Looking for my plumbing vent

[ponypony]

morning, still having issues with my drain which has been unblocked four times in the last 8 weeks. Doing some amateur plumbing research on google and all the signs point to a blocked vent.
I went up on the roof to look for the vent and besides the geyser there are no other pipes there. Any thoughts on what the vent should look like and where it could be? I have three toilets all going to the same drain.

thanks.
p.

 

[_Jakes_]

It could be something like the "freeflow" number 3 on this drawing. This is just a 1 or 2 way vent valve sitting on the 110mm pipe above the drain where it comes out the wall.

 

[web]

The vent will be on the top of the toilet pipe were it goes through the wall to the outside,

 

[ponypony]

Thanks Jake147, I think I found it. Mine appears to have been painted over. So I'm assuming it is blocked. Will try take it apart and see which crucial part has been painted shut.