How to Repair a Glass Block Window

Glass Block Window with one blue block and one yellow block, among the clear blocks
  • 3-4 hours
  • Intermediate
  • 120-220
What You'll Need
Duct tape
Utility knife
Putty knife
Rubber mallet
Glass window block
Plastic spacers
Silicone-based sealant
Lacquer thinner
Work gloves
Safety goggles
Waste bin
Glass block
Striking tool

When a glass block window has a damaged glass block or two, it really stands out and it can pose a safety hazard. Fortunately it is possible to replace the damaged block(s) without having to re-do the whole window.

Type 1: Mortar-less Glass Block Window

This type of glass block window is held together with plastic spacers and a silicone-based sealant. Here is a list of what you will need to complete the project.

1. Preparation

Cover both sides of the glass block with duct tape, leaving the grout line uncovered. Then use your utility knife to slice through the silicone-based sealant. Get as much sealant off of the block as you can without damaging the rest of the glass block window.

2. Remove Damaged Block

Put on your safety goggles and work gloves. Then hit one side of the damaged block with a rubber mallet until you hear it crack and break. Do the same for the other side. Pull away the tape and throw it and all of the glass pieces into the waste bin. Remove the old spacers as well. With the utility knife and a putty knife, completely clean out any remaining silicone and glass bits.

3. Placing the New Glass Block

Center the new block into the opening. Then, on all sides, insert plastic spacers in between the replacement and the surrounding glass blocks. If necessary, lightly tap the spacers with the rubber mallet. Once the plastic spacers are in place, spread a silicone-based sealant around the perimeter of the glass block. Immediately get rid of any excess silicone with lacquer thinner.

Type 2: Traditional Glass Block Window

This next section deals with a glass block window that uses mortar to hold it together. Here is what you will need to replace a damaged block in this type of window.

1. Removing the Old Glass Block

Put on your safety gear and then cover both sides of the damaged block with duct tape. Next, hit one side of the glass block with the rubber mallet until it cracks and breaks. Do the same for the other side. Throw away the duct tape and glass pieces. Be sure you have cleaned up every bit of glass before you continue. Then take your chisel and hammer and carefully remove the old mortar.

2. Putting in the New Glass Block

Mix up some mortar according to the directions on the bag. Then use a trowel to apply mortar to the bottom of the opening. It is important that the thickness matches the thickness of the mortar joints in the rest of the glass block window. Center the new block in the opening and start filling the rest of the spaces with mortar.

3. Finishing Touches

After 2 to 3 hours, use a striking tool to smooth and compress the now firm mortar. Then wipe any excess mortar off the glass block window with a soft wet cloth.