Replace a broken window pane without butchering the sash

Before you pick up a single tool, crosshatch the broken pane with duct tape on both faces. Cracked glass holds together right up until it doesn't, and a falling shard from a second-story sash is a trip to urgent care. Tape it, then go get gloves.

The rest of this is a single-pane wood sash job, which is the one most people can actually do at their own bench. Glass is cut 1/8 inch smaller than the opening in both directions, single-strength stock runs about 3/32 inch thick, and a local glass shop will cut it while you wait. Everything else is patience.

Gear up like the glass is out to get you

Leather gloves, not garden gloves. Safety glasses, not your sunglasses. Long sleeves. Put a cardboard box next to you and a dustpan under the work, because the shards you can see aren't the problem. It's the splinters that end up in a sock two days later.

If the house was built before 1978, assume the paint on that sash has lead in it. That changes one thing specifically: don't take a heat gun to it on high. High heat vaporizes lead. Work with a low setting, keep a HEPA vacuum nearby, wet-scrape where you can, and look up the EPA's RRP guidance before you start sanding anything. Kids and pregnant women out of the room, full stop.

Thirty seconds that tells you if this is your job

Look at the edge of the glass. One thin sheet with putty holding it in? That's a single pane and you're in business.

Two sheets of glass with a metal or gray spacer bar between them is an insulated glass unit. You can't fix that at home. The seal is the whole product. Measure it, or better, pull the sash and take the whole thing to a glass shop, and they'll order a replacement unit. Expect a wait.

If there's a small etched stamp in one corner, the pane is tempered or laminated. Tempered glass can't be cut, so that one gets ordered to size too. Any pane in a door, next to a door, or low in a bathroom is almost certainly tempered by code, and it needs to stay that way.

Vinyl and aluminum windows usually hold glass with a snap-in bead instead of putty. Pry the bead out with a stiff putty knife, note which piece came from which side, and reverse the process. No putty involved. Different job, same caution about the glass.

Get the sash out and flat

You can re-glaze in place. It's miserable. Putty wants gravity on your side, and working overhead on a ladder with a knife in your hand is how people get hurt.

On older double-hung windows, pry off the interior stop molding with a stiff putty knife and a thin pry bar, working slowly so the molding survives. The lower sash lifts right out. Cut any sash cords and knot them so they don't disappear into the weight pocket, or unhook the chain.

Carry the sash to a bench, workmate, or a sheet of plywood on two sawhorses. Broken side up. Now you can see what you're doing.

Take the old glass out

With the tape still on, tap the glass from the inside face with a hammer handle and let the pieces come loose into your box. Work the middle first and the corners last. Corners hold on hardest and they're where the sharp bits like to fly.

Wiggle the larger pieces straight out of the putty rather than levering them. Then pull the glazing points. They're little diamond or triangular metal tabs pushed into the wood, and needle-nose pliers grab them fine. Count them as they come out so you know roughly how many to put back.

Strip the rabbet to bare wood

The rabbet is the L-shaped groove the glass sits in. Old glazing putty in there's hard as a dinner plate and it has to go, all of it. Leave ridges and the new glass won't sit flat, which means a stress point, which means a crack the first cold snap.

A heat gun on low plus a stiff 1-inch putty knife works. So does a glazing chisel. Work along the groove, not into it, and keep the knife shallow so you don't gouge the sash. Scrape the bed side too, where the putty was under the glass.

Finish with 80-grit paper and a vacuum. You want clean, dry, bare wood.

Prime it. This is the step that gets skipped.

Bare wood is thirsty, and glazing putty is basically whiting held together with linseed oil. Set putty against raw wood and the wood pulls the oil straight out of it. Six months later the putty is chalky, gray, and falling out in chunks, and you get to do the whole thing again.

Brush a thin coat of oil-based primer into the rabbet. Let it dry per the can, usually overnight. If you're using a modern acrylic glazing compound instead, follow that manufacturer's prep, but the principle is the same: seal the wood first.

While it dries, measure. Bare rabbet to bare rabbet, both directions, then subtract 1/8 inch from each. Write the numbers on a scrap of wood with "H" and "W" so you don't transpose them at the counter. Ask for single-strength for small panes, double-strength for anything over roughly 3 square feet, and ask them to seam the edges.

Bed, set, point

Knead a ball of putty in your hands until it's warm and behaves like modeling clay. Cold putty from a garage in February will fight you the entire afternoon.

Roll a pencil-thick rope and press it into the rabbet all the way around. That's the bed. Set the glass in, press it down with your fingertips spread near the edges, and squeeze the bed until you see a thin even line of putty under the glass everywhere. Never press on the middle of a pane.

Push glazing points in with the edge of a putty knife, flat against the glass, about every 6 to 8 inches and at least two per side. They're what actually holds the glass. The putty is weather sealing, not structure.

The bevel, and then leave it alone

Roll another rope and press it into the outside corner where glass meets wood. Now hold your putty knife at a shallow angle, one edge riding the wood, the other on the glass, and pull it along in one continuous stroke. Firm pressure. Don't stop mid-run and don't fuss at it.

You're aiming for a clean 45-degree bevel that sits just below the sight line of the interior wood, so it's invisible from inside. If a run comes out lumpy, scrape it off and roll a fresh rope. Wiping a dry brush lightly over the bevel smooths small ripples.

Trim the squeeze-out on the inside face with the knife, clean the glass with a rag, and put it somewhere out of the weather for a week or two. Oil putty needs to skin over before paint. When it's firm to a fingernail, paint it and lap the paint about 1/16 inch onto the glass. That little overlap is the water seal. Miss it and rain runs straight into the joint.

Now the honest part: if you've got eight broken panes in a stripped storm window, a shop will re-glaze the whole sash for less than your Saturdays are worth. Do one pane yourself first. Then decide.