Hot Topics: Replacing Roofing Around Vent Pipe

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Original Post: Need to replace leaking roof around vent pipe...first step?

tiresharkdbb Member

I have a shingle roof, and have been putting tar around the vent pipe for 10+ years to stop leaks. The last time I crawled in the attic to check, the plywood had partially collapsed under the leak. You can see the underside of the shingles in spots. I've done some minor roof repairs over the years. I'm basically just going to have to cut out the old plywood, replace it with new, and get new shingles.

My question is if there is a good way to go about cutting out the bad section? Can I just get in the attic and take a Sawzall and cut from the bottom up through the plywood and shingles? Or would it be better to start up top, remove the shingles first, and then cut? Thanks for any advice.

XSleeper Group Moderator

You can, but you will eventually need to cut it again from the top side anyway. I would probably remove shingles first and find the edges of the sheet, pull nails, and remove it from on top.

tiresharkdbb Member

Thanks. Makes sense. Yeah, pulling up full sheets, or full cut pieces, would probably be better.

tiresharkdbb Member

OK, I removed some shingles and this is the situation. I believe the dark portion around the pipe (circled in red) is all I will have to cut out. The rest felt solid and took a nail good. Below the pipe will probably be more bad wood, though. How much, I dont know. To cut the bad wood out, I guess I'm going to have to expose the valley metal all the way to the top (which is only a couple feet more).

A roof that needs to be repaired around a vent opening.

Assuming I get lucky and the bad wood only goes down a half foot or so, am I going to have to expose the metal valley all the way to the bottom and replace it all, or can I cut it in half and replace? Talk to me like I'm dumb if needed...I have little roofing experience.

XSleeper Group Moderator

You "can" cut the valley tin below your rot. You will need to remove the entire valley and shingles in the valley on both sides ABOVE your repair. Then, lap the valley tin a minimum of 19" if your slope is 4:12 or less. As small as that area is, I would recommend you remove and reshingle it ALL.

tiresharkdbb Member

This is where I'm at with it. Planning to cut out the red triangle; the yellow lines are where the rafters converge.

A hole in a roof next to a vent pipe.

Rafters under a roof.

I'm open to any input, if anyone sees problems with my plan.

XSleeper Group Moderator

There's not very many rafters in that area. You might want to add a little blocking where your top red line is so that you can add an additional piece of rafter left of the vent pipe.

tiresharkdbb Member

I'm about 3" away from a rafter. There's nothing wrong with sistering two 2x4's to the rafter, to make the 3" gap, is there? I figure that's about the easiest way to do it.

A square next to a rafter showing measurements.

XSleeper Group Moderator

Well, you would actually need to sister on three of them in order to catch it. It's probably easier to add two pieces of blocking perpendicular to the rafters with one new piece of rafter between them that is 3/4" under the plywood. Or, just cut the plywood out to the center of the next rafter. It's only about a three- or four-foot run.

tiresharkdbb Member

I should have clarified that 3" puts it about halfway under the plywood, so I think they will reach. I'll probably do that, but I'll do what you suggested if two won't work.

XSleeper Group Moderator

According to the measurements on your square, it puts it 1/4" under the plywood...or not much to nail to.

tiresharkdbb Member

You were right about it not reaching. I thought I had it measured right, but I didn't. I just went over to the next rafter.

I got the hole patched. I suppose my plan now is to put some 30w felt down, extend new valley metal 19" on the top of the existing metal, and then shingle.

A roofing project with tools and plywood patching a hole.

As far as shingling the valley, I'm just going to try and replicate how they were originally installed. They had the method where it's cut diagonally all the way down. (Called closed valley or California cut, I think?) I think I've read not to nail within six inches of the center of the valley.

A patch on a roofing hole.

XSleeper Group Moderator

Sounds OK. Try not to nail in the valley tin at all. That's more than six inches, usually. California cut is tricky with three tabs. They tend to leak which is why it's critical not to nail into the fin. Be sure you are laying full shingles through the valley as they have done in your bottom photo. You don't want any butt joints landing in the valley. This may mean you need to add a single tab or a double tab in as you get close to the valley so that the full shingle spans the valley.

And it's too late now, but the piece of plywood you put in on the right is turned the wrong direction. Plywood always needs to be laid horizontally since that is the strength axis. If you step on the seam between joists, plywood is weak when laid the direction you have that right piece laid. The piece needed to be turned 90 degrees to have the grain running the right direction.

tiresharkdbb Member

Thanks Xsleeper, great info!

Yeah, I had thought about doing other methods for the valley, but since I'm tying into the old layout, I figured I was stuck with the way they did it. If it's possible to do a better method when tying in like this, let me know. If not, I'll just carry on.

I didn't know that about the plywood, but I did notice when I installed it that it flexed a ton when I stood on it. I couldn't understand why the one above it barely flexed, but the piece I installed did so much. I put blocking underneath it to give it support, but now I know what to do in the future. Thanks!

XSleeper Group Moderator

The blocking was a good move.

To read the rest of the thread, look here: https://www.doityourself.com/forum/roofing-gutters-all-waterproofing-anywhere/604495-need-replace-leaking-roof-around-vent-pipe-first-step.html

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