Standard practice is to specify PVCu windows 10mm smaller than the
opening, i.e. 5mm off each side. This seems to suggest that the window
will float 5mm off the bottom of the opening. You end up with a space
at the top that is rather too large for a bead of mastic. Surely it is
better to specify 5mm off the vertical direction. This will still be
more than enough for expansion of the plastic for most moderate sized
windows.
Or is there a flaw in my argument ?
Only thing I can think is to bed the sill in silicone, but I've never
seen anyone do this.
Simon.