Window Boards

Window boards are the internal horizontal platforms fitted underneath window frames to create a window sill or cill. They provide a decorative covering for the inside of the external wall below the window and a useful surface next to the window.

Window boards come in a variety of materials. Popular choices are softwoods like pine which are inexpensive but may warp or discolour over time, or hardwoods like oak which are more expensive but very durable. There is also uPVC and composite plastic window boards which never need painting. Or manufactured board like MDF which gives a smooth finish after painting, our MDF features moisture-resistant properties to protect your window sill from moisture and condensation.

1. Take precise width and depth measurements of the internal recesses underneath your window frames. It is important your window board will fit neatly into the space. You can allow a few mm of width on both sides so the board is not forced into place.
2. Using your measurements, mark then cut your window board to the correct size with a fine toothed saw. Check cuts are straight and neat, sanding the edges smooth if necessary. Softwoods can be cut with a power saw to achieve very clean edges.
3. Pre-drill and countersink holes at regular intervals around the window board if securing with screws. Alternatively, apply a continuous bead of panel adhesive around the edges and position in place pressed firmly against the walls. Small pins can also be used for extra support while the adhesive sets.
4. Fill and smooth any unwanted gaps around the edges with acrylic sealant/caulk, carefully wipe away excess with a damp cloth. Leave for at least 24 hours before attempting to decorate.

To calculate how much window board you need, carefully measure the total window recess width needing covered, as well as the depth of your window recesses. Multiply the two measurements to give you the total area in m2. Buy slightly more than your exact calculation to allow some contingency for cutting errors or damage. Remember if you have deep window recesses they may need two layers of boards.

For bay window boards, each section needs individually measured then neatly joined at an angle to match the bay shape. Mitre joints which intersect the wood at precise 45 degree angles are best for neat, narrow seams where the boards join. Alternatively, use several straight boards placed tightly together to make a stepped finish inside the bay recesses. Support long stretches of window board with timber battens attached at intervals across the recess walls.

1. Ensure window sills are cleaned of any dirt, grease or chipped paint flakes which could affect paint adhesion. Lightly sand the surface to improve paint key especially if they were previously glossed, wiping after to remove all dust particles. Fill any small holes, cracks or imperfections with a water based lightweight spackle filler, allow to completely dry and sand flat.
2. Apply one thin coat of good quality acrylic primer undercoat to seal the surface, leaving at least 4 hours drying time. Lightly sand again and wipe clean.
3. Using a small brush, apply two full coats of durable satin, eggshell or semi-gloss emulsion allowing the first coat to fully dry before recoating. Water-based trim paints give better durability for windowsills.
4. Consider applying a final finish of water-based varnish in satin or semi-gloss which will protect the painted surface from minor scuffs and marks.

In British English, both "window sill" and "window cill" are widely used terms for the ledge or platform underneath a window inside a building. "Window sill" tends to be more commonly used in modern properties while "window cill" remains popular in older properties, especially historic homes.