We have bay window that has sagged enough that that windows are out of square and no longer open or align properly to get a good seal. As such, they are drafty in the winter and we can't open them in the spring or fall to enjoy.

bay window

out of square

It seems the options for leveling a bay window are to suspend it using cables from the top, or build support from underneath. The cable method doesn't seem to be a good option because the roof of the bay window is basically flat and metal. Here are the steps I'm planning on taking with questions.

  1. Lay plywood down on the ground and using a floor car jack, slowly raise the bay to slightly overcorrect the level issue. This will allow some settling when adding the supports. Questions: will a car jack work here?
  2. Using pressure treated 2x4s, build 2 knee braces. enter image description here. Questions: will 2x4s look tacky? My plan is to paint them white
  3. Attach knee brace to cinder block foundation and the bottom frame of the bay window. Questions: how should I attach the knee brace to the foundation? Tapcon screws? Is this structurally sound? What type of fasteners should I use for the knee brace joints?

Here is a view the underside of the bay window: underside

Final questions: is this all a bad idea? Should I have supports in the ground instead? Or maybe the whole bay window just needs to be replaced? Any and all advice would be appreciated.

3 Answers 3


I cringe when I see this type of construction. How good are you at mixing concrete? I'd be digging out that area under the bay window, compacting it, adding a few reinforcing rods into the existing foundation, a little framing and pour a 6" slab/footer. Then use your jacks to slowly raise or even over raise the window, cinderblock and morter it in and cover with a coat of stucco. Don't forget to extend the vent with a prefab from your home store.

  • 1
    If you enclose the area under the window, don’t forget that you’ll need access to your new “crawl space”.
    – Lee Sam
    Sep 10, 2019 at 0:19
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    @Lee Sam Pour it solid? It's not that big an area. Did a few in Illinois and just filled them in with gravel.
    – JACK
    Sep 10, 2019 at 0:29
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    First off, any type of wood support will eventually fail. Knee braces are great for a book shelf, not for part of your house. Pressure treated wood is soft to begin with and it will warp. The screws holding the angle support would be a weak point for that type of load. Tapcons are out, they do not offer the holding power you'd need. You'd have to use lag bolts and lag bolt shields or drill all the way through the wall and use threaded rod, washers and nuts.
    – JACK
    Sep 10, 2019 at 17:54
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    If you don't have the "knowhow" you should work with someone who does. This is your house... your most valuable asset. No need to replace the bay window, just have it supported correctly, and permanently.
    – JACK
    Sep 10, 2019 at 17:59
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    @Andrew If nothing else, jack it up and stack some cinderblocks under the two "corners" until you figure out what to do.... it will only get worse.
    – JACK
    Sep 10, 2019 at 18:39

I like Jack's plan; however, I have to ask: Is there no concern that a contributing factor may be an inadequate or perhaps rotting header? It is too easy for water to get in and under these things and cause structural decay--which will also lead to the issues you are having.

  • I don't have any reason to believe there is a water penetration issue. I haven't seen any evidence of it and at this point I've inspected the window quite thoroughly. There is a small storage compartment in the bay window itself that I've pulled out an inner panel. What I see there is angled braces (similar to what I was suggesting) that are supporting the window and my guess Is these are failing.
    – Andrew
    Sep 13, 2019 at 19:03

My house is near 40 years old and has 2 front bay windows very similar to the one here. It sagged a little over the years. So last summer, just jacked it up with 2 car jacks on each side, and screwed in 2 new angle supports which are much sturdier than the old ones. Seem working fine.

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