Unscrew any opening window in the frame and reclaim any hinges and stays that might be reusable. If the frame has fixed lights, remove them if possible and save the glass for reuse. Glass left in place may get broken.
Use an old cold chisel or some similar implement to scrape and chisel away the mastic and mortar around the edge of the frame. Then insert a thin hacksaw blade between the frame and the brickwork and run it down the side of the frame until you find the fixings. Wrap the blade in cloth to protect your hand (or use a padsaw handle) and cut through the fixings.
Use a plugging chisel carefully to cut away the mortar joints around the brickwork holding in the horns. Remove the brick or bricks and save them (you will want to replace them later). Rake out the mortar bed under the window sill.
The frame should now be free so you can gently knock it out of the opening with a mallet or a hammer and a piece of wood held so that it protects the frame. It may be necessary to remove the window board (the internal window sill) before you can get the frame out.
Putting the old frame back
Before you start taking the frame out, you must decide how the frame is to be refixed. The choices are to build in the horns or cut them off, to use screws or nails driven into wooden plugs in the brickwork or to use screws driven into plugged holes bored into the brickwork.
This sequence of operations deals with a window that has horns built into the brickwork and is held in position with wooden plugs and screws.
If the existing plugs are loose, refix them into the brickwork or replace them with new ones. If the frame was held in position by fixing cramps, it may be necessary to cut new holes into the mortar between the bricks.
Check the fit of the frame in the opening and if everything is satisfactory lay a bed of mortar for the sill to rest on. You can shave off bits of the frame with a trimming tool or plane and pack out areas where the fit is poor. Any areas of bare woodwork exposed (and any new woodwork put in during the repair) should be treated with a preservative and primed before the frame is fixed.
Place the frame into position in the opening, hold it there, making sure that it is square, and screw through the frame into the timber plugs. It may make the job easier if the frame is pre-bored.
Replace the bricks into the wall around the hole cut for the horns. If the bricks are damaged, replace them. Point the mortar bed under the sill. Replace the glass into the fixed lights and rehang the windows.
Fill the gap between the frame and the brickwork with mortar to within 10mm of the outside edge of the frame. When the mortar has set (and possibly shrunk), fill the remainder of the gap with a non-setting mastic. Finally repaint the frame.
Repairing wet rot
Window sills often suffer from attacks of wet rot which weaken and soften the timber. Major attacks will mean replacing the whole sill or at least a section of it. Small outbreaks can be dealt with using a wood repair system, where hardener is used followed by special wood filler.
