hi roberto, i used to use a long pointed metal rod,cheers

hi roberto,yeah it`s a bit of a guess but a sharp point on a 5/6mm rod and a smack with the hammer will normally do it, once yo have the approx position you can adjust as you cut the hole from above,always worked for me,cheers

I've done the same thing for a lot of years myself, metal rod with a decent steel point which you strike with a hammer or such to get a decent reference point on the roof as something to start with. Once you've made a mark,Cut a 20mm hole in roof so you can see down through it then push an offcut of pipe if you have one, up through the soffit hole youv'e already made and wedge it in place. It will show you if you are off centre or not with your initial mark by 10 or so mm, maybe a tad more.

If you do it this way you are almost guaranteed to cut a nice tight hole every time.Works pretty well. Ive done a few where my initial mark is bang on. I think after you've done enough of them you can trust your mark made with the pointed metal rod is really close so never mind checking with a offcut piece of pipe etc. Having the pointed metal rod vertically level and centered with the soffit hole when you make your mark/dent on the roof helps a great deal in being close to centre when cutting the hole in the roofing material for the pipe to pass through.

If you cut the vent pipe at the same angle as the pitch of the roof, roof angle, you will end up with an elliptical shape on the end of your pipe. This shape can be used as a stencil to cut the exact shape in the roof sheathing required for the vent pipe to be true and plumb. The hole cut is usually an oblong circle shape, elliptical.

hey badger how about a .22 with good aim!
 but seriously I use an old motor cycle rear axle that has been sharpened to a point  and thinned down for about a quarter of its length aim for centre and perpendicular and give a good wack and viola straight through the iron!
( just a metal rod but thick and has a good striking surface) I have had good results with this method.

hi guys...Excuse me sir you do realise that you are not allowed to discharge a firearm within 50mtrs of a residential area, cheers
..p.s. perhaps we should have a section on `Tips and Tricks` to make our work easier.

hi guys, hey Badger i think i should `Ask` Plumber to add a tips& tricks rather than tell him or he may dig his toes in, you know what plumbers are like,, Ha-Ha, cheers and have a good one