How to Fix a Cracked Window: Temporary Seals, DIY Glass Replacement, and Glazing

A cracked window has a way of spreading faster than you’d expect—especially during temperature swings when the glass expands and contracts. What starts as a hairline crack in the corner can spider across the entire pane within a week. Beyond the aesthetics, a cracked window lets in cold air, rain, insects, and represents a real security vulnerability. The fix, especially for single-pane windows, is well within DIY range and typically costs under $30 in materials.

This guide covers every stage of window crack repair: stopping the crack from spreading right now, applying a temporary seal, removing and replacing the glass in a wood-frame window, and applying glazing compound the right way so it lasts.

What You Need

Before you begin any repair, gather these tools and materials:

• Window glass repair resin kit — UV-curing resin that seals cracks nearly invisibly
• Glazing compound for windows — for sealing glass into wood-frame sashes
• Glass cutter tool — for scoring replacement glass to size
• Glass suction cup handles — for safely handling cut panes
• Safety glasses and heavy leather gloves — glass cuts are serious; never skip PPE
• Linseed oil putty knife set — for applying and shaping glazing compound

You’ll also need clear packing tape for temporary sealing, a glass drill bit if stopping crack propagation, a stiff brush for cleaning the rabbet, boiled linseed oil for priming bare wood before glazing, and exterior-grade latex primer for painting over cured glazing compound.

Step 1: Stop the Crack from Spreading (Right Now)

Before anything else, slow the crack down. Temperature changes and vibration will drive it further if you don’t act.

Apply temporary tape seal: Press a strip of clear packing tape firmly over the crack on both the interior and exterior surface of the glass. Smooth out all air bubbles. This isn’t a repair—it’s a holding action that buys you days or weeks while you gather materials.

Drill stop holes (for longer cracks): If the crack is more than 3 inches long, drill a 1/16-inch hole at each endpoint using a glass drill bit. Keep the drill at very low speed and use light pressure—you’re not trying to go through; you want to blunt the sharp stress point at the crack tip. Mark the endpoints with a felt-tip pen first so you can see exactly where to drill. These holes prevent the crack from propagating further under thermal stress.

Don’t use the window: Avoid opening or closing a cracked window if possible. The flexing of the sash as it moves can shatter the pane entirely.

Step 2: Apply Window Repair Resin (For Cosmetic or Structural Cracks)

If the crack is a single clean line with no missing glass—common from impact or thermal stress—a UV-curing window repair resin can produce an almost invisible repair. This works best on small to medium cracks (under 12 inches).

Step 1: Clean the crack. Use rubbing alcohol on a cotton swab to clean inside the crack. Any grease, wax, or cleaning product residue will prevent the resin from bonding. Let it dry completely.

Step 2: Apply the resin. Most kits use a small syringe. Hold the glass horizontally if possible (tip the sash or lay it flat on sawhorses). Draw a thin bead of resin along the crack, letting capillary action pull it into the crack. Don’t overfill—a small amount goes a long way.

Step 3: Cure with UV light. Place the included UV curing strip or use direct sunlight. Most resins cure in 2–5 minutes in direct sun or 10 minutes under a UV lamp. The resin will harden clear.

Step 4: Remove excess. Use a fresh razor blade held at 45 degrees to scrape off any resin that cured on the surface of the glass. Work carefully and the crack will be nearly invisible.

This repair is structural enough to prevent the crack from widening, but it won’t restore the full strength of uncracked glass. For panes under significant stress—large sizes, storm-exposed windows—full replacement is still the right call.

Step 3: Remove the Broken Glass

Whether you’re doing a cosmetic repair that failed or diving straight into replacement, this is how to safely remove the old glass from a wood-frame window.

Step 1: Protect yourself. Put on safety glasses and heavy leather gloves. Lay a drop cloth below the window to catch fragments.

Step 2: Score and remove large pieces. If the glass is in large intact pieces, use a glass suction cup to grip each piece firmly. Score around the edge with a glass cutter if needed to break it into manageable sections, then gently work each piece free.

Step 3: Remove all glazing and glazier’s points. Old glazing compound will be dry and crumbly. Use a putty knife and heat gun (on low) to soften it section by section, then scrape it away. Be patient—rushing tears into the wood. Behind the glazing you’ll find small metal triangles called glazier’s points; pry these out with the putty knife or a flathead.

Step 4: Remove all remaining glass fragments. Tap small shards free with the putty knife. Run the putty knife around the entire rabbet (the recessed groove the glass sits in) to make sure no shards remain embedded.

Step 5: Clean and prime the rabbet. Brush out all debris with a stiff brush. If the bare wood is exposed and dry, apply a thin coat of boiled linseed oil and let it soak in for 30 minutes. This prevents the dry wood from pulling oil out of the new glazing compound prematurely, which causes glazing to harden too fast and crack again.

Step 4: Measure and Cut Replacement Glass

Accurate measurement is the most important step in glass replacement. A piece cut even 1/4 inch too large won’t fit; cut 1/4 inch too small and it’ll rattle.

Measure the opening: With the old glass and glazing removed, measure the width and height of the rabbet opening at multiple points—top, middle, and bottom for width; left, center, and right for height. Wood frames are rarely perfectly square. Use the smallest measurement in each direction.

Subtract 1/8 inch total from each dimension: This gives you a 1/16-inch expansion gap on each side, making installation manageable and allowing for movement.

Write it down as: Width x Height, in that order.

Order or cut the glass: Most hardware stores and glass suppliers will cut single-strength (3/32-inch) glass to size for a few dollars. For windows up to 15 square feet, single-strength is appropriate. For larger panes, use double-strength (1/8-inch). If you’re cutting yourself, lay the glass on a flat padded surface, mark your cut line with a felt-tip, score once with firm even pressure using the glass cutter, and snap along the score line by applying downward pressure on both sides of the line simultaneously.

Step 5: Install the New Glass

Step 1: Bed the glass in glazing compound. Roll a rope of glazing compound about 1/4 inch in diameter and press it into the rabbet all the way around the perimeter. This “bedding compound” cushions the glass and prevents rattling.

Step 2: Set the glass. Using suction cup handles, lower the glass into the opening and press firmly so it seats into the bedding compound. Center it with equal gaps on all sides.

Step 3: Install glazier’s points. Press glazier’s points flat-side down against the glass and into the wood every 6–8 inches around the perimeter using the tip of the putty knife or a glazier’s point driver. The points hold the glass mechanically while the glazing cures.

Step 4: Apply the face glazing compound. Roll another rope of compound and press it into the joint between the glass and the wood frame at a 45-degree angle. Use a wet putty knife to smooth it into a clean bevel. The compound should contact the glass and the wood frame with no gaps. Keep a cup of water nearby and dip the putty knife frequently for a smooth finish.

Step 5: Let it cure. Oil-based glazing compound takes 7–14 days to skin over before painting. Don’t try to rush it. Once it’s firm to the touch and doesn’t dent when pressed, it’s ready for primer.

Step 6: Paint and Finish

Unpainted glazing compound will crack and fail within a season. Paint is what seals it and extends its life for decades.

Step 1: Apply one coat of exterior-grade latex primer over the glazing compound. Let the primer lap about 1/8 inch onto the glass surface—this seals the glass-to-glazing joint against water infiltration.

Step 2: Apply one or two finish coats of exterior paint to match the window frame. Again, lap onto the glass by 1/8 inch.

Step 3: Once the paint is dry, use a razor blade to trim the paint edge on the glass to a clean line if desired.

When to Call a Professional

Some window repairs are genuinely beyond DIY scope:

• Double-pane (insulated) glass: Requires a custom-ordered insulated glass unit. The sash can often be removed and taken to a glass shop, but the job involves specific sealants and techniques.
• Tempered glass: Can’t be cut—shatters into fragments intentionally on breakage. Must be ordered to size from a glass supplier. Required in door lights, sidelights, windows near floors, and most bathroom windows.
• Laminated glass: Required in some skylights and specialty applications. Replacement requires professional tools.
• Large panes over 25 square feet: The sheer weight and awkwardness of handling large glass panels makes single-person DIY dangerous.
• Leaded or art glass: Requires a specialty glazier.

For everything else—the cracked bedroom window, the storm window that took a baseball, the old double-hung that’s been taped up for six months—you now have everything you need to handle it yourself.

Related Reading

• How to Weatherstrip a Window for Winter
• Home Maintenance Checklist for Every Season
• How to Fix a Cracked Window Screen