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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.

Mr. Boy
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    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 '22 at 08:42
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    @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 '22 at 08:47
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    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 '22 at 09:27
  • @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 '22 at 12:46
  • Judging by the previous question, these are non-opening windows, so non operational. – Tim May 31 '22 at 13:03
  • @Tim they are separate questions. It's rare anyone seems to connect the dots on the larger SE sites... I really like it on smaller sites where people follow 'stories' but hadn't considered it would cause confusion. Sorry for that! – Mr. Boy May 31 '22 at 13:59
  • So my 2+2=5! Are they opening windows then? How much wood is between them and the underside of the roof structure? I doubt much weight there, which is why I think a wooden lintel will suffice - but - insufficient info! – Tim May 31 '22 at 14:19
  • @Tim, it doesn't matter if they're operational. If you've ever looked at the construction of a vinyl window you know that there are hollows and channels everywhere. Pressure would easily distort frames and tear loose seals. It's not viable. – isherwood Jun 01 '22 at 13:19
  • @isherwood - that's fine. Except I've done it several times, with no problems. Particularly with wood above, thus not a lot of load. End of discussion! – Tim Jun 01 '22 at 16:00
  • Resumption of discussion. Of course it won't cause problems _every_ time. No one said it would. It's undeniably bad practice to fit a window tightly in an opening. Ask any manufacturer. – isherwood Jun 01 '22 at 16:02

3 Answers3

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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.

isherwood
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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.

crip659
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    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 '22 at 14:23
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Window frames can be warped substantially by low-expansion "door & window" spray foam. So no, they won't carry any load from the roof.

sribe
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  • 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 '22 at 12:42