I have water leaking from the sewer roof vent when it rains. The leak is not coming from the outside of the pipe but the inside. I can hear a drip from the inside of the wall in my upstairs bathroom and the water runs down and drips from the ceiling in the downstairs bathroom, luckily we have ceiling tiles and it is directly over the bath tub. Before I rip out my walls to look for the leak is it possible to put some sort of umbrella fixture over the top of the pipe?

Chances are it leaks somewhere around the flashing on the roof. Get on the roof and inspect roof flashing. Apply roof sealant, like Henry 208, around pipe and flashing.

If you really think your pipe leaks than it is lot cheaper to install a pipe cap on top of the vent pipe rather then ripping up ceiling and walls.

The leak is not coming from the outside of the pipe but the inside. Plastic or cast iron? Go up in the attic with a wade of toilet paper and start to wipe down the pipe looking for any moisture. If you find any get back to me with the location. I'm a great believer in locating and repairing the source of the leak but if you have to tear into walls to do it perhaps Milos vent cap, (see image) will save you the hassle. Good luck, tom