A cracked pane in a double hung window looks worse than it is. If the crack is stable and the glass is still sitting in the frame, you have time to do this properly rather than rush it. The job is straightforward for anyone comfortable with a screwdriver and a putty knife, and the modern no putty method means the window is paintable in days instead of the weeks old fashioned glazing putty needs to skin over.
Here is how to take out a cracked pane safely and set a new one in a timber sash window.
Make it safe first
Cracked glass moves in ways you do not expect, so treat it as fragile from the start. Put on a pair of proper glass handling gloves before you touch anything. Ordinary gardening gloves will not stop a shard, which is why we use dedicated Cut 5 glass handling gloves for this kind of work.
Run strips of masking tape or packing tape across the cracked pane in a criss cross pattern on both sides. This holds the pieces together if the glass gives way while you are working it loose, so nothing drops onto the sill or the floor. Tape right up to the timber on each side.
Two more things worth doing before you start. Lay an old towel or a sheet of cardboard along the sill to catch any small fragments, and think about the paint on the timber. Old sash windows can carry many layers of paint on the glazing rebate, and some of the oldest layers may contain lead. Do not sand or grind it. Keep the timber and any scrapings damp, wear your gloves, and wash your hands well when you finish so nothing transfers to food or family.
Get the old glass and glazing out
The glass is held in by two things: the bed it sits on at the back of the rebate, and the glazing on the front face that you can see. On an old window the front glazing is usually hard linseed putty, sometimes reinforced with small glazing sprigs or pins pushed into the timber.
Work the old glazing away with a putty knife or a stiff scraper. Start at a corner and lever gently along the rebate. Hard old putty often comes away in chips rather than ribbons, so be patient and keep your fingers clear of the glass edge. As you expose the sprigs, pull them out with pincers or long nose pliers.
Once the front glazing is off and the sprigs are out, the pane should lift away. Work from the taped glass, not the loose edges. Ease it toward you and lift it clear in one movement, keeping it flat so it cannot flex and snap. Set it down somewhere safe straight away, taped side up.
Now clean the rebate right back to bare timber. Scrape out every trace of old bedding putty and any flaking paint, then run a chisel or scraper along the corner so the new glass will sit flat. A clean, sound rebate is what makes the new pane sit true and seal properly.
Measure for the new pane
Measure the opening between the timber, height and width, at more than one point, because old frames are rarely perfectly square. Take the smallest of your measurements in each direction so the glass is not fighting the frame.
Then take a couple of millimetres off each dimension. Glass that is cut exactly to the opening will bind, and a pane under stress cracks the first cold night when the timber moves. A small gap all the way round is what you want, and it gets filled and sealed anyway. Write the finished sizes down as width by height and take them to a local glazier, who will cut the pane to order. Ask for the same thickness as the glass you took out unless you have a reason to change it.
While the glass is being cut, this is the moment to sort out the setting blocks. Small plastic setting blocks sit under the bottom edge of the pane so its weight rests on them rather than on the timber. Two along the bottom rail is the usual arrangement. They keep the glass level, hold the even gap around the edge, and stop the pane sliding down over time.
Bed and seal the new glass, the no putty method
Traditional linseed putty still works, but it takes weeks to cure hard enough to paint, and paint is what actually protects the glazing over the long run. A modern glazing compound skins fast and paints in days, which is why we moved to it for our own reglazing. If you want the full reasoning, we set it out in our test of hybrid polymer against linseed putty.
The method is simple. Run a bead of glazing compound around the back of the rebate to make the bed the glass will press into. Place your setting blocks along the bottom, then lower the new pane onto the bed and press it gently and evenly against the compound until it beds down with a thin, continuous layer squeezed out behind it. Do not force one corner harder than another.
Secure the pane with fresh glazing sprigs or glazing points, tapped into the timber against the face of the glass every so often around the frame. Slide the tool flat along the glass so you are not pushing toward it. With the pane fixed, run the front glazing bead around the outside, filling the rebate to the timber line and smoothing it off at a neat angle with a wet knife or a shaped tool so water runs off rather than pooling.
Clean the squeezed out compound off both faces of the glass straight away. Leave it to skin for the time the product states, then prime and paint the front glazing, carrying the paint two millimetres or so onto the glass to seal the joint against water. A Reglaze Window Bundle puts the compound, the tools and the setting blocks together so you are not chasing separate parts. For the finer points of the seal and why the paint line matters, see our guide to reglazing a window without putty.
Frequently asked questions
Can I replace a cracked pane without taking the sash out of the window?
Yes, in most cases you can do the whole job with the sash in place, working from inside or outside depending on which face the glazing is on. Taking the sash out gives you a flat bench to work on and better access, so it is worth doing if the sash lifts out easily, but it is not essential for a single pane.
Do I need to cut the glass myself?
No. Measure the opening, take a couple of millimetres off each dimension for clearance, and have a local glazier cut the pane to your finished sizes. Cutting glass by hand takes practice and the offcuts are sharp, so buying it cut to size is the safer and easier path.
How long before I can paint the new glazing?
A modern glazing compound skins and is ready to paint in days rather than the weeks a linseed putty needs. Follow the time on the product you use, then prime and paint, taking the paint line slightly onto the glass to seal the edge.
Is old window glazing dangerous to remove?
The main risks are the glass edges and the paint. Wear glass handling gloves and tape the cracked pane before you start. Assume old paint layers may contain lead, so do not sand or grind them, keep everything damp, and wash your hands thoroughly when you finish.