Replacing Broken Glass in a Steel Casement Window

Requires Specialized Tools and Processes

Replacing a broken pane of glass on a steel casement window isn’t like repairing wood sash. It takes different tools, special materials, and a fine-tuned process. But once the former are assembled, the job is easy enough for anyone with a little DIY experience and a willing attitude.

Step 1: Assemble your materials, starting with the glass. Unless your glass is ordinary 1⁄8″ double-strength sheet glass, purchase 3⁄16″ or 1⁄4″ plate glass at a glass supplier to match the existing. (The new pane of glass should be about 1⁄8″ narrower than the opening’s height and width.) Steel sash should be glazed with putty specifically made for metal windows, such as DAP 1012. (Wood window putty won’t last.) You’ll also need an awl and hammer, metal sash clips, duct tape, a wire brush, needle-nose pliers, a small window suction cup, and whiting.

Step 2: Wearing eye protection and a respirator and using basic lead-safe work practices, begin removing the old putty. Because old putty can be hard and may contain lead, Portland cement, or asbestos powder, use an awl and hammer, not heat, to loosen it. Look for areas where the putty is loose or missing, insert the awl point between the putty and steel, and gently tap the awl with your hammer; the force needed will depend on the condition of the putty and its composition. If the putty is intact, start from the middle.

 Step 3: Once the putty is removed and the edges of the glass are exposed, look for two metal spring clips on each vertical side of the opening; pull them out with the needle-nose pliers. These clips are fragile and easy to lose; if you can hold onto them and they remain intact, you can reuse them. Otherwise, purchase new ones at a glass company. Next, with the ground and floor protected to catch glass shards, old putty, and dust, cover both sides of the glass with wide masking or duct tape to contain breakage. If the pane isn’t badly cracked, pull it out using a small window suction cup, or by tapping gently from the inside out. With the glass out, finish removing the old putty. If the sash rebate is rusty, use a wire brush or a drill and wire brush attachment to remove as much rust as possible. Brush or vacuum the dust away, and prime the cleaned area with a coat of oil-based rusty-metal primer.

Step 4: Since DAP 1012 is usually oily on top of the can and dense at the bottom, remove all of the material and knead it into a pliable ball, using whiting if necessary to firm it to an appropriate consistency. (If the putty is dry on the top of the can, return it for fresh material.) The new pane of glass can’t touch the metal frame in any direction, so back-bed a thin layer of putty in the frame to seat the glass, protect it from thermal movement, and create a weather seal. Install the back-bed with your fingers or a putty knife, pushing the putty into the frame (warm putty is more pliable). Next, insert the pane of glass, and gently push it into the putty until it is well-seated. Using needle-nose pliers, install the metal sash clips on each side to secure the pane.

Step 5: Before glazing, remove excess back-bedding material from the interior side with your putty knife. Next, glaze the exterior—I like to push the putty into the frame and glass tightly with the putty knife, but the common method of rolling putty into a snake and then inserting it also works. Take care to respect the interior sight lines of the sash—putty and paint on the outside shouldn’t be visible from the inside. Tool the putty, pushing it tightly to the frame and glass while cutting the edge away at the sight line. To finish, sprinkle a little whiting or plaster dust on a clean cotton rag, and carefully wipe away all the smears. After the putty dries, paint with two coats of oil-based paint.