Replacing a window pane

A common job about the home is replacing a broken pane of glass. The technique of re-glazing is the same as for glazing a new frame — but with the need to remove the old glass and prepare the frame. Ideally, glazing is best carried out in warm weather, as low temperatures tend to make glass brittle and more difficult to cut.

In timber frames, the glass is held in a channel (called a rebate) with putty and either special glazing pins, called “sprigs”, or panel pins.

On metal frames, a mastic glazing material is used. Expansion must be allowed for by expansion pieces, made of plastic. These are set into the bedding mastic at the bottom of the frame. Their thickness must be added when measuring the size of the pane required. Sprigs are not used. Spring clips, rather like an elongated letter “S”, hook into the rebate and press on to the face of the glass. When replacing glass in a metal frame, save these clips as they are reusable.

Tools. You need the following tools to remove damaged glass and re-glaze a window frame: an old chisel, hammer, pliers, steel measure, T-square or straight edge, soft brush, putty knife, paint brush, goggles and protective gloves.

Materials. Apart from the glass, you need sprigs or panel pins, putty or mastic, spring clips, expansion pieces, metal priming (for metal frames).

Removing the old glass. Wear thick protective gloves and goggles when removing segments of glass.

1. Use an old chisel and hammer to hack out any remaining glass and the old putty.

2. Pull out the old sprigs or pins with a pair of pliers.

3. When the rebate is clear, dust out any debris.

4. When re-glazing metal frames, apply a coat of metal priming to the rebate before re-glazing.

5. Measure the rebate horizontally and vertically to find the size of the glass required. Check this measurement carefully.

Glazing a timber frame.

Prime the inside of the rebate and allow it to dry before glazing. Your putty should be soft and easy to work. Linseed oil can be added to standard glazing putty to make it malleable.

Once the putty is workable:

1. lay a 1/8 in. (3 mm) strip of bedding putty into the back of the rebate, using the thumb to push it firmly into place.

2. Position the glass in the bottom rebate at an angle, spacing it evenly on all sides, and press upwards into place. Press only along the sides; never from the centre. Continue to press firmly until all excess putty has squeezed out.

3. Leave a slight gap about 1/16 in. (or 2 mm) between the back of the putty and the rebate.

4. Remove excess putty with a putty knife.

5. Fix the glass into place with sprigs or panel pins placed at 6 in. (or 150 mm) intervals on the outside into the frame.

6. Apply an even strip of facing (or “weathering”) putty to the glass and rebate, using the putty knife at an angle; keep this moist so that it does not drag the material. Weathering putty should slope at an angle of 45 deg. to allow water to run away.

7. Trim off the surplus with the knife and mitre the corners.

8. Allow to dry out for two to three weeks and apply a coat of paint.

Wooden beading (square, splayed or quarter-round softwood) can be used instead of weathering putty, but the glass should still be set in a bedding mastic. The bead is set in glazing felt and fixed with ¾ in. – 1 in (19-25 mm) panel pins.

Metal frames. When re-glazing metal frames, the following rust-inhibiting precautions should be taken. Brush off any surface rust with a stiff brush, dust the rebate to remove any debris, apply a coat of rust inhibitor and then metal priming to the rebate.


There are numerous types of double-glazing systems suitable for fixing to timber or metal frames and all types of window and door. The main systems are: sealed units; the coupled sash; the applied sash, and the sliding sash.

Sealed units. The presealed factory-made unit, which can be supplied framed or ready to fit into your window frame, consists of a double sheet of glass, vaccum-sealed or filled with inert gas or dry air.

These units are twice as heavy as a single sheet of glass, so the window frames must be sound. A rebate of at least in. (13 mm) in depth, which allows a gap of about in. (or 5 mm) between sheets, is necessary if the unit is to be glazed into an existing timber or metal frame. Some units, however, are made with smaller gaps.

Fixing to existing window frames is best done with beading, which is tacked into position and then painted to match the rest of the framework. When fitting a sealed unit, use a non-hardening glazing mastic.

Although expensive, sealed units avoid the summer storage problems presented by detachable double-glazing frames. There are only two sides of glass to clean and none of the condensation or ventilation troubles that may occur with detachable frames.

Many manufacturers supply double-glazed units with a pre-cut hole for an extractor fan.

Coupled sashes. Supplied in kit form, applied and coupled-sash systems are popular forms of double-glazing.

A coupled sash, which consists of a removable glass panel fixed to the existing window frame, is simple to install. The sash unit should be -1 in. (1 .9 cm) less overall than the dimensions of the window opening and the glass in. (8 mm) less than the dimensions of the frame. Most coupled-sash systems employ four main channels, four corner hinges, locking nuts and screws and neoprene sealing strips.

To fix the sash:

1. Cut the glass to size and lay it on a flat surface.

2. Cut the main-frame channel strips to size.

3. Roughly position the four main channels and corner hinges, fitting the locking nuts and screws loosely.

4. Cut the neoprene sealing strips to size and mitre the corners.

5. Insert the strips into the channels.

6. Before positioning the glass, clean both surfaces. A sliding foot, fitted into the groove of the bottom channel, is provided as a window stay.

7. Tighten up the unit and offer it to the existing window frame to locate the position of the hinges.

8. Fix these and then hinge the double-glazed frame into place.

Coupled-sash sliding units normally consist of a head track, side member and sill track. The fixing of the head track decides the final position of the window. When determining the head track position, allow for the depth taken up by existing window furniture.

A true frame is necessary to ensure smooth sliding windows. Where the frame is not true, packing pieces can be used to achieve this.

Applied sash. Applied sash systems are of various types. One consists basically of a unit which is slotted together and screw-fixed directly to the window frame. The joint between the unit and the window surround is sealed with a neoprene sealing strip. Other systems consist of snap-fitted PVC extrusion which simply fits round the existing window surround or plastic channelling to hold the glass — which is then fitted to the frame with swivel clips.

Acetate sheets. A simple but effective way of double-glazing is to fix acetate sheets to the window surround with double-sided tape. Place the sheeting over the adhesive tape and ease out the wrinkles. Use a trimming knife, held against a straight edge, to make a neat edge.

A disadvantage of this system is that it needs renewing each year and limits ventilation.

Condensation. Hermetically sealed units present no problems of condensation but moisture may collect between two separate panes of glass.

One way of counteracting this is to place silica-gell crystals inside along the bottom of the unit. The crystals act as “blotting paper”, absorbing the moisture. When the crystals become saturated they should be removed and dried. This can be done in a kitchen oven at low heat.

Another method is to drill a series of small holes, upwards in the base of the frame, to provide ventilation. Moisture is drawn into the fibre of timbers even when these are painted. A strip of adhesive-backed aluminium foil can be used to seal the reveal between the glass to keep out the moisture.

Note: Whatever type of double-glazing is used, provision must be made for adequate, draught-free ventilation. A sealed atmosphere is unhealthy and could be dangerous

Warped frames. Before tackling any form of double-glazing. It is important to ensure that the frames are in good condition and fit properly. A badly warped frame will prove a source of heat loss and discomfort.

  • Wooden frames that are badly warped or rotted will have to be replaced. Small sections of rotted timber may be cut out and replaced with new timber correctly knotted and primed.
  • It may be possible to straighten a warped frame by removing it from the sur round and fixing it in cramps. You gradually tighten these to straighten the frame.
  • Badly rusted metal frames should be brushed down to remove rust particles. Treated with a rust-inhibitor and painted with metal priming.
  • Frames that are very badly distorted may have to be replaced.

Leave a Comment