How to fit window board

Window board

Window board is the ledge on the inside of a building at the base of the window, used to close the gap between it and the wall.  Condensation in a room will hit the window pane and then drip down towards the bottom of the glass. We would recommend using SAM moisture resistant (MR) primed MDF window board. This will divert this moisture and prevent damp and mould. Window board is often mistaken for windowsill, which is the ledge below the window on the outside of a building that allows rainwater to be diverted away from the window and run off the surface instead.

Not only is window board practical, it also looks great. It’s the perfect place to arrange your ornaments, photos or plants!

Choosing your window board

When you purchase your window board, it will usually come with a tongue already cut into the back, so it can slot into place easily. You can save time and money with SAM primed MR MDF window board as it is ready to paint. This is unlike softwood timber which needs a lot of preparation to get to this stage. If you would prefer to cut out painting altogether, SAM Zero fully finished window board offers a seamless, high quality finish. Therefore, allowing you to get on with the more important things in life!

SAM MDF window boards come in a wide variety of made to measure profiles. They are also moisture resistant and complete with a bullnosed edge for a smooth and polished look.

Fitting your window board


To start, you will need to ensure your window board is the correct size to fit the space underneath your window. Measure the width of the opening/window and add 100mm on for the overhang, so 50mm for each side of the window board. Add the thickness of plastic or plasterboard that’s going on the walls and cut the window board to that width. If you have chosen our SAM MR MDF window board, you may order it as ‘made to measure’, with a variety of sizes up to 600mm width (minimum order quantities apply).


Window board can be glued down with adhesive or expanding foam. Then, weigh it down with a heavy object until the glue sets. To get the best finish, it is best to avoid visible fixings like nails or screws. Unless the whole window board is being painted.

Once you have checked that the window board fits in the gap between both walls, you can then apply your adhesive to the bottom surface. Apply your adhesive evenly across the window board and lay it carefully onto the surface, pushing down firmly to hold it into place. You can use a spirit level to ensure that the window board is perfectly horizontal. You may angle the window board slightly if desired to allow any excess condensation to run off towards the tip of the board.

Finish Fitting Your Window Board

Once the adhesive is dry, use some expanding foam to fill in any remaining gaps. This will also ensure that the window board is fully secured, preventing it from loosening over time.

And that’s it! Your window board is secure and ready. If you have opted for SAM primed MDF window board, you can now paint it in any colour of your choice.

For more information contact our team on +44 (0)28 9442 8288 or email