More Building Science
I got the shingles replaced on my roof this week. I’ve needed it for a while but held off for a couple of years. Why? Because there were a couple of things I needed to do at the bottom of the house before attacking the top.
You may recall that I wrote about roof penetrations a couple of years ago. And I used my house as the poster child for the water problems they can cause. The main problem I pointed out there is that penetrations lower down on the roof are more likely to leak because more water hits them.
Of course, the most likely places to leak overall are the places where things change. That could be the penetrations for plumbing stacks, furnace flues, bath fans, and other stuff. It’s also at intersections between two roof sections or a wall and a roof.
Changing the plumbing stacks
Let’s look at some of these penetrations more carefully. The two lowest ones on my roof were plumbing stacks that ran up through exterior walls. You can see one of them in the photo below. The other was on the other side of the sunroom. Both were low on the roof. Both leaked.
But I couldn’t just remove them. Plumbing stacks serve an important purpose. And that’s where the work at the bottom of the house came in. In total, my house had four plumbing stacks penetrating through the roof. Plumbers say you always need at least one, so I just needed to figure out how to rearrange the venting so I could remove the two at the eaves.
Back in February, I had PV Heating, Cooling, & Plumbing do a bunch of work on the drains in my basement as part of my basement remodel. They also made the necessary changes to the venting.
For the vent connected to bathroom plumbing, they connected the drain to one of the two other stacks going through the roof. For the kitchen, they installed an air admittance valve (aka, a Studor vent). With that done, we were ready for the new roof.
Removing the plumbing stack penetrations
After the roofers stripped the roof, they cut the boards around the two plumbing stacks I wanted removed. Then they used a hammer to break off the cast iron pipe so that it was below the roof deck height. You can see one of them in the photo below.
(By the way, I saw that plumbing stack when we had our soffits and fascias replaced in 2019. I could see the edge of the spray foam that had been sprayed into the attic and learned an interesting lesson.)
But leaving those pipes open at the top wasn’t a good idea because at least one of them is still connected to the drain pipes in the basement. An open stack beneath the roof deck could result in sewer gas odors getting into the house, and nobody wants that! So we sealed them with PVC caps and tape, as you see below.
![Plumbing stack capped beneath roof deck [Photo by Bell Roofing]](https://www.energyvanguard.com/wp-content/uploads/2024/05/plumbing-stack-capped-beneath-roof-deck.jpeg)
But wait; there’s more!
If you compare the lead photo showing our new roof to the second photo of what it used to look like, you’ll see two other penetrations have gone missing. We removed the powered attic ventilator because, of course, there’s almost never a good reason to have one. And it was already disconnected and covered with spray foam in the attic.
Then we had the furnace flue removed because we went all-electric in 2019, shortly after we bought the house. When we bought it, the house had a gas furnace, gas water heater, and gas logs in the fireplace.
We replaced the furnace with Mitsubishi ducted air handlers in the attic and a ductless unit in the sunroom. We replaced the gas water heater with a heat pump water heater. And we just removed the gas logs in the fireplace and will get a wood-burning insert for emergency heat.
Lessons learned
Everyone knows the old adage that where there’s smoke, there’s fire. A related one holds here: Where there’s water, there are leaks. By removing penetrations from my roof, I’ve reduced the chance of leaks occurring.
And there’s a second lesson here, too: Going all-electric can make your roof less likely to leak by allowing you to remove penetrations.
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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn. Images courtesy of author.
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15 Comments
how do all of your bathroom vents exit the house? or do you rely strictly on an erv?
my vented roof looks like swiss cheese. idk how but i ended up with ~10 roof vents instead of a few larger domes, 4 bathroom vents, kitchen vent, 3 plumbing vents, gas water heater :(
Looks like his house have gable ends. Bath exhaust and kitchen exhaust vents can be vented out walls and gables, (and if done properly, out of soffits).
cs55: The two bath fans have roof penetrations to the right and bit below the old furnace flue. You can't see them in the photos I put in the article.
Unfortunately, your situation is common. When I look at roofs, I often see way more penetrations than I'd want up there.
ty, also more than i want up there. a vented attic with a hip roof, no way to send anything out of walls.
I elected to use AAVs (Studors) and only one plumbing stack vent. Bath exhaust will be tied into HRV and vents are high on walls under large roof overhangs. Direct vent gas fireplace and under slab/radon vent are also high on walls under gables. Kitchen range exhaust is with the smallest commercial roof mounted unit from Fantec. Makeup air for the kitchen is on an adjacent wall with a motorized damper.
We have a standing seam metal roof and the only 2 penetrations are enclosed in a faux chimney at the high end of the roof, covered with a steel chimney cap. I felt that was the best way to fight potential water infiltration and address the aesthetics of roof penetrations.
This is something I've been contemplating as I will likely replace my 2nd story roof in the next year or two and would like to reduce the number of plumbing penetrations. Of the 4 penetrations, 3 are in outside walls and only one of those 1 is visible from the attic as it goes up the gable side, while 2 are on the side of an eve. There are no open soffits on the eves to worry about but bathroom windows are likely too close. In zone 3a we rarely have much if any snow to worry about. For the 2 on the eve side I may add an access panel for a studor AAV right below the 2nd floor ceiling . The one by a gable could likely vent out the gable wall.
I recently had my roof completed and had the plumber tie the two vent penetrations into a single stack (they wanted two to just go straight up and through).
I also added a secondary (sacrificial) $6 rubber roof collar over the integrated boot so the UV can eat that up over the next decade. It just slid on over the PVC pipe w/ little soapy water
mordors_eye,
The less holes the better, but I understand why plumbers favour separate, straight stacks, which make them both less likely to get clogged by debris over time, and easier to unblock.
This is a great springboard! I am in the process of preparing for my roof to be redone this Fall. I will have my asphalt shingles replaced with Standg Seam Galvalume, on a simple traditional Colonial with only two dormers on the front roof face and it is a 12/12 pitch in zone 5 (New Hampshire). I want to remove my only roof penetration, which is my 3 inch Plumbing Vent Stack and I want to block and seal that and replumb that in my 3rd floor open walk-up to run horizontally to the exterior wall to a side-wall penetration. I have reviewed the code...but, I am struggling to find good examples to copy and the best way to deal with the end of the vent, which would terminate on the outside wall. I am looking for any recommendations or examples from someone else doing this.
The reason I want to eliminate the vent stack penetration is to eliminate the risk of roof leaks and vent stack damage from snow/ice melt and the challenge of roof access for my roof from ground to gable point is 42 feet. Additionally, I am considering PV arrays being installed.
Thank you in advance!
I too am looking for ways to accomplish this and it seems the code does allow for a wall exit for plumbing stacks. The code (at least what I see in GA) seems to indicate as long as the exit is protected from nesting animals, that It is not close to a window (10ft I believe) , and that it doesn't exit under a soffit vent, that should be acceptable.
I am planning for an unvented roof assembly and standing seam metal as well, so would love to have zero penetrations to leak. At a minimum, you should be able to vent through the wall and then extend past the overhang and turn the vent pipe vertical to extend above the roof.
Patrick,
You may find this discussion useful:
https://www.greenbuildingadvisor.com/question/plumbing-vents-through-the-wall-instead-of-roof-downsides
I don't know much about horizontal venting in colder climates, but it appears freeze up could be a problem.
Thank you! I have tried to do some extensive research into this "method", which presently doesn't seem to be very commonly practiced - yet.
The town I live in utilizes and does not add to IPC and Section 903, provides the requiremetns and guidence I did see that for horizontal runs, there seems to common practice to increase pipe size for termination, due to frost up cocnerns. So, my intension is to "horizontally" run the PVC in my conditioned third floor with adding a pitch to the horizontal run and then exit the sidewall with some sort of PVC flange configuration versus a boot (as would be used for a roof penetration) this way I can better flash and seal to mitigate moisture and bulk water incursion. Then, the exterior PVC run will not conflict with window opening requiremetns nor vent to a soffit or rake eves. I'll have to figure out what type vent opening and screen configuration I can some up with. I do see animal nesting being a concern.
Patrick,
I would flash and detail it much like other vent terminations. So run it through a mounting block, and use a similar screened termination to the other ones on the house.
My preference would be to slope the horizontal run back towards the vertical one, rather than plan on it possibly draining down the facade of the house.
I visited Japan for the first time last spring, and was impressed with their buildings (though there was very little new construction). Japan is a beautiful country. They seem to build things to last and one element of that is that they use mostly simple tile roofs (not a lot of dormers, angles, etc.). One thing I noticed that I don't understand that I'm hoping someone on this forum will know is how they can get away with having no roof penetrations. They use a lot of mini splits, so that can explain the lack of chimneys, but there were no plumbing penetrations either! I would think physics would work the same. And I couldn't find penetrations through sidewalls either.
kjames,
Interesting. I wonder if they simply oversize pipes to avoid the traps emptying?
I find traditional Japanese architecture beautiful too, but for more than a century Japanese houses have been seen as expendable, intentionally built for a predicted lifespan of about 25 years. From an energy standpoint they continue to be very poorly insulated, and their extensive use of concrete makes their carbon footprint quite high. Apart from aesthetics, I'm not sure there are many useful lessons there.
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