How to Install Preassembled Glass Block Windows


Glass block windows are exceptional in quality and they look fabulous. You can also order them pre-assembled so all you have to do is slip them into the opening. Glass block windows require ½-inch gap on all four sides to fill with mortar. Therefore, measure the opening and subtract 1-inch from both the width and the height. Remember; measure twice to be certain the measurements are correct. Take the measurements to your local building supply store and choose the style that best fits your needs. Most of the pre-assembled glass block windows will come in a metal frame. If your needs calls for something with fins, then make sure to relay that information to the person ordering your pre-assembled glass block window. Also, keep in mind that these windows can weigh as much as ten pounds per square foot.

Things You'll Need

  • Pre-assembled Glass Block Window
  • Cedar Shims
  • Portland Cement
  • Sand
  • Trowel
  • Sponge
  • Dry, Clean Cloth
  • Caulk Backer Rod
  • 100% Silicone Caulk
  • Lacquer Thinner
  • Measure the preassembled glass block window to be certain it will fit in the opening. Next, place some of the shims on the bottom, or windowsill, of the opening. These need to hold the window off the sill ½-inch. It’s best to set these 3 or 4-inches in from the corner to allow for mortar in the corners.

  • Apply some mortar on the sill without disturbing the shims. Remember that this window is very heavy, so it will pack the mortar. Be certain to place enough mortar on the sill so the weight of the window will pack it down to ½-inch. If you discover a few gaps, fill them without removing the window.

  • Place shims on either side of the window to even the gap on the sides. Be certain they match the gap on the windowsill as well.

  • Fill the areas on either side of the window with mortar. Remove the shims on the sides as you come to them, but leave the ones on the bottom (windowsill) of the window for last. Do not place mortar on the top of the window. Clean up excess mortar before it sets with a damp sponge and a clean, dry cloth. Wait 24 hours before caulking the top.

  • Fill the gap at the top of the window with foam caulking backer rod. Leave it back from the face of the window ½-inch. Apply the caulk in the gap so it finishes out to the face of the window. Clean excess immediately with lacquer thinner.

Tips & Warnings

  • Always wear safety glasses.
  • Allow at least a day for installing a preassembled glass block window in a medium sized opening.
  • Do not apply mortar to the top of the window as this can cause the glass to break over time.
  • Keep work area free from clutter.
  • Clean up hands and tools with warm soapy water.
  • Learn all you can about the different types of mortar available for glass block windows.
  • A slight variance in the mortar gap around the window is acceptable and hardly noticeable.
  • Do not try to pick up a pre-assembled glass block window on your own.

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