How big does the roof opening need to be?
The size of the roof opening must match the clear width of the roof access hatch. If trim is to be installed, the size of the roof opening must be increased on all sides by the thickness of the trim material (e.g. 18-mm-thick multiplex).
The trim must fit flush to the underside of the mounting flange. When a roof access hatch is installed in combination with a retractable ladder, the size of the roof opening is determined by the dimensions of the casing for the retractable ladder.