A Flat Roof Skylight installation would normally have a totally different install to those that are for Pitched Roof.
Flat Roof Sky Lights would have to be ‘out of plane’ of the roof and would require a minimum of 150mm waterproofed upstand detail to make sure that ponding water or heaped snow won’t enter the building. The waterproof materials would be products like Roofing membranes, Asphalt, EPDM or Liquid Solutions & traditional materials like Lead etc.
A Pitched Roof Skylights would normally be more ‘in plane’ with some sky light options being very much flush within the tiles, slates etc.
In most occasions the Sky Light would require the Flat Roof to have a structural opening larger than the widths of the supporting timbers. This would mean, on wooden roof decks, the timbers would need to be cut and then doubled up to create a strong ring beam to support the Flat Roof Skylights chosen. Concrete/Metal can also be cut into for Skylight fitting but please discuss with a Structural Engineer before cutting any structural openings in any roof.
It would also be important to make sure any Flat Roof Skylight has a Non Fragility test such as an ACR or CWCT. The Health & Safety Executive would also require you to consider ‘Health and safety in roof work’ HSG33 too.