This page covers a few different ways of fitting window cills and fixing them to different wall types including cavity walls and timber stud work walls too. If possible it's better to install them before the plaster or plasterboard goes on.
You can buy window boards straight off the shelf made from pine, hardwood or MDF (ideally moisture resistant). They'll come with a tongue already cut into the back like the picture below to slot into a groove in the bottom of the window.
If you're making your own cill which sometimes better if you want the timber to match because the window and board are both being stained, or you need a deeper than usual board then you'll need a router with a fence, round-over bit and a straight cutter for the rebate. It is possible to make the rebate with a circular saw if you don't have a straight cutter.
The measurements will depend on whether the wall is being plastered, plaster-boarded or covered with something else. I always aim to finish up with around 50mm extra width on each side, and a 25mm projection at the front.Step 1: Width
Measure the width of the opening/window, add 100mm for the overhang (50mm each side), add the thickness of plaster/plasterboard that's going on the walls and cut the window board to that width.
Those projections look fine and are in proportion, unless you are replacing an existing sill in which case you should match to the rest of the house.
The only other time I change that is when I'm working on Victorian box sash window boards where the architrave is mitered round the window and sits down onto the window cill.
In this instance I add extra depending on the size of the architrave being used. There's more info about architraving Victorian sash window cills on the bottom of the architrave page.
Use a router with a rounding over cutter bit and guide to machine a Bullnose edge onto the board. Some people prefer to push the router away from themselves I prefer to walk backwards pulling the router towards me, obviously making sure the coast is clear first! The pic below shows the direction I use the router in while standing at the bottom. Once one side is done, flip it over and machine the bottom of the window cill too before finishing off with sandpaper.
Once the edge is rounded over you can cut the notches out of the back corners and rip the back off to the correct depth (Depth of groove in window + cill + overhang).Step 3: Rebate
You can use a straight cutter in your router to cut the rebate or set the circular saw to the desired depth and then use the fence to set the width of the tongue. I always make it 1-2mm shorter than the depth of the groove and put loads of wood glue on the back before sliding the tongue into the rebate on the window.
There are several different methods you can use to fix boards in place. You can simply glue them down with Gripfill or expanding foam and weigh them down with something till the glue goes off.
I try to avoid visible fixings on finished work unless its being painted but sometimes it's necessary to drill pilot and countersink holes in the top of the board before screwing down into the masonry or stud work.
When I'm fixing window boards to masonry that hasn't been plastered yet I slot the tenon into the window rebate and mark the edge of the brickwork along the underside of the window board. Remove it, turn it over and screw a couple of Lateral restraint straps (really cheap) or L - brackets to the bottom of the board no more than 300mm apart.
Using lateral restraint straps to fix window boards means you can adjust the front edge of the board up and down before screwing the strap to the brickwork.
Sometimes I put a very slight fall on the window cill so if there's any condensation it will run toward the front of the board not towards the bottom of the window (top green arrow above) where it will sit, eventually rotting the timber or causing mould.If working in a stud-work wall, once the window boards are installed the plasterboard can be cut and fitted.
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