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Making a Roof Less Likely to Leak

Removing Penetrations Can Make A Roof Less Likely To Leak

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

But my focus here is on roof penetrations.  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.

Roof penetrations before removal during reroofing
Roof penetrations before removal during reroofing

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.

Plumbing stack pipe cut down below roof deck
Plumbing stack pipe cut down below roof deck

(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]
Plumbing stack capped beneath roof deck

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.


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.


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This Post Has 35 Comments

  1. The biggest problem that I have seen with roof penetrations is that many roofers do not know how to properly flash them.

    1. Roy: Proper flashing is a problem everywhere, not just at roof penetrations. I had some discussions with my roofers about that, and we agreed to disagree. I’m going to be watching closely and will take advantage of the warranty if it leaks.

      1. Roofs that intersect walls and fireplaces are the biggest problems that I have seen. The principle behind flashing is quite simple: water flows downhill. But many roofers take short cuts.

        1. Roy, it flows downhill, except in the snow regions and wind-driven rain.

          Well executed roof penetration flashing, in my experience, lasts longer than the surrounding shingles.

          I also recall reading a (white?) paper a while back talking about moisture levels rising dramatically underneath asphalt composition shingles after long wetting. I’ll need to be the Eeyore here and say: most of the time we’re lucky asphalt shingles keep the house dry. In the big picture, they’re just a horrible material, filling up the landfills every 10-25 years, depending on the homeowner’s eagerness to replace.

  2. Hopefully your roofing contractor used a roofing membrane rather than the 20lb paper that’s still so common in the industry. We had to replace our roof after a hail storm and upgraded to the newer class 4 impact resistant shingles as well as the peal and stick underpayment and water and ice shield in the valleys and the roof edges and under the solar array because of the roof penetrations to attach the rooftop solar system. We also removed our powered attic fan and had then redeck that area thanks to your guidance.

    1. David: I had a roof replaced about 15 years ago due to hail damage, but the insurance company wouldn’t pay the extra cost for impact-resistant shingles. They would give me a discount on future premiums if I had them installed at my cost, but it would not even come close to paying back over the normal life of the shingles. Were your shingles covered by insurance and were they willing to pay for the higher grade ones?

      1. Sadly no the insurance company didn’t cover the extra cost for the shingles. We paid the cost out of pocket for the shingles along with several other upgrades. This is the 4th roof to be installed on this house that was built in 1980. I wanted to insure that if at all possible it’s the last one that I’ll have to pay for.
        If I could have I’d have installed a metal roof but the price differential and the battle with the neighborhood association over dead restrictions prevented that.
        The shingle roofs in the gulf coast area just come apart in the excessive heat and hurricane winds we get.
        When I was still working we had a tile roof installed on the house we had built. That was 25 years ago and it still looks new. The current owner has never had a problem with it and shouldn’t for at least another 25-50 years.

      2. Roy, to be clear the insurance company did pay to have a new roof installed, minus the deductible. They also paid to have the solar array removed and replaced but we covered the cost of the shingle upgrade and the upgraded underpayment.

  3. I hope that the roofers used a lead boot for any remaining plumbing stacks. The lead boots last decades whereas the rubber ones in your picture last about six years at the outside. Then, they leak! This was told to me by the president of a major manufacturer of those rubber boots. The only reason not to use lead boots is if your region has squirrels who like to chew the lead ones. There are other modern alternatives to lead which still long outlast the rubber boots.

  4. Another good story. Curious, while choosing the new shingles, have you considered their solar reflectance and thermal emittance?

    When they used a hammer to break off the cast iron pipe, hopefully no chunks fell into the pipe.

  5. Allison,
    Now that contemporary and/or modern single family houses have become much more abundant in the Atlanta, Georgia Metropolitan Area, as well as throughout the USA, please devote one or more energy vanguard articles to “Making a FLAT Roof Less Likely to Leak”.
    Thank you.

  6. Quoting ….. “We removed the powered attic ventilator because, of course, there’s almost never a good reason to have one. ”

    Perhaps I missed the reason for this conclusion in your previous articles, but can you briefly provide why? Attics can get VERY hot and humid in the summer months and a temperature and humidity controlled roof ventilator can surely bring that down.

    1. I’m curious to what Allison says, but I did some analysis years ago with an average blown-in insulation attic and found it better to spend the watts at the air condition side rather than spend it at the attic blower. In other words, the BTUs saved / blower watt (cooler attic) was a lot less than the BTUs / AC watt. We never ran those blowers again.

    2. This topic has a lengthy history on this blog with a lot of lively discussion in the comments. Not to put words in Allison’s mouth, but I would sum it up like this:

      1). If the envelope of the spaces below the attic is not sealed properly, then the powered attic ventilators are going to be pulling conditioned air up out of the house into the attic. This WILL keep the attic cooler, but it would be at the expense of the system(s) cooling the spaces below.
      2) If the attic NEEDS to be cooled, then it would be better to have insulation at the roof deck and bring the attic inside the conditioned envelope.
      3) If the attic DOESN’T NEED to be cooled, then it would better to make sure the ceiling of the spaces below is sealed as airtight as possible and sufficiently insulated from the attic. If that is done, then it won’t matter how hot (or cold) the attic gets in the first place.

      Where this really gets into the weeds is when the discussion turns to the possible effect of attic heat on the life of the shingles and weighing the first costs vs the lifetime cost savings of insulation, conditioning, etc.

      1. Wesley, Thank you.

        Assuming one of the lower CMF (100 to 200 CFM) powered temperature/humidity controlled roof ventilator fans ………………… how about a situation where it’s an vented attic (with plenty of locations for incoming air (soffit, dual gable vents, ridge vent) so even if the attic floor isn’t terribly well sealed, little feed-air should be drawn from the blow-attic spaces?

        1. Paul, you’re right – the heat transfer between the attic and the living space below absolutely depends upon the temperature in the attic and upon the delta T between the attic and the living space. I was too casual and imprecise in saying that the attic temperature “won’t matter”.

          To sum up the previous blog post Allison linked, sealing and insulating the ceiling is a superior choice to adding powered attic ventilators. If the ceiling IS properly sealed and sufficiently insulated, then the air conditioning system serving the living space below will be much better able to handle the heat load coming through the ceiling from the attic. I know nothing about residential heat load calculations, but the heat load coming from the attic would have already been considered when sizing the air conditioning system serving the living space below, right? Powered attic ventilators won’t be needed.

          If, on the other hand, powered attic ventilators are added WITHOUT properly sealing and insulating the ceiling, they will simply be cooling the attic at least in part by sucking conditioned air from the space below. In this case, the attic temperature will be lower, the delta T between the attic and the living space will be lower, and the amount of heat transferred from the attic to the living space through the ceiling will be reduced. But this will be accomplished by pulling conditioned air from below, which ultimately won’t save any energy.

          If we WANT to condition the attic, then that’s OK too. In that case, the attic should be brought into the conditioned envelope of the house and a properly sized air conditioning system should be added to serve the attic.

    3. His attic is conditioned (Spray foam in the rafters) so there’s no air for the fan to pull.

      The problem with attic fans is that they can pull conditioned air into the attic and as long as your have sufficient insulation on the attic floor it doesn’t matter how hot the attic gets.

      1. JC, of course it matters how hot the attic gets. Refer to ∆T in the conductive heat transfer.

        This blog will be viewed by hundreds, maybe thousands of people who after reading over and over again that it does not matter as long as there is “proper” insulation might form a belief that it indeed does not matter. That would be unfortunate.

    4. Barry: Below are the articles I’ve written about powered attic ventilators. They always stir up a lot of comments, so take a look there, too.

      Don’t Let Your Attic Suck – Power Attic Ventilators Are a Bad Idea

      The #1 Reason Power Attic Ventilators Don’t Help

      Power Attic Ventilators Banned by New Georgia Energy Code

      Is It Ever Helpful to Use a Powered Attic Ventilator?

  7. Allison,

    I am wondering if you demolished the chimney all the way down to the basement or if you just removed the roof penetration portion and if you left it did you just spray foam it as well?

    I am getting ready to do a gut renovation on a 100 yr old house in Massachusetts so would love to follow your example and remove the extra roof pentration.

    1. Brian: I assume you mean the plumbing vents, not the chimney. We haven’t removed the chimney, as you can see in the photo. So, to answer the question about plumbing stacks, no, we didn’t remove them all the way down…yet. When we remodel the kitchen, we’ll do that one. Then we’ll have to do some work in the bedroom and open the wall there to take that one down. But it’s on the agenda of future improvements.

      If you look the section titled “Removing the plumbing stack penetrations,” you’ll see how we handled the pipe left beneath the roof deck.

  8. Unfortunately those who roof mount their solar panels learn this the hard way. To add insult to injury the panels are sometimes installed on shingles approaching their useful life. Soon the solar panels and related wiring must be taken done then reinstalled. My advice to homeowners is either ground mount your solar panels or don’t install them at all.

    1. John, not sure if it is direct experience that you’re speaking of, but from what I have seen, panel rack mounts come with superior flashing that is on a whole different level compared to anything else on the traditional roof. If the shingles could speak, they would tell you that they thought the rack mount flashing was just another shingle.

      Any respectable solar installer will let you know that your shingles are approaching their useful life. Your advice to ground mount or else is, unfortunately, ill-conceived.

      1. Paul, there are roof rail supports like you described that if flawlessly installed will not leak. However, solar installation is a predatory industry and low bids often prevail resulting in substandard materials used and poor workmanship. Beyond the roof penetrations, there are other considerations for ground mounted such as optimal orientation, ease of panel replacement, cleaning, and snow removal in northern climates. We were the first residence in Ohio to deploy solar panels (Sharp 175w) in two ground mounted arrays. BSEE and general contractor.

        1. John, very impressive. If you’re lucky to have the right space, otherwise, roofs are still the default in Anytown, USA, despite non-optimal azimuths, tilt angles, possible roof dead load issues etc. Roof mounts have been nothing short of phenomenal since the mid 2010’s. I’d rather remain optimistic that the solar installation industry will strive for improvement, and educate consumers about their installation options and best BOS. This is the only way. HOA’s make it hard enough.

          When Allison mentioned minimizing roof penetrations it surely encouraged comments such as yours, and yet I sleep soundly not thinking about the dozen and a half roof mounts on my asphalt composition shingle roof (in HOA territory).

  9. Allison, Are you planning to add rooftop solar? If not, why not? If so, what will you do to ensure any penetrations associated with it will not result in leakage?

    1. John: Yes, I’d like to do that now that we have a new roof. But first I need to finish the basement renovation. I’m hoping to have it done in the next five years. I haven’t looked into attachment systems yet, but I think there are some good ones that minimize the chance of leaks. Of course, they have to be installed properly and that’s often the rub.

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