The sill is a fundamental part of a window frame, and if one is afflicted by rot it can mean major repair work.
A casement-window frame is constructed in the same way as a doorframe and can be repaired in a similar way. All the glass should be removed first. The window board may also have to be removed, then refitted level with the replacement sill.
Make sure that the damp-proofing of the joint between the underside of the sill and the wall is maintained. Modern gun-applied mastics have made this particular problem easier to overcome. Some traditional frames have a galvanized-iron water bar between the sill and sub-sill. When replacing a sill of this type without removing the whole frame you may have to discard the bar and rely on mastic sealants to keep the water out.
Do not simply replace a sill by cutting through it and fitting a new section between the jambs. Even if you seal the joints with mastic, any breakdown of the sealant will allow water to penetrate the brickwork and end grain of the wood, and you may find yourself doing the job all over again.
Serious rot in the sill of a sash window may require the whole frame to be taken out. Make and fit a new sill, using the old one as a pattern. Treat the new wood with a preserver and take the opportunity to treat the old wood which is normally hidden.
The traditional stone sills that feature in older houses may become eroded by the weather if they are not protected with paint. They are also liable to crack if the wall subsides.
Repair cracks and eroded surfaces with a ready-mixed quick-setting waterproof mortar. Rake out the cracks to clean and enlarge them. Dampen the stone with clean water and work the mortar well into the cracks, finishing flush with the top surface.
Undercut any depressions caused by erosion to help the mortar adhere — a thin layer of mortar simply applied to a shallow depression in the surface will not last for long. Use a cold chisel to cut away the surface of the sill at least 25mm (1 in) below the finished level and remove all traces of dust.
CASTING A NEW SUB-SILL
Cut out what remains of the old stone sill with a hammer and cold chisel.
Make a wooden mould with its end pieces shaped to the same section as the old sill. The open top of the mould represents the underside of the sill.
Fill two-thirds of the mould with fine- aggregate concrete, tamped down well. Add two lengths of mild-steel reinforcing rod, judiciously spaced to share the volume of the sill, then fill the remainder of the mould. Set a narrow brickwork. Apply a bead of mastic sealant to the sill, then replace the complete frame in the opening from inside. Make good the damaged plaster.
It is possible to replace the sill from the inside with the frame in place. Saw through the sill close to the jambs and remove the centre portion. Cut away the bottom ends of the inner lining level with the pulley stiles and remove the ends of the old sill. Cut the ends of the new sill to fit round the outer lining, and under the stiles and inner lining. Fit the sill and nail or screw the stiles to it.
Make a wooden former to the shape of the sill and temporarily nail it to the brickwork. Dampen the stone, trowel in the mortar and tamp it level with the former, then smooth it out. Leave the mortar to set for a couple of days before removing the former. Allow it to dry thoroughly before applying paint.
Use a piece of wood such as a dowel, into notches cut in the ends, of the mould. This is to form a ‘throat’ or drip groove in the underside of the sill.
Cover the concrete with polyethylene sheeting or dampen it regularly for two to three days to prevent rapid drying. When the concrete has set (allow about seven days), remove it from the mould and lay the new sill in the wall on a bed of mortar, packed from underneath with slate to meet the wooden sill.