How To Replace A Window Sill
Our photo guide to swapping a tired or water-damaged sill for a new UPVC or timber window board using basic hand tools.
The short answer
To replace a window sill, cut the old sill back flush with the wall or prise it out, then measure the length, depth and horns for the new window board. Scribe the wall angles onto the board with a sliding bevel, cut and test fit it, then bed it on a grab adhesive such as Gripfill and weigh it down while the adhesive sets.
Internal window sills in UK homes are made from painted timber, MDF or white UPVC board, and replacing one is a job you can do yourself with a handsaw, a sliding bevel and a tube of adhesive. The fiddly part is getting the new board to sit tight against walls that are rarely square, so that's where a lot of this guide focuses.
We've fitted UPVC window boards on all of our own sills, and the photos throughout come from one of those jobs. Below we cover the choice between timber and UPVC, how to measure up and each stage of the fitting.
Wood Or UPVC Window Sills
Timber was the traditional choice for internal sills and it still has its fans, but UPVC board has taken over in a lot of homes because it needs so little looking after. Neither is the wrong choice, and the right one depends on the room and the finish you like.
A wooden window board has these points in its favour:
- Cutting: timber is easier to cut and shape for a tight fit
- Colour: you can paint it to suit any scheme, and a coat of hard-wearing satinwood paint stands up well to pots and blinds
- Repairs: dents and chips can be patched with a decent wood filler for holes and dents and repainted
UPVC window board brings a different set of strengths:
- Durability: it's hard-wearing and shrugs off knocks
- Cost: it's cheaper to buy than a timber board
- Cleaning: a wipe with one of the best UPVC cleaners keeps it bright
- Finish: there's no sanding or painting, now or later
- Moisture: it won't rot, shrink or swell when water sits on it
In some rooms, water makes the decision for you. The photo below shows an MDF sill from one of our properties that had taken on so much water that the front edge swelled and crumbled, and it was beyond repair, so we replaced it with a UPVC board. In bathrooms, utility rooms and anywhere condensation runs off the glass, we'd fit UPVC every time.
Elsewhere in the house it comes down to taste. Some people don't like the slight shine of UPVC next to painted walls and woodwork, and a painted timber board blends in better there. We chose UPVC for all our sills because it's tough and easy to keep clean, so this guide shows a UPVC board going over the top of an existing wooden sill. A new timber sill is fitted differently in one respect, as the old board has to come out completely first.
Tools And Materials For Fitting A Window Board
- Window board in UPVC or timber
- Sill end caps for a UPVC board
- Grab adhesive and a sealant gun
- Tape measure
- Sliding bevel
- Jack saw or jigsaw
- Pencil or a non-permanent pen
- Hammer and chisel if the old sill is coming out
Measuring Up For A New Window Sill
Measure before you buy anything, because window board is sold in set depths and long lengths. Run the tape along the full length you want to cover, including the horns. These are the ends of the sill that extend past the window reveal onto the wall, and they're purely decorative, so make them whatever length looks right as long as both sides match.
For the depth, measure from the window frame to the front edge of the old sill and add the overhang you want. That's a personal choice, though from our experience an overhang of about an inch looks right on most walls.
Fitting UPVC over an existing sill means measuring its thickness too. The front lip of the new board has to cover the old one, along with any damaged plaster below it that you'd rather hide than patch. Prices for window board vary a lot between suppliers, so shop around once you have your sizes.
How To Replace A Window Sill Step By Step
1. Cut Back Or Remove The Old Sill
The old sill either comes out or gets cut down. As we were laying a UPVC board over the top, we left ours in place and trimmed it back with a jack saw, which is the stage in the photo below. Keeping the old sill in also meant there was no risk of it cracking the freshly plastered wall as it came out.
A timber replacement needs the old sill removed altogether. Knock out some of the plaster around it with a chisel to free the board, then lever it away from the wall. It's slow, messy work that takes far longer than cutting back, which is why we'd overlay wherever you're fitting UPVC.
2. Take Your Final Measurements
With the old sill trimmed or removed, measure for the new board using the method in the section above. Boards of 3 metres and longer are easy to find, so plan your cuts before you pick up the saw. Getting two sills out of one length and keeping the offcut saves buying another board.
3. Mark The Wall Angles With A Sliding Bevel
Walls are rarely square to the window, so a board cut at a neat right angle leaves gaps at the ends. Hold a sliding bevel against the wall where the board will meet it, lock the blade at that angle, then lay it on the board and draw along it with a pencil, as in the photo.
Repeat this for both sides and for the inside edge of each horn. Stick to pencil or a non-permanent pen, since you'll want the lines to wipe off once the sill is down.
4. Cut The Window Board And Test Fit
Cut along your lines with a jack saw or a jigsaw, keeping the cut as clean and straight as you can because any wobble shows at the ends of the finished sill. Long boards flex and bounce as you saw, so get a second person to hold the far end.
Offer the board into the opening and check how it sits. Expect to trim it a few times before it fits tight, and shave off a little at a time rather than risk cutting it short. Even experienced fitters rarely get it right first go.
5. Apply The Adhesive
Once the fit is right, run adhesive over the old sill. We laid ours in a zigzag bead along the length of the sill, which spreads it evenly without waste. Too much squeezes out under the front lip, and too little leaves hollow spots that let the board lift later.
Plenty of adhesives will bond a window board. On this job we used Evo-Stik Gripfill, a gap-filling grab adhesive that copes with an old sill that isn't perfectly flat.
6. Press The New Sill Into Place
Don't leave the adhesive sitting for long before the board goes on. Lower it into position, press down firmly at each end and work your way towards the middle, keeping the pressure on so the adhesive spreads out underneath.
7. Weigh The Board Down
Put some weight on the sill while the adhesive grabs so it beds down along its whole length. A stack of books or a few tools will do, as long as nothing scratches or marks the surface.
8. Fit The End Caps
End caps are optional on a UPVC sill, but they're a tidy finishing touch. They cover the cut ends, hide any small imperfections in the cut and conceal the edge of the old sill underneath.
9. Clean Up The Sill
Peel off any protective film and packaging, then wipe away the pencil lines left from marking out. A thin bead of white sealant along the joint with the frame closes up any hairline gap, and our guide to the best bathroom sealants covers ones that stay white.
Getting The Ends Of The Sill Right
Replacing a window sill is well within reach of a confident DIYer, and the cutting and gluing are straightforward. Marking out the ends is the part that takes patience, because those bevel angles decide whether the board sits tight to the wall or leaves a gap you'll be filling afterwards.
If you've gone for timber, seal any knots with a knotting solution before priming so resin doesn't bleed through the paint. Anyone weighing up the same choice for the rest of the woodwork will find our comparison of MDF or wood skirting board useful, and where the plaster around the window is new, check how long plaster takes to dry before you paint the reveals.
More Questions Answered
Can you fit a new window sill over the old one?
Yes, you can fit a UPVC window board straight over an old timber sill, and it's far quicker than taking the old one out. Trim the old sill back so it isn't proud of the wall, then bond the new board on top with a grab adhesive. The front lip of the UPVC board covers the old edge, and end caps hide the cut ends.
What adhesive do you use for a UPVC window sill?
A gap-filling grab adhesive is the right choice for a UPVC window sill, and we used Evo-Stik Gripfill on our own. Apply it in a zigzag bead across the old sill or base, press the board down from the ends towards the middle and weigh it down while it sets. Gap-filling adhesive copes with a base that isn't perfectly flat.
Do you need to remove plaster to replace a window sill?
You only need to remove plaster if the old sill is coming out completely, which is the case when fitting a new timber sill. Chip out the plaster around the board with a chisel to release it, then make good afterwards. Overlaying a UPVC board avoids this, as the old sill stays in place and the plaster isn't disturbed.
How far should a window sill overhang the wall?
A window sill overhang is a matter of taste, and about an inch past the face of the wall looks right on most windows. Make sure the new board's front lip also covers the edge of any old sill underneath. The horns at each end can be any length, as long as both sides match.
Would you choose a UPVC or timber window sill for a bathroom?
We'd fit UPVC in a bathroom every time, and the same goes for utility rooms and any window that streams with condensation. Standing water won't make it rot, shrink or swell. One of our own MDF sills had soaked up so much water that the front edge swelled and crumbled, and swapping it for UPVC meant that problem wouldn't come back.
Why does my new sill leave gaps at the ends?
Gaps at the ends mean the board was cut square to walls that aren't. Most window reveals sit at a slight angle, so copy each one with a sliding bevel, lock it and pencil the line onto the board, including the inside edge of both horns. Then trim a little at a time and keep offering the board up until it sits tight.
How do I stop the new sill lifting later?
Even adhesive coverage is what stops a sill lifting. We ran a zigzag bead of Evo-Stik Gripfill along the old sill, which avoids the hollow spots that let a board rise later without so much that it oozes out under the front lip. Press the board down from each end towards the middle, then leave some weight on it while the adhesive grabs.
Do I need to fit end caps on a UPVC sill?
You don't have to, but we'd fit them. A pair of end caps tidies up the cut ends of a UPVC sill, covers any slight wobble in the saw line and hides the old sill where it pokes out underneath. Once they're on, peel off the protective film and run a thin bead of white sealant along the frame joint to close any hairline gap.
Richard has spent years renovating properties and turning them into rentals and Airbnbs. Like a lot of people he started out as a beginner, and these days there isn't much DIY he won't take on.