Velux Sizing

[mpaxton78]

Ok so I've been looking at the Velux sizing and I'm a little lost as I can't really tell if the sizes quoted are related to the finished measurements or the spacing to which they fit.

Joists are spaced at 400mm centres so I was looking at the Mk06 (780x1180mm) and planned to use the Velux flashing kit, but is this window the right size or do I need a smaller one?

Thanks and sorry if I'm asking what seems like a silly question, but I'd rather be sure and would like to avoid respacing joists if possible.

 

[catlad]

All measurements are external and velux recommend
a 20mm gap around the windows ideally.

 

[noseall]

would like to avoid respacing joists if possible.

Someone has made off with a few of them already!

 

[Nick Pain]

All the information you need is here...
https://www.velux.co.uk/professional/products/installation-instructions

Be aware the gap is bigger than window size +20mm if your using the insulation kit.
Instructions very clear!