A Fernco can be used to connect the extension.
In the roofing department, you can find a gasketed flashing that goes around the vent as it exits. It will have to be laid in properly with your roofing to be leak proof.
No bend. No screen is required.

Depending on how much snow you get, about 2 foot above the roof where it goes through.

Begin in the attic space.
Use a string & plumb bob to extend a vertical line from the center of the pipe to the underside of the plywood roof decking and mark the spot.

Drill a hole through the roof decking from the underside up through the roof then push a wood dowel, piece of wire or other object through that hole that you will be able to see when on the roof. That will mark the center of hole through the roof.

Now go on the roof and locate the center index marker you pushed up. For a 2" pipe I would layout a 3" hole centered on that mark, then cut the hole out with a jig saw.

Drop your PVC pipe down through the hole and connect it to the cat iron pipe.

If the cast iron pipe has a hub you can get a "Fernco" rubber donut gasket to seal the PVC to the hub.

If the cast iron pipe is cut off so that it has a spigot end (raw pipe diameter) you can use a "Fernco" PVC x CI rubber coupling to connect the pipes.

Next you will install a "Roof Flashing", which is a flat aluminum plate about 1/8" thick and about 2' x 2' square. The flashing has a hole through the center to fit over the pipe and it has a flexible rubber gasket that seals it weather tight against the pipe wall. The flashing is slid down the pipe and placed tight against the roof. The shingles on the upper side of the pipe should be carefully folded up to allow slipping the upper section of the flashing under the shingles. In this manner any rain water coming down the shingle will go on the top of the flashing plate and continue running down over the flashing and onto the roofing below. The bottom corners of the flashing should be nailed in place to prevent the flashing from moving.

Now looking at the pipe from the side you will note the roof angle. The vent must extend 6" above the roof measured from the high side of the pipe on the roof angle. If you live in a region subject to snow the vent must extend 6" higher than your average snowfall. (Check your local AHJ because most communities have set a specific length to allow for snow in your community)

SHAME ON ME!!! I am surprised that no one has noticed the serious mistake on my last post.

The instructions on how to install the vent through the roof are fine, however I forgot to mention that if you live in an area subject to frost the code requires that you increase the size of the line to 3" at least 1' inside the structure to prevent frost closure on the pipe. (some local codes require 4")

memarcus - you could go out through a wall - but you still have to go up. Also, vents going out the side are usually ugly - depending on the structure. Codes differ - but there are some I've seen that vent out the side and then up (usually older structures). BTW - lazypup gave excellent instructions on doing this for someone who hasn't done this before. Nice.

"SHAME ON ME!!! I am surprised that no one has noticed the serious mistake on my last post."

I just noticed this one and the first thing I thought of when reading through was the up sizing of the line.

The height above the roof deck also needs to be considered.

In deep snow areas vents need additional height to remain clear.

You still do a great job Lazypup.

I deal with all areas of construction as PE, but routinely have to look up specific details.

HORRIBLE ADVICE... "The bottom corners of the flashing should be nailed in place to prevent the flashing from moving." Nooooooo. NEVER put nails in the bottom corners of the flashing. You are creating unprotected holes through the shingles and the roof that will eventually leak.

Jay,

First this is an EIGHT YEAR OLD THREAD so there was no need to resurrect it.

Second, the method described is absolutely, positively, without any doubt whatsoever, the correct way to install pipe flashing. the shingles are laid back down over the those nails on the edges - no unprotected holes to leak. Telling people not to install those nails is terrible advice, contrary to both building code and common sense.

A) It doesn't matter how old the thread is...people are still finding it daily via Google and thinking it is sound advice. It needs to be clear that it is wrong. This is as much a how-to wiki as it is a discussion forum, and it's coming up on the first page of Google when searching for info on installing vent pipe flashing. As such, it is just as relevant today as it was 8 years ago.

B) You missed my point entirely... the advice given was to put nails in the BOTTOM CORNERS of the pipe flashing, where there are almost always no shingles to cover the nail holes. This is absolutely incorrect. Nail holes in the top corners, yes...the shingles will cover those. Nail holes in the bottom corners? Absolutely not. The bottom corners of the flashing almost always protrudes below the shingles...leaving those nail holes exposed to the elements. Go buy a vent pipe flashing at Home Depot. I bet you $50 there are indented holes suggesting where to put the nails on the top corners, and on the sides...and NO indents along the bottom edge. Know how I'm so sure? I just bought and installed 3 of them today.

I promise there is no building code that says your vent flashing has to have nails in the BOTTOM corners. If it's nailed across the top edge (under the shingles), it's not going anywhere ("common sense?").

The poorly thought out advice above is going to make people think that what's depicted in this image is acceptable...and it is most certainly not...

http://www.ready2roof.com/wp-content/uploads/2014/08/pipe-flashing.jpg

Or this...

https://oldhousecrazy.files.wordpress.com/2012/02/old-house-crazy-cutting-a-hole-in-your-roof-installing-a-vent-pipe-diy-10.jpg

Or this...

http://www.indoor-air-health-advisor.com/images/radon_exhuast.jpg

I assure you I am more than capable of reading and comprehending and understood you perfectly. You gave poor advise. You won't meet windload requirements in most locations without those nails on the bottom corners. Just seal the exposed nail heads properly.

You can keep doing it your way, and I'll keep refraining from penetrating roofs with exposed nails. With nails in the top edge and the sides of the flashing, and the bottom edge secured to the roof with a proper roofing adhesive, it's not going anywhere unless you've got winds strong enough to take part of the roof with it.