How do I Mull Two Aluminum Windows Together?

These windows have been mulled together.
These windows have been mulled together. (Image: fenêtre double et table image by from

Oftentimes windows are mulled together to make one larger unit. Windows add value and elegance to a home, and bigger windows can make houses even more beautiful. Mulling two regular size windows together gives the impression of one larger window. Aluminum windows can be joined just as easily as any other type of window. Mulling windows can be a bit of a challenge, but with the proper materials two windows can be mulled into one much larger unit.

Things You'll Need

  • Tin snips
  • Utility knife
  • Hammer
  • Pliers
  • Level
  • Clamp
  • Hacksaw
  • Drill
  • Screws
  • Drill bit
  • Caulk gun
  • Staple gun
  • Staples
  • Silicone sealant

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Remove the nailing fin from each window by cutting it with tin snips and a utility knife. Cut one corner up to the frame with snips to get it started and continue down the window, scoring the flange with a utility knife. Run the knife all the way around the window.

Bend the flange with pliers once it has been scored and hammer it down until it breaks off. Repeat the process for each side of the flange and remove the jamb from the second window the exact same way.

Open the inside corners of the window by cutting them with a hacksaw. Cut vertically across the corner down to the bottom groove and then cut horizontally to complete the opening. Cut all four inside corners of the window.

Run three beads of silicone sealant along the inside jamb of the two windows being mulled together. Place one bead where the nailing fin was located and the other two at the top and bottom sides of the jamb.

Slide the two windows together and check to see that they are level. Clamp them together to ensure they are properly tightened. Staple the jambs together. Insert a staple 3 inches from each end and then every 5 inches up the entire jamb.

Attach a mull plate to the exterior side of the jamb where the two windows meet. Drill it into the jamb with pan head screws. Remove the clamp that is holding the two windows together. Check to see the window is level and plumb. Make sure everything is properly sealed and the window is functioning correctly.

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