How do you measure for uPVC Double Glazing

27 Nov 2006
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United Kingdom
It seems the general rule of thumb is to measure the opening from brickwork to brickwork in three places, use the smallest measurement and deduct 10mm to allow for the frames 5mm tolerances and for squaring up with a level.

However, what if the walls are rendered? Should the window be slightly bigger than the opening to allow it to butt against the inside edge of the render?

Any advise on measuring the height of a window which has existing external tiled sills?
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I would chop of the render and measure top,middle and bottom, then deduct 10 mm of the size in tightest place. Do the same measuring from head of the frame to the sill, same again deduct 10mm. Measure internal plaster in same way as outside. Bathrooms and kitchens worse place for this. Where tiles are close to glass lines and casements. As for the tilled sill, 2 things you can do. 1. No cill and face drain and put a 30mm frame extension on bottom of frame, so the window frame is smaller but with the frame extension makes it the right size. Its so the frame fits but moves glass lines up because of the internal window board. Hope that makes sense. Also if your worried about render being close to casements put frame extensions on the sides aswell. 2. Put a stub sill on, so still got concealed drainage and put frame extensions on side for render. Hope this helps. If in doubt but trust your measuring, go and speak with someone at a local double glazing firm about what i said.
it depends on if your your house has a reveal or reverse reveal, or straight brick with render as the check

measure in between the outside ingoes, top and bottom, also from the underside of the cill to the lintol

then on the inside, what i do is chisel away a bit of the plaster/plasterboard or what ever you have and measure the window that's in just now, also do this for the height.

you'll see from the sizes what kind of reveal you have, if it's a normal reveal, add 10mm to the size you got from the outside.

a wee tip about tiled cill's, sometimes they can go up/stick up further than you think, i replaced the windows in my mum & dads house, it had tiled cill's, the back edge of the tile stuck up a bit into the old cill/window frame

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