How to Replace Broken Glass in a Single-Pane Window
All it takes is an errant baseball, a pebble thrown by a lawnmower blade, or a misguided bird to break the glass in a window. Occasional glass breakage is a normal part of owning a home. If the window uses insulated "thermo-pane" glass consisting of two glass panels around an empty space, repairing it requires a repair call from a glazier or a visit from a window company to replace the entire sash. You can determine if you have multiple-pane windows by shining a light on the window from the inside while it is dark outdoors. If the window has multiple panes, the light reflection will be repeated. For example, on double-pane windows, two reflections will show up, one next to the other.
But if you have older wood-framed windows with single-pane glass, it is relatively easy to fix broken glass yourself. You just need a few simple tools and materials, some of which you may already have on hand. Other materials are easily obtainable at home centers and hardware stores.
Remember that working with glass is always inherently hazardous. Be sure to always wear safety glasses and thick gloves.
Anatomy of a Single-Pane Window
Before starting this project, it is important to understand how a single-pane window pane is fastened into the wood frame. If you have ever worked with glass photo frames that have bendable metal stays in the back, the construction of a single-pane window will be familiar. Instead of metal stays, the single-pane window uses metal triangles called glazing points. These tiny metal triangles are essentially the "nails" that hold the glass to the wood frame.
These points are then covered up with a bead of glazing compound that hardens into a solid molded wedge that seals the glass and hides the glazing points. When painted to match the window, this system is virtually invisible.
Note: Some types of windows will be held in place with a wood or vinyl bead molding held in place by small nails. If this is the case, the molding can pried loose and reused or replaced with new. A small bead of caulking can be used around the edges of the new glass before nailing on the bead. Slightly set the nails in the molding with a hammer and use a nail set to finish driving them in so as not to impact and break the new glass with the hammer head. A power brad nailer would work well if one is available.
Replacing a broken pane of glass is a process of removing this hardened glazing and the glazing points, removing the shards of glass, then installing a new pane of glass and new glazing points and glazing.
What You'll Need
Equipment / Tools
- Leather work gloves
- Eye protection
- Putty knife
- Wood chisel or razor scraper
- Tape measure
- Pencil (optional)
- Metal straightedge (optional)
- Glass cutter (optional)
- Rubber mallet
- Fine-grit sandpaper
- Linseed oil or clear wood sealer
- Replacement glass cut to size
- Glazing points
- Glazing compound
- Exterior-grade paint
Remove the Broken Glass and Clean the Frame
Using a putty knife, pry up the hardened glazing compound from around the glass. It should come off in large chunks, as it is usually quite dry and brittle. Where bits of compound are stuck to the wood, scrape them away.
Use a putty knife, pliers, or a thin flat-head screwdriver to pry out the old glazing points. These tiny fasteners pin the glass into the frame's recess, with sharp points that are embedded into the wood. Wearing eye protection and with thick gloves, remove all of the broken glass.
Using a chisel or razor scraper, scrape down the L-shaped channel around the frame where you removed the glazing compound and glazing points. Be careful not to gouge the wood. Sand the wood smooth with fine-grit sandpaper, then seal any bare wood with linseed oil or clear wood sealer applied with a paintbrush. Let the sealer dry completely.
Measure the Frame and Buy the Glass
Measure the width and height of the window opening, measuring to the outside edges of the L-channels. Subtract 1/8-inch from each measurement to use as the glass size. Slightly undersizing the pane makes it easier to install and provides room for seasonal expansion and contraction.
Take these measurements to the hardware store or home center and have a piece of glass cut to this size.
Alternate Method: Cut Your Own Glass
Alternatively, you can cut a piece of glass yourself if you happen to have a large sheet of stock glass.
Place the glass on a clean, flat work surface. Make marks for the cutting line with a permanent marker. Place a metal straightedge on the mark and score the cutting line onto the glass using a glass cutter. A glass cutter has a small metal wheel for scoring the glass. Run the cutter along the straightedge while pressing the wheel down firmly. Make only one pass with the cutter. Be sure to wear safety glasses and gloves for the next step.
Slide the glass along your work surface so that the scribed line on the glass is aligned with the edge of the surface. Holding the main portion of the glass with one hand, use the other hand to push down sharply on the waste portion of the glass to snap it along the scored line. Be careful when handling the cut glass, as the edges will be quite sharp.
Mount the Glass
Roll out glazing compound into long, very thin ropes, about 1/8-inch in diameter. Push the ropes into the L-channels around the perimeter of the window frame where the glass will rest. Gently press down around the perimeter of the glass with the tip of a putty knife, slightly compressing the glazing compound and bedding the glass.
Press two glazing points into the bottom of the frame against the glass, forcing them into the wood with the tip of the putty knife. If you need extra force, gently tap the handle of the putty knife with a rubber mallet to drive in the points. Install additional glazier's points, two per side, for a total of eight glazing points per window pane.
On the backside of the window, use a putty knife to scrape off any glazing compound that seeped out.
Glaze the Window
Roll more glazing compound between your hands, forming ropes about 1/2-inch thick. Apply these ropes of compound to the L-shaped space where the glass meets the window frame, over the glazing points. Use your finger to press the compound against the glass and wood.
Using a putty knife held at a 45-degree angle, run the blade of the knife along the bead of glazing compound, forming a flat, angled wedge along the joint between the glass and the window frame. If you notice gaps in the compound, apply a little more and smooth the joint with another pass of the putty knife.
When you've achieved a perfect wedge-shaped seal with the glazing compound, carefully remove any small pieces of compound on the glass, using the tip of the putty knife. Be careful not to touch the finished bead of glazing compound.
Glazing a window can be tricky, but it can be mastered with a little practice. The goal is glass that is sealed in place with a perfect wedge of compound around all four sides. The compound will stay soft for a fairly long time, so if you mess up the job, just scrape out the glazing compound out and start over.
Let the Glazing Cure, Then Paint
Let the glazing compound dry and harden, following the manufacturer's recommended drying time. This can be as much as 5 to 7 days.
Once it is dry and fully hard, paint the glazing and any exposed wood with exterior-grade paint. Professionals like to overlap the paint onto the glass by the barest of margins—perhaps 1/16 of an inch—to ensure the best possible weather seal.
After the paint has fully dried, clean the glass.
Glaziers, Occupational Outlook Handbook. United States Bureau of Labor Statistics.