Hope someone can help. I have been trying to work out how i can accurately place a skylight on a pitched roof with has a flat ceiling below. The only way i can see is creating a cut out in the slab. This is proving extremely time consuming and frustrating! I also cannot seem to get it just right, measurements are always out. I have attached a screenshot to explain what i mean. Essentially i want to 'box in' the skylight, but struggling to find a suitable way to do so.
The ceiling slab is not related to the roof, so yes you will have to manually cut the hole in the slab.
There are techniques to cut the hole in the slab but use the roof plane as your editing plane and project the hole onto the slab.
So effectively you are picking points on the roof (the extents of the sky-light) and you are cutting the hole in the slab - projecting vertically or perpendicular to the roof.