How to Replace Broken Window Glass

Ron had an old window at his home that was in relatively good shape, but in great need of reglazing, so he took advantage of the opportunity to show us how it is done.

Remove old window glazing
The first step is to remove the old glazing material from the windows, and there are several ways to do that.

One method is to use a very stiff putty knife to break loose the glaze where it contacts the wood. It helps if you use a file to create a very sharp edge on the end of the putty knife. Heat from a heat gun in combination with a putty knife can work well because the heat can actually soften the glazing even if it has become hardened from age. Be careful not to overheat the glass or it can crack.

A second option is called the putty chaser. It's made by Prozi and its sole purpose is to remove old glazing compound from windows. It attaches to a drill and has a flat blade almost like a blunt screwdriver. As it spins, it knocks away the old material.

Ron's original intention was to demonstrate only reglazing, but in the process, he inadvertently cracked a pane of glass and needed to replace it.

Replace glass
Begin by putting duct tape on both sides of the window pane to keep it from shattering when you try to remove it.

Whenever you are working with broken glass panes, always protect your hands with leather or Kevlar gloves, and protect your face with a plastic shield.

With the masking tape in place, tap the glass with the end of a putty knife -- just enough to make it shatter. Most of the broken pane should lift out in one piece.

Treat exposed wood
After all of the old glazing is removed, there is sometimes a lot of exposed wood remaining. You will want to paint this exposed wood with an oil based primer for two reasons. First, the wood in older windows is often very dry and you don't want it to draw the oil out of the glazing prematurely, causing the glazing to crack and shrink. Secondly, anything that's exposed needs to be weatherproofed and sealed, which the primer will do nicely. Oil based paint needs to dry overnight before putting the new glass in and replacing the glazing.

If you prefer, you can take measurements for the replacement glass and have your local hardware store or glass company cut the replacement pane for you. But some do-it- yourselfers prefer to cut their own glass. It is really not difficult to do.

Cut glass
The first step, which is more important than you might think, is to clean the surface of the glass. If it is not clean, it won't score properly and just won't cut very well.

Begin by marking your measurements onto the glass. If you're using a metal straight edge, it can slip on glass. To prevent this, put a piece of masking tape on the back.

After you have drawn cutting lines directly onto the glass, you are ready to score the glass. Hold the glass cutter with the teeth or notches facing downward and your first finger resting on the back of the cutter.

Dip the cutter in oil, which acts as a lubricant, then tip the cutter up at an angle, get a good grip on the straight edge and then make just one scoring cut.

It is important to break the glass as soon as possible after you score it. To make sure you get a straight cut, put a piece of dowel under the score and then press down on two sides. Repeat the process to cut the glass to length.

You will need to apply a bedding coat of glazing putty to act as a cushion for the glass and to seal the glass pane into its frame.

Window glazing
Putty is easier to work with if you knead it a little first and then form it into a rope shape.

Lay the putty into place with your fingers and then use a flexible putty knife to press it firmly onto the lip of the window frame.

Set the glass pane in the bed of putty and press around the edges to make sure it gets a good seal.

Next, attach the glazier's points, which are small metal clips that hold the glass in place while the putty is drying. Embed the glazier's points into the wooden frame using a putty knife.

With the glass in place, apply more glazing putty to the outside of the window. This will form a weather tight seal to keep out water and drafts. Dipping the putty knife in paint thinner will allow it to glide more smoothly over the putty, leaving the finished surface neat and level. The excess putty lifts off easily.

Glazing in a tube
In addition to the classic putty, you can also purchase window glazing in a caulking tube. The caulking tube nozzle is designed to lay a bead that is the perfect width for the job.

The product is water based so you can dip the putty knife into water instead of paint thinner. The caulk-like material is a bit thinner and stickier than glazing putty so you might be better off waiting until it's partially set before trying to remove the excess.

There are pros and cons to both types of putty. On one hand, the old fashioned kind cleans up a little more easily, but the caulking tube variety is very easy to apply. But the truth is, both are quite easy to work with, and you may be surprised how easy it is to replace your own glass and glazing, if you ever need to.