BEST WAY TO INSTALL AN INTERNAL WINDOW SILL

Window sills are available in hardwood, softwood, and PVC options. It comes down to personal choice and budget, however MDF window sills are often deemed the most appropriate.

Window Sill size

Window sills can be bespoke to any size, but most homeowners will choose a standard size for financial reasons and convenience. There are three elements to consider; length, height, and thickness. Both the length and the height will be decided by the size of your window. Here’s how to measure it in the most effective way.

As for thickness, you’ll want to consider the height of your windows. If they are quite low, adding a thick window board could look a little strange.

How to Install the Window Sill

Once you’ve found the right window sill, the next job is to install it correctly. Window sill fitting isn’t the hardest DIY task imaginable, but it does require accuracy. Here’s how you can find the right size window sill and install the MDF window board in style;

1. Make a ¼in reveal mark on the window jambs.

2. Make a reference line on the sides of the window.

3. Take the measurement of space between the two wall makes, then add 1.5in to cover the overhand needed on each side.

4. Cut and sand the window board.

5. Hold the window board against the window and mark where it meets the window and wall, as these areas need to be cut out.

6. Trim the stool by cutting out the unneeded areas.

7. Fix the stool into the desired gap and make any necessary cut adjustments to ensure a snug fit.

8. Use 6d or 8d finishing nails to fix the MDF window board into place.

9. Add the new side window trim.

10. Use 6d finishing nails to nail trim casings to the side jambs and into the vertical casing of a tight joint.

Apply the Finish

You can do this before or after fixing the sill in place, but we recommend doing this before so you can apply the finish in a well-ventilated area with suitable protection, and avoid spillages on wallpaper or carpet.

Whether you are finishing with stain or paint, always follow the manufacturer’s instructions to ensure a professional finish. To prevent damage and improve durability, add 2-3 coats of a clear finish and allow to properly dry.

Note: If you have chosen a fully finished product, you can skip this step.

Top Tip!

When installing the window sill, give it a slight, unnoticeable tilt down from the window to ensure any moisture runs away to the front of the board, preventing the formation of rot or mould.