I am planning to install UPVC double-glazed windows into a timber structure I'm (re)building. So far we only built the main super-structure to give design flexibility for window placement, the idea being that timber cladding will also give more rigidity later.

Are the frames of these windows designed to provide any support/rigidity at all or is it important to ensure the window aperture is completely stable (no sagging, no movement) before installation? I know glass is strong and I have one or two apertures slightly on the limit whether I need additional structural support, but I don't know if UPVC frames are strong or flimsy.

  • 1
    You have chosen an answer that presumes there's no frame round the glass. In an earlier question you ask about just that. But there's no indication in this question that you haven't changed plan to include the plastic or aluminium frame that usually surrounds these windows. Please make the question more clear. If the roof is already supported, and there is indeed a frame, the weight should not be a problem - there'sa not much wood above, and you'd put a wooden lintel over anyway. But at replacement time, everything will be too tight to remove the lot.
    – Tim
    May 31, 2022 at 8:42
  • 2
    @tim my question title explicitly refers to UPVC windows and I ask "Are the frames of these windows designed to provide any support" I'm not sure how I can make it clearer I am talking about UPVC windows rather than glazed panels. I don't think the accepted answer is referring to glass panes but windows generally
    – Mr. Boy
    May 31, 2022 at 8:47
  • 1
    O.k. My experience shows that the frame is completely separate from the glazed unit, and has a certain strength of its own. The unit inside doesn't support the frame, nor the frame support the unit. The answer presumes you're going to rest a load straight on the edge of the glass, or unit - which it appears now you're not. Hence my comment. Provided there is a lintel - here, wood will suffice - there's no problem, as the unit can be fitted and removed easily as it is never load-bearing. Thus I questioned the answer as being provided from unclear information, that's all.
    – Tim
    May 31, 2022 at 9:27
  • @Tim, I disagree with your interpretation and with the notion that the frame is a separate thing. It takes fairly little deformation of a frame to affect performance (operation and insulation). While there's strength in the verticals of a frame, the spans are very flexible and the miter joints in the corners aren't designed for load. It's a non-starter to rest anything on them. All this is generally true of aluminum windows as well.
    – isherwood
    May 31, 2022 at 12:46
  • Judging by the previous question, these are non-opening windows, so non operational.
    – Tim
    May 31, 2022 at 13:03

3 Answers 3


No window is designed to carry structural load. In order to prevent binding and damage, the opening around the window must be completely supported and stable.

You stated that "glass is strong", but that's comically oversimplified. It resists compression very well. It doesn't carry weight on edge well, nor would the frame, which contains spaces and voids that would easily crush.


Glass does not bend well.

About the only structural support windows give is to keep out weather from rotting the structure.

Most windows/door are suppose to go in openings slightly larger so there is a gap between the window frame and the opening in the wall.

Usually use wooden shims where the mounting screws are used to level and plumb the window in the opening.

This is mainly to prevent any movement of the structure to stress/press on the window.

  • 18
    It should be made clear that the tops of windows should never be shimmed. Doing so transfers load to the window when settling or seasonal movement occurs.
    – isherwood
    May 30, 2022 at 14:23

Window frames can be warped substantially by low-expansion "door & window" spray foam. So no, they won't carry any load from the roof.

  • I've never seen this happen. I have seen conventional spray foam do it, but not the window & door version. I certainly recommend it over jamming some fiberglass in there like we used to do. It's vastly superior in both insulation and air blocking.
    – isherwood
    May 31, 2022 at 12:42

Not the answer you're looking for? Browse other questions tagged or ask your own question.