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How To: Replace Broken Glass in Old Windows

By The Craftsman on Mar 25, 2013


Whether it’s an errant baseball or a tropical storm, windows are bound to break on occasion.

Before you call in a window specialist to make the repair, consider doing the job yourself. After all, if you have an old house, then most window specialists today won't even know how to fix your broken window glass.

What’s Different About Historic Windows?

Old wood windows (typically pre-1950s) were almost always putty glazed. (You can watch our video on how to putty glaze windows here.) Today, the average window contractor doesn’t know how to putty glaze and when confronted with a historic window, uses caulk or some other inappropriate product. When that happens, a host of problems can result. It’s always best to replace materials on a historic home with in-kind materials so that the repair blends seamlessly and doesn’t stick out like a band-aid. Here’s how to do it!

1. Remove the Broken Glass. Wearing gloves and glasses, carefully remove the remaining pieces of broken window glass and dispose of them safely.

2. Clean the Rabbet. Clean the old glazing putty from the rabbets on the outside of the window. Use a firm putty knife, being careful not to gouge the soft window wood as you scrape out the putty.

3. Remove Any Glazing Points. There may be some glazing points (little metal triangles or diamonds) remaining in the rabbets. Using needle nose pliers, pull them out and dispose of them too.

4. Wipe On Boiled Linseed Oil. Once the rabbets are clean of old putty and glazing points, wipe them down thoroughly with boiled linseed oil, available in the paint section of any hardware store.

5. Cut Your Replacement Glass. Measure the width of the opening for your replacement glass. You can either cut your own window glass or take the measurements to your local hardware store and have them cut it for you. Important: Be exact on these measurements! You have to cut the glass 1/16″ smaller than the opening, otherwise the glass may break as the wood swells in certain weather. Glazing rabbets are very small, so your measurements have to be precise. This is the hardest part. Just remember the old saying, “Measure twice, cut once.”

6. Glaze the Rabbet. Add glazing putty to the rabbet. This will create an airtight cushion for the glass to rest in. For repairs outside the shop, I recommend Sarco Dual Glaze putty. There are other acceptable types of glazing putty, but this one handles the elements better than others.

7. Bed Your Glass. Put your replacement window glass into place and press it gently but with firm pressure around the edges into the glazed rabbet. This will cause some putty to squeeze out over the inside of the glass (to be cleaned later).

8. Set the Glazing Points. Glazing points are what hold the window pane securely in place. Depending on the size of your replacement window glass, you may need more or less points. A good rule of thumb is to use one point for every 12″ of width and at least one point on each of the four sides. Bottom line, too many is better than too few.

9. Glaze the Window. Spread a fair amount of putty around the edges of the window and really push it into place. It doesn’t have to be pretty at this point.

10. Tool the Putty. Run your putty knife along the length of the window to smooth the putty at a bevel. Come back and make nice clean corners. You can mess with the putty as much as you want. If it doesn’t look good, keep tooling it until you are happy with the results. For a more in depth explanation of glazing, watch our video: How to Reglaze Old Windows.

Clean the Glass. Brush on some whiting to clean the window glass of oily fingerprints (this video will show you how), and you're done!

You’ll have to let the putty sit a few weeks until it “skins over” and is ready for paint. Don’t wait too long to paint, though. Without paint, glazing putty won’t survive long. It can mildew and fail in a few short months when not painted. But when painted it should last decades without much maintenance.

For more on working with old windows, visit our resource page How To Repair Old Windows.

Step-by-Step in Pictures

Step 1 Remove Glass

Step 2 Clean Rabbet

Step 3 Remove Points

Step 4 Apply Oil

Glazing Putty

Step 6 Glaze rabbet

Step 8 Set points

Step 9 Glaze window

Step 10 Tool Putty

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