Window Frame 'Knock-Ons'

[CPEA]

I've just had new Rehau double glazed units fitted and noticed that the frames have what is called 'knock-ons' (?) at the top, making more plastic visible and making the window size smaller - not by anything much but obviously with UPVC double glazing the less plastic and more glass the better.

Why would these be needed when the surveyor measured brick to brick, I thought new units should be made to measure as one unit?

Any good or bad reasons appreciated - do they have an impact on the performance of the double glazing?

 

[Forest Grump]

It doesn't have any impact on performance.
Could be that you have a thick plasterline so the build up would be required. Or could be an unsure surveyor. A lot of people use the knock ons as a safety net, in case they cock the measuring up !

 

[libby lou lou]

Your surveyer should of allowed about 10mm

 

[noseall]

often, if there are trickle vents fitted to the top rail of the window, then knock-ons will be used to give the head plaster a chance.

 

[CPEA]

Thanks for the info. The windows have a 10mm clearance, with the knock-on included. Still doesn't make sense though as if we needed a knock-on at the top of the window for the possible reasons given, why would the width be measured to fit correctly without a knock-on?

We do have one window with a trickle vent, but thats the only one which doesn't have a knock-on and is made to measure...

 

[Forest Grump]

It is a lot easier to get the width right, but the height can be a bit trickier, with cement beds at the bottom or hidden lintels at the top.