Glass Block Window Installation Tips

Glass block windows are a fun project to tackle. Minimal tools are needed. If all goes well even a "newbie" DIY'r can install four glass-block windows in a day. Here is what I have learned over the years about installing these neat windows.

Measure Twice and Draw

Make a simple sketch of each window opening you intend to transform to glass block. Measure the narrowest point side to side and up and down to the masonry or where you "think" the masonry will be once you remove the frame. If you are in doubt, take a photo of different windows. You will need to show these to the company that will make your glass block panels. Check your measurements. Go to the fabricator and ask if everything is in order. Show him/her using your photos and drawings where the measurements were taken. Getting the right size is the most critical aspect of the ENTIRE job. Let's take our time and do this right.

Big Slabs of Glass

When the windows are built, they will be a giant slab of glass. Some fabricators put a band clamp around the windows so they can't break apart. If you don't have a clamp, be CAREFUL! If you bump a corner, you can pop a glass block off. Ask the fabricators to wrap the windows with a band clamp if possible.

Removing the Old Frame

This is the hardest part of the job. If you get the frame out easily, celebrate! If your window frame is a full width metal frame that was cast with the concrete, then you better ask the fabricator for his recommendation. I always leave those frames in place. It is a NIGHTMARE to remove them.

Once the old frame is out, use a brush to clean away dust and debris. I also recommend that you sweep up the floor and get all unnecessary stuff out of the way.

If you are positive that all of the new glass block panels are the right size and will fit, then remove as many frames and old windows as you think you can deal with in a day. I prefer to get all the demo work done and do things in stages. I don't like to do these windows one at a time. It really breaks up your momentum.

Setting the Panels

Now for the fun part. I hope you bought several bundles of cedar shims, as we are going to need them. I also assume that there will be a 1/2 inch gap of mortar around the window. If so, then place several shims on the bottom frame that extend up 1/2 inch. Place these in from the corners about 3 inches or so. Slightly dampen the exposed masonry jambs with a spray bottle of water.

Trowel some of the cement mortar onto the sill. Don't use bricklayers' mortar. Use Portland cement and sand. It will be stronger and more waterproof. If mortar is your only alternative, it will work, but I prefer cement.

Now tilt or slide the window onto the shims. Take two other shims and place them between the top of the window and the jamb. Slide them until they are just snug. Check the window for plumb and square. You are also looking for equal reveal side to side. You want the window parallel with the inside and outside walls, not twisted.

Massive Quantities of Mortar

Use rubber gloves, small trowels, sticks, whatever it takes to pack the cement mortar around the window. Fill any pesky gaps at the bottom. Don't put any mortar on the top of the window. Downward pressure or stress over time can crack these windows. The top gap can be successfully filled with caulking tomorrow.

Your mortar may be too wet to tool as soon as you install it. Don't worry. Within a short period of time the mortar will begin to harden. You can tool it with an old spoon, a stick, or with your fingers. Use a damp sponge to remove mortar smears from the glass.

Shim Removal

Usually within about two hours, you can gently remove the shims. Apply pressure against the glass as you pull the shims. Immediately, fill the bottom holes with cement mortar. Blend it in to match the texture and tooling of the rest of the joint.

Caulk the Top

Don't try to caulk the glass on the same day it is installed. I get the best results waiting 24 hours. Don't try to fill the entire space with caulk! Use a foam caulking backer rod to get within 1/2 inch of where the caulk joint will finish out. Use 100 percent silicone caulk. Wipe up smears immediately with lacquer thinner.

Dear Tim,

I was looking at info about building glass block windows and you were talking about using lacquer thinner to clean up silicone residue. I thought I'd e-mail you to offer the suggestion that the next time you are using silicone you try de-natured alcohol. My tile man taught me this and it works really really good! Probably less toxic too! I share your interest in building...I live in what was the Harwell fire department at the turn of the last century and remodeled it into something a little more modern. John Figueras, Cincinnati, OH

Dear John,

Thanks for this tip.