Replace a worn uPVC window gasket to stop draughts and condensation, working around the frame section by section.
WG-01Confirm it's the gasket at faultRun your hand around the closed window on a windy day to feel for draughts, or look for a flattened, cracked or perished rubber strip in the frame. If the uPVC frame itself is warped or the hinges are loose, a new gasket won't fix that. Gaskets are cheap, so it's worth replacing if in doubt.
WG-02Identify the gasket type and measure upOpen the window fully and find the rubber gasket seal running in a groove around the frame or the opening sash. Note whether it's a bulb shape (rounded) or a wiper/fin shape, as they aren't interchangeable. Measure the full perimeter it runs around, and add 100mm extra for trimming and joins.
WG-03Buy the matching replacement gasketCut a short offcut of the old gasket, about 50mm, and take it to a glazing supplier or use it to match against products online. Gasket profiles vary between window manufacturers, so an exact match matters for a proper weatherproof seal. Buy it by the metre, ordering the length worked out in the previous step.
4 m · Replacement uPVC window gasket (co-extruded rubber, per metre)
WG-04Set up your work areaLay a dust sheet on the sill and floor below the window. Have your flat-blade tool, knife and cloth within reach before you start, as the job goes faster if you don't stop halfway round.
WG-05Remove the old gasketStarting at a corner, slide a flat-blade tool or old flathead screwdriver under the end of the gasket to lift it clear of the groove. Pull the whole strip out by hand, working around the full perimeter. Old gasket can be brittle, so it may come away in short pieces — that's fine.
WG-06Clean out the grooveWipe out the empty groove with a cloth dampened with mild detergent to remove dirt, old adhesive and grit. Dry it fully before fitting the new gasket, as trapped moisture can stop it seating properly.
WG-07Fit the new gasketStarting at the same corner, press the new gasket into the groove by hand, working it in a little at a time rather than stretching it tight. Go all the way around the frame or sash, keeping even pressure so it sits flush and doesn't bunch up at the corners.
WG-08Trim and join the endsCut the gasket to length with a sharp knife once you're back at the starting point, leaving a small overlap. Mitre the cut at roughly 45° so the join sits neatly rather than butting square, which shows a visible gap.
WG-09Test the seal and the window operationClose and lock the window fully, then check the gasket is compressing evenly all the way round with no visible gaps. Open and close the window a few times to check it still operates smoothly and the gasket isn't snagging or twisting anywhere.
WG-10Tidy upBin the old gasket offcuts and fold away the dust sheet. A light spray of silicone lubricant on the hinges and locking mechanism now will keep the window moving freely.