My son bought an old house in DC which has a flat tar roof. There is a very small space above the top floor which may be considered an attic. He would like to install a whole house fan which would draw hot summer air from the house thru the roof. The attic area is not accessable and does not appear to be vented presently. What could he use to remove the hot summer air? Could it also be used to vent the attic space? We thought maybe he could use a wind turbine but don't know if they could be used on a flat roof. Any ideas?

A mushroom-type fan can be mounted on a flat roof. Usually, it is mounted on a wood or prefabricated curb which is flashed with neoprene and mopped into the roof with tar, and the base of the fan is a counter-flashing with a drip edge. The fan capacity and speed should be selected by someone who is familiar with such fans and their noise properties. You don't want the fan howling up there. It can be controlled by a thermostatic switch in the attic if desired.

There should have been side wall vents in this crawl attic space. You do not want to cut into the flat roof, unless you understand how to properly curb and flash it. Not to many folks do. Its a lot more then just cutting and framing a box then throwing a little tar on it.

You should consider a few things.
1. Paint the roof with aluminum roof coating. That will lower the temp a lot. It will also preserve the roof as well.
2. Cut into the side of the attic walls vents. One or two on each side if you can. That should be more then enough to do the trick

A lot of folks turn to power fans and vents to ventilate their attics, regardless of shape. What they do not realize is that it can cause other issues as a result.
Mold is a very common condition when a attic is power vented. These fans draw more humid air into the attic then what would have normally entered with natural ventilation. The end result is mold growth. Also with motorized fans you are using electrical current which is not needed with a natural ventilated attic that is properly insulated.