Window Glass Replacement

Windows are as much a part of the wall as brickwork, and a broken window must be one of commonest of household accidents. Fortunately it presents no difficulties in repair. The only difficulty lies in cutting the glass to size. Many people find this operation tricky. Mostly, though, the difficulty lies in having a bad cutting tool. If you can get hold of a new glass cutter and use it as we show in our illustrations, you should have no difficulty.

Window glass is not all the same. There are a number of different thicknesses. Also, one sort called plate glass is made a special way to give a perfectly flat surface, ideal for shops or very large picture windows. Ordinary glass tends to have slight irregularities which are detectable as ripplings when one looks through. These minor defects are unimportant for most house windows.

Glass is sold, strangely enough, by weight. There is a 24 oz glass and a 32 oz. glass, the heavier one naturally being thickest. There is no hard and fast rule as to which one to choose but certainly the larger the window the heavier the glass should be. Also, windows inside the home, as in glass doorways, should always be of the heavy kind. They are more likely to be subject to shocks.

Plate glass is very expensive and the fitting of it can be more difficult, largely due to the size and weight of the pieces.

Therefore, in general, we do not recommend that you try to replace a plate glass window. For the ordinary sort though, there should be no qualms at all.

The procedure is very simple. The old glass is removed completely, the old putty scraped away, the exposed wood painted and a fresh bed of putty laid for the new sheet of glass. This is then pressed into the putty and held to the frame by small steel tacks. Finally, a neat covering of putty is applied to seal the edges.

Metal window frames require a special kind of putty and, of course, no tacks. Otherwise the procedure is much the same.

Glass cutting may be done by the supplier. This is certainly easier, provided you have made very accurate measurements of the size required. The sheet should fit into the frame with in. to spare. You cannot measure to this accuracy with, say, a flexible dressmakers tape. A short foot rule is also difficult because of the movements you must make with it as you go along the length of the frame. The best measuring device is certainly a steel flexible tape.

Incidentally, it is a good idea to ask the glass supplier to nibble off the corners of the sheet with his glass cutter, because it is strain at these extreme points that often causes cracking as the glass is being fitted. A very slight rounding of the corners greatly reduces this risk.

In addition to plain glass there are many kinds of decorative glass. These are nearly all cut and shaped in exactly the same way as plain glass, even those which contain a reinforcing wire (though you are not likely to need these very often in the home).

One often neglected part of window repair is the preparation of the frame itself. This must be well scraped and cleaned, and preferably also sealed with aluminium paint. After fitting, the sealing putty should always be painted or it will crack and allow water to seep into the frames.

One job on windows that may sometimes be required is a hole cut in a pane in order to fit, say, an extractor fan in the kitchen. You can have this done professionally by removing the glass and taking it to a glazier. Alternatively you may find a worker who is willing to come and cut the glass in your window without removal. In either case you will probably be surprised at the cost.

Not that you will succeed every time. Indeed, professional glaziers themselves have difficulties. But if you follow the instructions given with patience then we are confident that you will be able to do the job successfully. Do not, however, try to do this on a fixed window. It is far better to remove the pane, or to get a new one, and cut the glass on a table.

One particular sort of house window requires very great care in its repair. This is any window above the ground floor.

Nearly all windows must be attended to from the outside and therefore work upstairs must be done on a ladder or scaffolding. If you are unused to heights and also new to handling glass, the chances of an accident are considerable. The glass may be dropped and broken or you may yourself have a nasty fall in trying to save it.


(a) The key to successful work is to have a good tool, which has been well cared for. The ordinary 6 wheel cutters are perfectly satisfactory but often they are kept in situations that attract fine rust. It is very important that they be very lightly greased after use, using vaseline or something similar and preferably then wrapped in waxed paper. This will prevent rust and ensure that the tool is always in good condition when you need it.

It takes a little practice to make a clean scratch. Press downwards on the tool with one of the small wheels rolling against the glass surface. Then, maintaining the pressure, draw your hand back using a straight edge as a guide. Since, this type of tool cuts by pressure, not by speed, you must maintain a steady even pressure all the time you move. You can quite easily feel when the cutter is cutting well. There is a ‘biting’ feeling. A little practice on some waste glass will soon enable you to cut a straight, even line.

You may find that you have missed part of the scratch by failing to maintain the pressure evenly or by allowing the angle of the tool to vary so that the little roller is not pressed against the glass. These gaps will probably prevent the glass snapping cleanly. Unfortunately it is extremely difficult, though not impossible, to fill these gaps.

Any fresh cut you make must be in exactly the same line as the rest. By very careful work you may be able to do this and the cut will be successful. It is far and away better though, to make a clean, even cut the first time.

(b) Next you must snap the glass apart. When a small piece is being cut from the end of a sheet of glass you can flex it between your fingers. Hold your thumbs at each side of the scratch and bend it firmly. The crack that starts at the end of the scratch will flash along its whole length without any difficulty. If you try to bend it in the middle of its length the cut may turn out irregular.

With a large sheet of glass you can place the scratch immediately over the edge of a table and then press firmly down, on its outer edge. This will usually snap away the surplus glass quite cleanly.

In all cases glass cutting is best done on a surface that is firm but not hard. A double layer of blanket on a smooth table is ideal. The small slots at the edge of the glass cutter may be used to chew away any surplus glass where the cut has become irregular and, as we noted above, it pays to take off the extreme corners of the pane, before fitting it.

Refitting the glass to a window

1 First of all remove all old broken glass and putty. The latter may be very hard. To shift it, lay an old chisel flat against it and tap sideways with a hammer. This will force the old putty free 2 Any remaining traces can be scraped away with a broken table knife or some similar tool For best work give the exposed wood a coat of aluminium paint.

3 Next make a putty bed for the glass by spreading softened putty into the rebate of the frame, pressing it well down into the corner.

4 Press the glass firmly into this bed till the putty squeezes up on the inside of the window frame. This gives a solid base, trimmed off neatly inside with the knife.

5 Finally, tap fine nails into the frame above the glass to hold it in position and cover them with a smooth fillet of putty. This must be well painted after about a week, when the putty will have hardened.

Cutting a circle from glass

This is one of the more difficult of glass cutting operations but with care and patience you can certainly succeed with it. You may break one or two pieces of glass in practising so it is best to try the job out on old glass before tackling a large important window.

1 You can mark the circle you require with the glass cutter drawn about some circular object such as a saucer of the right size. Glaziers have special suction-held tools which make this curve but you are not likely to have one of these about. Then, with the circle marked, criss-cross the inside of it with closely spaced scratches.

2 The scratches must not stretch outside the original circle. Both sides of the glass are similarly marked as you see here.

3 Take a reasonably sharp-sided metal tool and very gently tap the glass around the circle. After a few seconds a very fine crack will appear as a white line around the circle. Follow this around till the circle is completely cracked. It will not, however, drop away, although the glass is now completely separated. It is still too closely fitting to come free.

4 Next make a ring of putty or plasticine, rather smaller than the circle. Press this to the back of the glass.

5 Turn the glass over and very gently tap the cross hatching in the centre of this putty ring. The putty will prevent the cracks from spreading too far or too fast. Very gradually the cross hatching will shatter apart until finally, tiny squares of glass are forced away and the hole appears in the centre of the ring.

6 Now you can use the slots at the edge of your glass cutter to gently prise away the glass to the edges of the ring, not breaking too much at a time.

7 This will expose a perfect ring in your glass without further trouble. You will see that this is an operation much easier to carry out on a table top than in the window itself !

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