Flat roof widows are sized based on structural opening dimensions, with the external frame sizes being 220mm larger in both directions. Flat roof windows of all variations sit 'on top' of the opening.
Pitched roof windows are sized based on external frame dimensions, so fitting tolerance must be allowed for at an average of 10mm all round (depending on the flashing type and pitch etc). Pitched roof windows sit 'within' the opening.
For more information and help on fitting FAKRO roof windows, or about sizes, please contact firstname.lastname@example.org.