Inevitably, old wooden casements and sash windows will have deteriorated to some extent, but regular maintenance and prompt repairs will preserve them almost indefinitely. New frames, or frames which have been stripped, should always be treated with a clear wood preserver before they are painted.
The bottom rail of a softwood sash is most vulnerable to rot, particularly if it is left unprotected. Rainwater seeps in behind old shrunken putty and moisture is gradually absorbed through cracked or flaking paintwork. Carry out an annual check and deal with any faults. Cut out old putty that has shrunk away from the glass and replace it. Remove flaking paint, make good any cracks in the wood with flexible filler and repaint. Do not forget to paint the underside of the sash.
Where rot is well advanced and the rail is beyond repair it should be cut out and replaced. This should be done before the rot spreads to the stiles, otherwise you will eventually have to replace the whole sash frame.
Remove the sash by unscrewing the hinges or, if it is a double-hung sash window, by removing the beading.
With a little care the repair can be carried out without removing the glass, though if the window is large it is safer to do so. In any event, cut away the putty from the damaged rail.
The bottom rail is tenoned into the stiles (1), but it can be replaced, using bridle joints. Saw down the shoulder lines of the tenon joints (2) from both faces of the frame and remove the rail.
Make a new rail, or buy a length of moulding if it is a standard section, then mark and cut it to length with a full- width tenon at each end. Set the positions of the tenons to line up with the mortises of the stiles. Cut the shoulders to match the rebated sections of the stiles (3) or, if there is a decorative moulding, pare the moulding from the stile to leave a flat shoulder (4). Cut slots in the ends of the stiles to receive the tenons.
Glue the new rail securely into place with a waterproof resin adhesive and reinforce the two joints with pairs of 6mm (1/4in) stopped dowels. Drill the stopped holes from the inside of the frame and stagger them.
When the adhesive is dry, plane the surface as required and treat the new wood with a clear preserver. Reputty the glass and apply paint as soon as the putty is firm.
REPLACING A FIXED-LIGHT RAIL
The frames of some fixed lights (windows) are made like sashes, but are screwed permanently to the jamb and mullion. Such a frame can be repaired in the same way as a sash after its glass is removed and it is unscrewed from the window frame. Where this proves too difficult you will have to carry out the repair in situ.
First remove the putty and the glass, then saw through the rail at each end, close to the stile. Use a chisel to pare away what remains of the rail and chop out the tenons from the stiles. Cut a new length of rail to fit between the stiles and cut housings in its top edge at both ends to take loose tenons (1). Place the housings so that they line up with the mortises and make each housing twice as long as the depth of the mortise.
Cut two loose tenons to fit the housings and two packing pieces. The latter should have one sloping edge (2).
Apply an exterior woodworking adhesive to all of the jointing surfaces, place the rail between the frame members, insert the loose tenons and push them sideways into the mortises. Drive the packing pieces behind the tenons to lock them in place. When the adhesive has set, trim the top edges, treat the new wood with clear preserver, replace the glass and reputty. Repaint once the putty is firm.