How to Add Support to a Sagging Bay Window

Place the jack under the outside edge of the bay window at its lowest point. Locate the heaviest part of the framing and place the jaw of the jack there. Crank the handle to put upward force on the frame.

Hold a level on the underneath side of the bay window. Jack the window up until the window is square. Crank the handle two more times to lift the window slightly past level.

Crawl underneath the window and inspect the framing. Find the optimum place for two supports and move the cement deck anchors under them. Measure the distance with a tape measure from the center of the deck anchor to the place you have chosen for support. Add 3/4 inch to the measurement.

Cut the two 4-by-4s to the measurement with a handsaw. Crawl under the window and insert the end of the 4-by-4s into the hole in the deck anchor. Bring the 4-by-4s upright until it stops, they will be slanted. Use a hammer to tap the 4-by-4s into place. If they won't go without doing damage, crawl back out and crank the jack handle up two more times and try again until the 4-by-4s fit under.

Level the 4-by-4 posts with the level, tapping them in the direction needed. When they are level, flip the lever on the jack and lower the window until it is resting on the posts. Remove the jack.