Whether you want to install a roof vent, a skylight or any other structure that spans the space between your house and your flat roof, the first thing you'll need to do is create a hole. When making a hole in a flat roof, the biggest concern is making sure that you don't accidentally cut through a ceiling joist, as this can badly damage the stability of your roof. Also, you only cut one side of the roof at a time, so you don't sever any wires and ensure that the holes line up.
Place your stud finder on the flat roof ceiling to locate the ceiling joists running above you. Mark the joist locations with a pencil.
Choose where you want to create the hole in the roof. Position the hole so that it does not intersect any of the joists. Install a long drill bit into your drill and place the drill at the exact centre of your desired hole. Drill through the ceiling and continue until you feel the drill bit clear through the roof.
Get on top of your roof and located the hole you created. Use a utility knife to cut and remove the roofing material around the hole. Use a template or a measuring tape to ensure that the piece of felt you remove is exactly the size of the hole you need.
Place your jigsaw into the hole you drilled and use the saw to cut the wood beneath the roofing material. Cut along the line formed by the roofing material that you removed.
Place your drill at one corner of the hole in the roof and push it down until it touches the ceiling material. Turn the drill on and create a hole. Do this for all four corners of the hole, or if your hole is circular, at points signifying four opposing lines around the outside of the circle.
Move back underneath the cut and use your jigsaw or utility knife, depending on the type of ceiling material that you have, to cut from circle to circle to remove the ceiling and complete your hole.