new upvc windows warping in corners

10 Jan 2017
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United Kingdom

last spring we had all our windows replaced with new upvc windows. over the last couple of months all of the windows's main frames have bent outwards in each corner, in some cases opening a gap of 3mm (the larger windows are warping more than the smaller ones). I guess that the outside of the window is maybe shrinking in the cold. This has led to some openings not sealing properly on the hinge side, as the top and bottom of the frame are bending out. All the windows are like it to varying degrees, all all sides of the house.
The people of the window company came and had a look, admitted there was a problem, and are sending someone round to fix it. they want to fill the gaps and force the windows to seal but adjustment. is this the correct solution or is there a fundamental problem with the windows?


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Sure, hopefully one attached. Sorry, not a great description from me, but basically every corner, on every window, has bent out to some degree. Enough in some cases so that the window doesn't close fully on the hinge side (there isn't a straight edge on the frame, as the top and bottom has curved out). Hope that is clearer,
Can you see any fixing screws when you open the window sash? If so how many ... and are there any anywhere near the corners that are bowing?
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Upvc windows don't tend to warp, so I'd look elsewhere first. There's a gap down the edge of the window, but not in the sealant around the bullnose at the bottom of the window frame.

I'd check the screws are in securely, and then look to remove the sealant, and use a beading to seal that gap first, and then see how it goes.
I heard that all Black UPVC need steel reinforcing in it due to the high heat that can build up in it when in the sun. maybe they didn't put it in?
Interesting point, but that still wouldn't explain why the sealant hasn't moved on the bullnose.
Are you sure the windows are bowing? Have you used a long straight edge flat against the frames to check?

Its true that darker colours will expand and contract more than white. White PVC is around 1.6mm per linear meter, I cant remember for coloured off the top of my head but it was around double that.

Based on just that one photo I would say that is natural shrinkage of the sealant combined with the thermal expansion of the frame.
They will of course need to make any adjustments before resealing the windows.

Hard to tell from that 1 small pic but it looks like they have done a fairly neat job (with the sealant at least) and they are coming back so they do seem to be a decent company.

If there truly are bows in the frame they may need to add some extra fixings if not just some adjustment and sealant.
thanks for the replies.

There are three screws down the edge: one in the middle, one about 6 inches from the top and one about 6 inches from the bottom.

Although the photos doesn't show it, the sealant on the bullnose does have a gap, but it is covered by the trim; which is stuck to the frame.

I have placed a straight edge against the frame and it is bowed, with the curve increasing nearer the corners. There is no gap at the middle point, air blows in at the bottom. We had about 12 windows put in, there are all like it to varying degrees. On the worst the curve of the bow means that the windows can't close properly. I am no materials expert but it is like the outside of window has shrunk, bowing the window out: does that happen?

To be fair to the company, they did a good job and have an excellent reputation; i can't really complain about them. I just wanted to work out how unusual or severe the problem was, just to have peace of mind that they fixed the issue properly.

Like i said I think they neglected to put the reinforcement in. I got a few quotes for black upvc and I had to ask why it was quite a bit more for black and the answer was the extra reinforcing. Not sure how they can really correct it now no matter what the issue is. If the window is bent as you say it likely needs replacing I would have thought?
Get a strong magnet and check around the frame, bare in mind reinforcing steel stops short of the corners by a few inches, this is to allow for welding, both the frame and sashes should be reinforced especially woodgrain foiled pvc as they do suck up more heat than white and it's not unheard of to see a bit of movement. I don't think there's any hard and fast rule on what is deemed acceptable, I would accept a few mm (3mm) but any more then I would start asking questions. Also please go easy on the fitters, windows warping in the heat is not their fault and is not a fitting issue, windows of poor quality, lack of reinforcement and such should be aimed at the manufacturer
I wonder if the window has been fitted in too tightly, and there's no room for expansion, hence the warping and bowing effect.

Tried the magnet thing, but only got a slight pull so it probably wasn't powerful enough. It would be pretty amazing if they hadn't reinforced them properly, but, i guess, not impossible.
I did think that maybe they had been fitted too tightly, but not sure, 12+ windows all fitted too tightly. One guy from the company said that they'd had a lot of problems over the summer with new windows expanding more than expected, but it wasn't until it got colder that we noticed the frame problems.
And, yeah, i don't intend to give the fitters a hard time, for me it is clearly a manufacturing issue, i just wanted to be ready just in case they tried to bullshit a bit.

Thanks again.
It's a manufacturing issue if they haven't fitted the reinforcement, and the surveyors fault if they've been fitted too tightly, but the fitters should also have been aware of that; and it's also possible that they've been sized for white Upvc instead of black.

If there's a slight pull on the magnet, that implies there's steel behind the frame, but you could check by putting a small drill through the frame; You'll either drill easily into the wall, or get stopped by the steel.
Just take one of the fixing screws out, you'll probably see the steel.

As for the fitters then they can get blamed for a lot, they're between a rock and a hard place sometimes, part of being a fitter or a good fitter is the ability to solve problems and find work around's, it's not to get straight onto the phone to the boss and tell him his windows are s***, or they've been measured wrong, I'm lucky cos I'm salaried but generally fitters are on a percentage OR a price per frame, either way if they don't fit anything that day they won't be paid so it's not in their interests to stop the job and query reinforcement, that's my view anyway

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