How to Lay a Sill
Sills are commonly found at the base of an opening whether it be a window or a door. If the correct dimensions are at hand laying a sill can be done with relative ease. If you have created a new window or door opening, perhaps in a garage area or as part of an extension then this guide will offer a simple methodical approach. This ‘how to’ shows you how to properly lay one.
1) Take note of the window or door frame which is to be sat into the opening as its depth will determine the underside of sill height. Most common sills measure a depth of 140mm and come in varying lengths, therefore the make up beneath the sill should allow for sill depth, a mortar bed and tolerance to the frame.
2) Take a length of 2”x1” timber and use it to create a staff, marking up underside of sill and top of sill. The difference between underside and top should be 10mm (bed joint) + 140mm (sill depth).The staff should be cut long enough to suit the depth dimension of the opening. Therefore, the staff should be the following length; depth dimension of frame + 5-10mm tolerance to frame + sill depth + mortar bed. The underside of sill marking will show you how high the external blockwork or brickwork needs to be built if it does at all.
3) Once the underside of sill has been achieved, cut a length of 450mm damp course 300mm longer than the sill itself and centre it to the opening. It is a good idea to tack the damp course to the top of the external skin to prevent any slipping when the sill is laid. The extra 150mm at either side of the sill is used to form a corner in the damp course.
4) Using a saw, the staff can now be cut at top of sill height. Lay the mortar mix on the external skin at a depth of roughly 15mm as the bed will reduce under the weight of the sill. Once the sill is laid place a wedge underneath the sill at either end to support it. Using a rubber mallet or gently moving the sill from side to side, reduce the mortar so that the staff fits between the underside of the window opening and the top of the sill. Be sure to level the sill front to back and side ways. Use the wedges to help achieve the desired tilt.
5) Work out what amount of overhang is appropriate. The overhang can be determined by one of many methods; set the overhang to suit any existing sills around the building; often a sill will be pushed tight against the cavity insulation; roughly 50mm away from the blockwork as this will ensure the drip is still effective once the the wall has been rendered.
6)The excess 150mm damp course at either end can now be folded to form a corner.
Be sure to make a guage staff as this helps you with all your heights and is much easier than repeatedly using a tape measure and maintains accuracy
Use wedges to help support the sill and prevent it falling forward.
Make sure you allow enough damp course at either end of the sill, usually 150mm
Only use a rubber mallet to prevent damaging the sill