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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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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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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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