A flat roof is difficult to properly ventilate, as the roof doesn't have the soffits which are usually there with a gable or pitched roof, so a flat roof vent has to be managed and maintained regularly. As the flat roof vent is having to work harder to perform its task, it is more likely to develop problems, and more likely to need repairing or replacing. If you find that your roof or attic space is not being ventilated properly, then you might consider replacing your old flat roof vent with a ceiling exhaust vent, but short of that, you can only attempt to repair the vents that you have.
Step 1 - Inspect the Roof
Before you remove the vent and attempt to repair it, you should take the time to inspect the roof, and the attic underneath it. Look for pools of water, or for signs of mold around the edges of the vent. This will suggest that the vent has been a problem for a long time, If you do find pools of stagnant water, you should leave this alone, and call in a professional. Stale water in a warm atmosphere is a prime breeding ground for Legionnaire's Disease, which can kill. Mold can also be toxic, and is better handled by professionals also.
Step 2 - Remove the Vent
If all seems alright, remove the vent from the wall by unscrewing it. You may need to prise it off of the wall, particularly if it has been in place a long time. Try not to disturb any of the shingles. Take the vent down to a well-ventilated area, and look at it under strong light. A flat roof vent might become clogged with debris, which will stop it from working. If you see that the vent is blocked with leaf litter or similar, wash it out. Dismantle the vent, and place all of the pieces into soapy water. Use a scrubbing brush to give it a good clean. Leave to dry.
Step 3 - Refit the Vent
Once the vent is thoroughly clean, inspect it closely. If there is nothing else wrong with the vent, then you can quickly refit it back into the same position. Use new screws to refit the vent, as this will ensure that it remains in place.
Step 4- Replace the Vent
You may find cracks on the surface, or signs of rust if it is a metal vent. A flat roof vent which is cracked will not work, and needs to be replaced. Throw out the old vent, and buy a new one of a similar size. Push this into the opening, and screw back into place. The new vent may be slightly larger or smaller, so be prepared to adjust the vent's plywood braces. Once the new flat roof vent is installed, caulk around the inside edge, and then replace any of the shingles which have been knocked off in removing and refitting the vent.