Cutting a Hole in Your Roof – Installing a Vent Pipe – DIY

Holy Toledo! This is a scary job for a DIYer. Think about it. You’re cutting a hole in the only thing that is protecting your home from an endless stream of the most devastating force nature has to throw at your most important, and expensive, investment: water. But worry not. Like most home repairs, this really isn’t all that complicated or difficult. However, if you do it poorly, it could cost you thousands in water damage.

Can you find the other code violation? It's blue and doesn't have a cover.

A friend asked me to help him eliminate a sewer odor that had been coming and going since he lived in his house. You might have just thrown up a little in your mouths, I know. It was one of those things where, just when it got bad enough to hire someone to fix it, it would go away for months at a time. Then, I started this blog, and he’s all like “maybe that guy knows how to fix it”.

After rooting around in the crawl space and attic, we discovered the culprit. Some A-hole plumber (yes, many professionals are A-holes that don’t do the job professionally) installed a sewer vent that vented directly into the attic. In their defense, they did install a vent filter to pass code. Poorly. Here’s the deal: whenever you run a drain to the sewer, you’re required to run a sewer vent up and out. In well planned new construction or renovation, you can link these vents together to minimize the number of pipes coming out of the roof. In crazy old houses, which we love, there are often many generations of pipes coming out of the roof. The really old ones are cast iron. The newer vent pipes are PVC. We needed to finish the job of this old plumber.

New vent pipe extension

The first thing we needed to do was remove the old PVC coupling so that we could add a new one. The only way to separate PVC that has been bonded is to cut it. That PVC “glue” you’ve seen isn’t actually glue. It is a solvent that dissolves the PVC a little and chemically welds the two pieces together. They cannot be separated after this has occurred. We used a small hand saw to remove the old coupling, then slid a new pipe extension over the older pipe. As you can see in the photo, we angled it up using two 45 degree elbows so that our roof hole would be a little higher up the slope of the roof.

Once we had the pipe extension connected, we figured out where the new vent pipe would need to pass through the roof using a piece of scrap wood held flush with the pipe. Where the scrap wood hit the understructure of the roof, we drilled a hole straight through to the outside. Standard practice is to drive a nail through, but my friend was heart set on using a drill.

Down the attic ladder we went, climbing up the roof ladder to find our drill bit like little kids hunting for Easter eggs. Hooray! After we located the bit, we began separating the asphalt shingles and cutting out a space for the flange to go.

The flange needs to fit snuggly around the vent pipe and should go under as many layers of shingles as possible. Furthermore, the flange should only be exposed on the bottom half (the upper half should be covered by shingles). Asphalt shingles are brittle when new, so we were extra careful not to damage any shingles that we didn’t need to. These were not new, so this took some care.

Once the shingles were prepared and some nails were removed (lift up the shingles and pull out any nails that are blocking your flange from sliding up), we used a drill to make a pilot hole, and then a jigsaw to cut out a circle roughly the size of our pipe. The hole needs to be taller than your pipe diameter, as the pipe isn’t passing through the roof at a perpendicular angle.

Now that the hole was cut, my friend ran back down the ladder and up into the attic so that we could connect the vent pipe through the roof. This can be a little tricky and required some readjustments and recuts of shingles to make it all work. Don’t sweat it. That is normal. After the pipe was through the flange and the flange was under the shingles, we used roofing nails to secure the flange to the roof.

Use roofing nails to secure the flange to the roof

But wait! We just spent all this time being careful not to damage any shingles and then we stupidly drove nails through them!?! Isn’t that more holes in the roof. Yes. It is. But that’s why they make roofing tar, quite possibly the nastiest stuff on earth. Wear old clothes because once this stuff touches your clothes, it’s over. They’re ruined. We bought a tube of tar (you need a caulk gun) and used it to reglue the upper shingles down, cover the nail heads, seal all of the edges, and patch a few other vents on the roof that had started to crack. This stuff is also great for sealing leaking gutters or any other places where water leaks.

I know it’s ugly. After applying the tar, use a piece of scrap shingle or wood to smooth it out a bit. This stuff is really nasty, so don’t use a finger or anything you plan on keeping. Thanks for reading – Robby

New roof vent, sealed with tar.

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This entry was posted in Attic, DIY, Don't Hire a Professional, House Repairs, Plumbing, Roof, Tools and Techniques and tagged , , , , , , , , . Bookmark the permalink.

12 Responses to Cutting a Hole in Your Roof – Installing a Vent Pipe – DIY

  1. mlpowell520 says:

    Thanks Robby.
    Keep em coming.

  2. good info but I would not have nailed the three bottom nails. Just tar under the plastic flap would have sufficed.

    • The Bennetts says:

      You’re probably right, Joseph. Looking back over the photos, I tend to agree. However, the flange instructions tell you to drive those three nails to secure the bottom. I’ll probably leave them out and just glue it with the tar next time. Thanks for reading.

  3. bambam says:

    iam buying shares a silicone company,,,thx

  4. Pepita says:

    Can the vent go from under the floor up the side of home? I have a metal roof. Does the vent have to go higher that roof to vent properly.

    • The Bennetts says:

      Those are good questions. I have seen vents come out of the side of houses, but I haven’t done this myself. I’d recommend contacting your local code enforcement or checking the building code online if it’s available. You could also call a plumber and ask what they’d do. Some will tell you. Some won’t. Good luck.

      • Travish says:

        2010 Uniform Plumbing Code states: “Each vent pipe…shall extend through its flashing and shall terminate vertically not less than six inches above the roof nor less than one foot from any vertical surface.”
        You can run the pipe at the exterior, but you probably should extend above the roof line.

  5. Ed H says:

    Of all the plumbing vent installations I’ve ever seen, I’ve NEVER seen a plumber put nails thru the exposed portion of the rubber boot. The whole purpose of the rubber flange is to extend UNDER the shingles at the top end and OVER the shingles at the lower end. If you nail the boot at the upper end and along the upper sides, where they’re covered by the upper row(s) of shingles, how’s it possibly going to go anywhere? And the black tar? Possibly a small amount underneath the lower end of the boot is not a bad idea, but, again, I’ve never seen a plumber use tar around a boot where it can be seen. If you cut your shingles correctly around the boot, you should never have to goop down the installation to seal it.

    • Robert says:

      Thanks for the insight. As stated, I’m an amateur who reads and follows directions on product packages and building codes. While most plumbers may not do the things I’ve done, the product manufacture recommends on the packaging exactly what I did. I think your logic is sound, and I will probably do it your way next time this comes up.

  6. Paul R says:

    I wonder about the top of the boot…seems like after a few years that rubber would harden and crack and then you’ve got a leak. I’m an amateur but would want to put tar around the neck of the boot.

  7. Pingback: Sewer Odor Experts for Roof Top Sewer Vent Stack Filters | homedesignsidea.info

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