Sagging timbers, floors, ceiling joists and windows are a common problem in older homes, as gravity takes a toll on any structure no matter how well-built. Foundations sink and that can result is a visibly sagging bay window. Support must be added to the exterior of the window in an aesthetically pleasing way. You can do this by adding 4-by-4 timbers and cement deck anchors that do not detract from the exterior appearance of your home.
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 centre 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.
If the high lift won't go high enough, measure and cut a 2-by-4 and use it as a prop that sits on the jaw of the jack running to the bottom of the window. If you don't like the look of the supports, paint them or add a cedar lattice framework to the outside of the bay window. You can get concrete deck anchors at any home improvement store. The measurements are for examples. Buy your 4-by-4s to any length to best optimise the lumber.
High lift jacks are capable of intense pressure, don't break your window frame when you are cranking up the window. If it begins to crack, move the jack to a different position.