Window sill cover boards allow you to easily cap over existing window sills. These 9mm capping boards are exterior grade uPVC, so unlike wood they are not affected by damp and moisture so can be used in wet areas and rooms of high humidity. External Board Width: 225mm Internal Board Width (Coverage): 216mm The following bullet points provide some guidance for fitting: 1. Cut capping length to match overall length of window board (including side edges) 2. Cut the back edge of capping to equal window board depth plus 9mm (allowance for thickness of downward leg) 3. Measure length of reveal to cut out any side edges as required 4. Apply quick grab adhesive to window board before placing capping firmly on top 5. Apply decorators caulk around edges for a neat finish if required