July 13, 201610 yr I just need to double check I have the right info. I am looking at specifying Velux roof windows and as you all know they have various sizes. Now I believe the sizes relate to the external frame size. I need to confirm with the timberframe supplier what structural opening I require. I think it will be slightly larger than the external frame size. For example one of the window sizes will be 78x160 cm. Does anyone know if there is a measurement/allowance i have to add for the opening in the roof? I am guessing there will be some for the flashing. It will be the slate flashing. Not recessed.
July 13, 201610 yr We added 50mm all round in the od days to allow us to add 50mm of sheet insulation to all four sides top trimmer is best horizontal and bottom one vertical an there is a lot of working out to get it right so that 50mm of insulation with plasterboard still locates into the groove in the window frame.
July 14, 201610 yr Depends on the type of window you are fitting. Tolerance's are shown here: http://www.velux.co.uk/professional/products/installation-instructions
July 14, 201610 yr Author thanks Barney. Just as I thought, looks like I need approx. 50mm wide and 45mm for the height
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