How to Mull Windows

Cover a clean, sturdy surface with a blanket. Lay the windows wood side down on the blanket. Match the sills and heads of the two windows together.

Snap the nail fins off the sides of the windows that you are joining together. Pull the nail fins forward and then backward with a pair of pliers until the nail fins snap off. Push the windows together so the sills and heads are flush to each other.

Insert the mull pin in the middle of the nailing fin channel and slide it down all the way. Use a rubber mallet to gently pound the mull pin into the nailing fin channel if needed. Place the mull plug between the two windows with the injection hole facing the exterior.

Loosen the two screws that are located on the bottom area underneath the mull pin. Place the bracket over the bottom two screws that you loosened. Tighten the two screws.

Use a power drill with a 3/16-inch drill bit and drill a hole in the windows. Use the hole at each end of the bracket as a guide to where to drill the holes. Install rivets in the two holes you just drilled with a rivet gun.

Install the mull cover over the gap between the two windows. Use a rubber mallet to gently pound the cover if needed. Flip the joined windows over carefully and install the mull cover on the other side.

Load a sealant container into a caulking gun. Place the nozzle inside the mull plug and inject the sealant. Stop injecting the sealant when it leaks out of the fin channel.

Start at the corner of the frame that is perpendicular from the mull and slide the nail fin until it is flush against the other, and cover the mull plug.

Apply the connector and structural brackets as stated in the manufacturer recommended installation.