fitted over an old stone sill often projects less than the original did. Check the projection against the wall face, not against the old sill.
If the damp inside is at the sides of the window rather than under it, look at the reveals and the frame-to-wall joint instead, and at whether the cavity closer is intact.

Common mistakes

Sealing along the joint where the sill meets the wall below — That joint is often the escape route for water that has already got behind. Sealing it traps the water in the wall instead of letting it out, so the damp gets worse rather than better.
Painting the underside of the sill without protecting the groove — Two or three coats close the groove, and the sill stops working. Mask the groove or clean it out afterwards.

If it doesn't work

Damp band directly under the window, full width

Cause: Drip groove filled or absent — Fix: Rake the groove clear, or bond a proprietary drip bead to the underside. Cheapest and most likely fix.

Damp worst at both ends of the sill

Cause: Sill cut flush with the reveals, water running into the wall at the ends — Fix: Fit end caps or stools, and seal the reveal junction. On a stone sill this may need a mason.

Damp on the internal wall under the window after rain

Cause: Failed sealant between the sill and the window frame — Fix: Cut out all old sealant, dry the joint, and reapply a low-modulus exterior sealant in one pass.

Damp under a bay window sill only

Cause: Bays concentrate water and often have a poorly detailed sill run with joints — Fix: Check every joint in the sill run and the junction with the bay's structure, and check the roof above the bay.

Questions people ask

Can I add a drip groove to a sill that has none?

Yes. A router with a small cutter will cut one into a timber or a soft stone sill, and for anything else there are stick-on aluminium and plastic drip beads made for the purpose. Both work.

Should I seal the gap under the sill against the wall?

Generally no. That gap is often a deliberate or accidental drainage path, and sealing it is a common way to make a damp patch worse. Fix the water getting in above it instead.

My internal window board is damp. Is that the same problem?

Not necessarily. A damp internal board with black mould and no rain correlation is condensation running off the glass. Watch for a week and see whether it tracks the weather or the mornings.