How to Build Braces for a Window Air Conditioner

Measure the width of the window with a measuring tape. Measure the width and depth of the channel in which the window slides on both sides of the frame.

Transfer the window width measurement to a 2-by-4 inch piece of lumber with a pencil and cut the lumber to this width with a jigsaw.

Transfer the width measurement of the channel to the lumber's 4-inch face from each end. Draw a line to mark the channel width measurement. For example, if the channel is 1/2-inch in width, mark a line 1/2-inch from each end.

Subtract the depth of the window channel from the 4-inch width of the lumber. Measure and mark half of the result from both edges of the lumber's 4-inch face at each end along the width lines and mark it with the pencil. For example, if the channel depth is 1 inch, measure and make a mark 1-1/2 inch from each edge of the lumber.

Cut all four corners of the lumber, using the L-shaped marks on each corner of the guide. After cutting, you will have a centered notch at each end that will fit in the channel of your window.

Open the window and insert the lumber so that the notch fits into the channel on both sides. Lift the lumber so that it meets the bottom of the window pane and position your air conditioner in the window.

Lower the lumber and window so that the wood sits directly on top of the air conditioner unit. The top lip of the air conditioner should rest against the lumber.

Drill pilot holes through each hole in the air conditioner's top lip and into the lumber. The pilot holes will help make inserting screws easier and should be slightly smaller that the shafts of the screws.

Insert wood screws through the holes in the air conditioner's top lip and into the pilot holes. Tighten the wood screws with a screwdriver.