How do I replace the rubber gaskets on a uPVC window?

The three gaskets on a uPVC window, which one causes which symptom, and fitting a replacement without stretching it or disturbing the glass.

Difficulty: beginner
Time: 1 hr
Read: 3 min

Short answer

There are usually three: the wedge gasket holding the glass in, the frame gasket the sash closes onto, and sometimes a second sash gasket. A draught means the closing gasket; water inside means the glazing gasket or a blocked drain.

uPVC gaskets are extruded rubber that takes a permanent set after fifteen years or so. They rarely look broken — they simply stop springing back, which is why the diagnosis is by feel rather than by eye.

What you'll need

A sample of the old gasket
Replacement gasket by the metre
A blunt plastic tool
Soapy water
Scissors

Step by step

Work out which gasket is failing.Air coming through when the window is shut and locked means the closing gasket between sash and frame. Water on the inside cill means the glazing gasket or, far more often, blocked drainage.
Do the paper test around the sash.Close the window on a strip of paper at eight or ten points. Where it slides out with no grip, that section of gasket is no longer compressing.
Compare a hard-worked section with a sheltered one.Gasket behind the hinge side sees less compression. If the two sections feel different in thickness and springiness, the gasket has taken a set and replacing it will work.
Rule out adjustment first.A sash that has dropped, or keeps that have wound back, produce identical symptoms. Check the sash sits square before buying rubber.
Pull a sample out.Start at a corner. Most closing gaskets pull out by hand. Take 100mm to a uPVC supplier and match the barb profile, not just the visible face.
Leave the glazing gasket alone unless it is the problem.Replacing the wedge gasket that holds the glass means taking the beads off and disturbing the sealed unit. Do not do it as a precaution.
Clean the channel.Vacuum the groove and wipe it. Twenty years of grit in the channel holds a new gasket proud and undoes the whole job.
Fit from the top, without stretching.Push the barb into the groove with a blunt tool and a little soapy water, feeding the gasket in loose. Stretched rubber contracts within weeks and opens the corners.
Butt the joint under slight compression.Cut the last end 5mm long and squeeze it in, and put the joint at the top of the frame rather than the bottom.
Recheck the sash operation.A new gasket makes a window stiffer to close for a week or two. If it is still hard after that, adjust the keeps rather than trimming the gasket.

Tips

Silicone grease on gaskets once a year keeps them supple, stops them sticking to the frame in frost and measurably extends their life.
Buy 10 percent extra. Corners consume more length than you expect and a short piece cannot be joined invisibly.
Never paint a gasket. Paint stiffens the rubber and it stops sealing almost immediately.

Common mistakes

Replacing the gasket when the problem was the drainage — Water on the inside cill is far more often a blocked drain slot than a failed gasket. Replacing rubber does nothing, and the water reappears at the next heavy rain.
Removing the glazing beads to change the glazing gasket unnecessarily — Taking beads off risks cracking them, disturbing the packers under the sealed unit and letting the glass shift, which causes a whole new set of problems.

If it doesn't work

Draught with the window locked

Cause: Closing gasket flattened — Fix: Replace the sash or frame closing gasket.

Water on the inside cill after rain

Cause: Blocked drain slots, not gaskets — Fix: Clear the drainage slots on the outer face of the frame first.

New gasket wavy after a month

Cause: Stretched while fitting — Fix: Remove, let it relax and refit with extra length.

Window very stiff to close

Cause: New gasket at full thickness — Fix: Give it a fortnight; if still stiff, adjust the keeps rather than the rubber.

Questions people ask

How do I know which gasket to buy?

Take a sample. There is no universal profile, and suppliers match them from a physical piece in seconds. Photographs alone rarely work because the barb shape matters.

Can I just add self-adhesive draught strip?

It is a stopgap that stops the sash compressing properly and often makes the handle hard to close, which loads the gearbox. Replacing the gasket is not much harder.