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Which Roof Penetrations Cause Water Damage?

Penetrations Through The Roof Are Inevitable, But Where They're Located Can Make A Big Difference

Which roof penetrations cause water damage?  They all do, of course.  But some are more likely to leak than others.  The photo above shows the back of my house.  We got a tiny bit of snow on Sunday, and some of it even stuck around for a few hours.  With the snow there, you can see the penetrations more easily.  There are five of them:  a chimney, furnace flue, and powered attic ventilator near the ridge (disconnected!), the kitchen range hood midway down, and a plumbing soil stack down near the eave.  One of those is far worse than the others.

Low roof penetrations are more likely to leak

The zoomed in photo below should make it clear to you which one I’m talking about.  The soil stack comes up through the exterior wall to the right of the kitchen window.  And look at that brick wall!  That discoloration just appeared a couple of weeks ago when we had about 6 inches of rain in 36 hours.  Ugh!

Low roof penetrations cause water damage
Low roof penetrations cause water damage because more water hits them than hits high roof penetrations. Note the discolored area on the brick wall.

So, of the five penetrations showing on my roof, the soil stack is most likely to leak…and it is indeed the one that’s leaking.  Why?  Because it’s lower on the roof.  Lower parts of the roof get a lot more water.  At the ridge, the only water a penetration gets is the water that falls right there.  The three vents on my ridge are therefore less likely to leak.  The soil stack, at the bottom edge of the roof, gets all the water that lands on the whole slope above it.

Double your pain

Unfortunately, that soil stack at the kitchen isn’t the only stupid roof penetration they put in my house.  The photo below shows a second soil stack at eave above the bathroom.  And guess what?  That 6 inch rain caused the flashing to fail there, too.

The bricks beneath another low roof penetration also show signs of water damage.
The bricks beneath another low roof penetration also show signs of water damage.

Here’s the drywall just below that soil stack roof penetration.  It’s expanding out over the crown molding right where the pipe goes through the exterior wall because of the water that it has absorbed.

The drywall inside has swollen where the plumbing soil stack goes and penetrates the lower part of the roof
The drywall inside has swollen where the plumbing soil stack goes and penetrates the lower part of the roof.

So, I’ve got two leaky roof penetrations to deal with now.  But having these plumbing vents near the eave isn’t the only problem.  The pipes that you see coming up through the roof go through exterior walls.  That means they’re taking space away from insulation, too.

I’ll have to fix the leaks soon.  I’ll call a roofing company to repair and reflash the penetrations for now, but I really want a longer-term solution.  I’ll first need to relocate the soil stacks in the basement and crawl space.  This is the year I gut and remodel the basement, so that’ll be part of the process.  The hard part will be finding places to take them up through the first floor and into the attic.

A note about other roof penetrations

For those of you who know this blog and what I preach, you may be wondering about the roof penetrations for the powered attic ventilator and the furnace flue.  We have a spray foam insulated attic, so I’ve disconnected and sealed the powered attic ventilator with foam.  And we have an all-electric house with heat pumps now, so I’ve also disconnected and sealed the furnace flue.

On the bedrooms side of the house, we also have roof penetrations for two bathroom exhaust fans and two more soil stacks.  Three of those four are close to the ridge, and one of the soil stacks is midway up the slope.  When I get my new Zehnder ERV installed this year, I’ll monitor indoor conditions and maybe remove the bath fans when we get a new roof.  That would leave me with four fewer penetrations after I remove the furnace flue and powered attic ventilator.

Let my pain be a lesson to you.  All roof penetrations cause water damage when the flashing fails, but low roof penetrations are more likely to fail because they have to deal with more water.  Put all roof penetrations as high up on the roof as you can to minimize problems.  And of course, minimize the number of roof penetrations, too.


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 writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.


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This Post Has 43 Comments

  1. I am going to reroof this summer. My roof has the traditional number of drain line roof vents – one vent for each drain/water trap. Plus vents for natural gas furnace and water heater. Plus bathroom exhaust fans. Is it possible to combine some of the plumbing vents to reduce the number of roof penetrations? Any plumbers out there?

    1. Thomas, per current IPC here in GA (we’re at 2018), a single 3″ vent will be more than enough for most houses, and 2″ or even 1 1/2″ for many. To be sure, call you local building inspections to confirm they don’t want something specific. As you combine the vents in the attic, make sure everything is sloped, do not reduce pipe size upstream, and use proper fittings. Obvious main goal: you don’t want to end up with rain water standing inside the vent pipe somewhere and blocking it (and possibly freezing, depending where you are). And while you’re up there, at a minimum seal all the vent ceiling penetrations.

  2. I have been waiting for an article like this. I have several comments.

    It would be interesting to see a close-up of this vent installation to see if it was flashed and shingled properly. I have seen many roofers who do not know what part of the flashing should be under the shingles and what part should be over the shingles. I have even seen new houses where they did some of them right and some wrong on the same roof. I just can’t imagine a properly flashed plumbing vent leaking no matter where it is located or how much it rained.

    When I worked for a builder several decades ago in the north. We manifolded all of the plumbing vents together in the attic so that we would only have one roof penetration with a 4″ pipe, typically near the peak. But where I have lived in the south, I notice that they do run many separate smaller vents which just doesn’t make sense, especially when you have an open attic. Is this a code issue?

    1. Roy: I agree that if it’s flashed and shingled properly it should last as long as it can last. But how long is that? The quality of the materials matters. And the amount of water hitting it should matter, too. It’s like a light switch. The more you use it, the shorter its lifetime.

      I don’t know if it’s a code issue, but I wouldn’t think so. When the house was built in 1961, they probably could do whatever they wanted as long as they vented the drain lines.

      I like the idea of combining the soil stacks, but I still want to get those two pipes out of the exterior walls.

    2. Many roofers do a 2 to 5 year install. Most laminate shingles are 50 products. For plumbing penetration to last 50 years it must be installed with ice and water target patch installed on top of underlayment. The pipe needs sealed to that ice and water with a heavy bead of roofing cement or epdm wrap. The shingle with head that passes around pipe set in roofing cement. The bottom of flash need the set in a bed of roofing cement. Then the shingles on top of that flashing shall be set in roofing cement. Recommend a sheet metal storm collar set above flashing to protect gasket below. Then every roof needs walked year to ensure flashing have not raised and caulk replaced. That is a leak proof 50 year detail

      1. I like the idea of the secondary storm collar. I’m a believer of …. “a properly flashed roof penetration won’t leak”.

        However, sometimes the product itself fails over time. I have had several minor leaks arising at pipe vent penetrations, but never due to bad flashing or shingling. What has happened, is that the rubber boot cracks (perhaps from UV rays , or just natural age limitations) and a strong wind-driven rain will eventually come through the cracked boot. My “solution” has always been the same and has always worked. I lay fiberglass tape mesh (of the drywall taping variety) into a bed of roofing cement over the boot, then cover (and smooth out) a 2-nd layer of roofing cement on top. I’d bet that a storm collar would have prevented the deterioration of these boots.

  3. Your company helped with HVAC of our house that we built in 2016 I worked with Alex Bell I try to keep up with your blog and building science. Somethings I would like to do differently, but can’t change now. and I probably won’t build another house. But one of the things that we did right was the only penetrations in my roof is the ridge vent. I built a gable end house so we put all the pentation’s through the walls or gable ends. My plumber was hesitant but when I contacted the state of Wisconsin they said I did not need a variance that passed the Wisconsin plumbing code. very glad we did it with no roof penetration’s. Am waiting to get your new book and to share copies that I bought as gifts.

  4. What do you think of the one piece pre-welded metal boots (like at to get rid of as many leakable joints as possible?
    Maybe I’m weird, but I actually preferred an asphalt roof over a “lifetime” metal roof so I can check all this type of flashing sooner than later.
    Because I have a Zehnder my only penetrations are plumbing stacks, and a radon vent.

  5. Great article. I strongly agree that thoughtful consideration of roof penetrations is essential! It might not be obvious to non-professionals such as myself how to follow the advice of “minimize the number of roof penetrations.” Perhaps you can weigh in on other ways to do this, but I am aware of just three.

    First, instead of going through the roof, go through gable walls. This solution works great for bath and kitchen exhaust fans. This will typically lengthen the duct run, however, so it is definitely best to use hard duct to maximize air flow.

    Second, when designing or altering a plumbing system, use a “horizontal wet vent” design. This is an old-school approach to combining plumbing vents in the basement or crawlspace so that you have only a single (larger) plumbing vent stack. For example, I have a “Jack and Jill” bathroom (two bathrooms side-by-side, connected by a door) that use this design. There are two sinks, two toilets, and one shower that all use the same vent stack. This results in only a single roof penetration. Here is a link describing this strategy:

    Third, whenever you hire other people to do work on your house, make sure that you are both on the same page regarding roof penetrations. For example, I recently hired an electrician to upgrade my home to 200A service. He assumed, without consulting me, that he would run the new service entrance through a new roof penetration rather than the gable wall. This was unnecessary but easier for him to do. Fortunately, I walked through each step of how I wanted to job done and specifically discussed a new service entrance mast should be installed on the gable wall and NOT the roof. Similar issues can arise when any service people that run wires are working on your house: watch out for the cable company and satellite installations. They may not only make an unnecessary penetration in the roof, but they may do it poorly because they are not really roofing professionals!

    One other bit of advice on roof penetrations: when hiring a company to reshingle a roof, ensure that you are on the same page with pipe flashing. I’ve heard of some roofing companies saving a few dollars by re-using old vent boots instead of using the opportunity to replace them. Perhaps more likely is that they will purchase new vent boots, but use the cheapest ones available. The difference in cost between the cheapest vent boots and the highest-quality boots is nothing compared to the cost of repairing water damage from a leak! Allison: a blog post on choosing this type of building product would be great, IMO. I am partial to old-school lead flashing instead of plastic or metal, but maybe my preference is misplaced and there are modern replacements that are even better than lead.

  6. Allison, Good article & good timing for me. my daughter-in-law is replacing her roof due to Florida’s new law that allows insurance companies to cancel/deny insurance for roofs 15 yrs/older (ignorant of higher quality shingles). She’s also planning to refinish her master bath which has chronic mold problems as there’s no vent … so we were going to have new roof vent added. Is it better to connect new vent to the one in the other/adjacent bath so we’re not adding another roof vent?

    1. Does this Florida law allows insurance companies to cancel/deny insurance for roofs that are presently over 15 years old or for a roof that is installed today? If it is for a roof that is installed today, this would be a major factor in the economics of solar panel installation? If a roof is good for 30 years and panels for a similar (or better) duration and both are put on at same time it does not make sense to have to choose between insurance coverage and remove and replace.

    2. Tina, I encourage you and your daughter to look into installing an IBHS FORTIFIED Roof ( Florida is one of the states in which insurance companies reward resilient building strategies and offer lower insurance rates for homes with verified FORTIFIED Roofs. The insurance savings will often offset any additional charges for compliance, and the roof will withstand high winds and heavy rains much more effectively. This program is really a reflection of best practices.

  7. I spent many years in coastal Alaska where 2 feet of wet snow could slide off the roof and take out stacks near the eave. Since then I have a conscious respect on where stacks and vents should be located. I am working on a home design for attainable housing so maintenance is a concern. This design has a vertical exterior wall as part of the attic and above the attached garage roof. My plan would be to take the vent stacks, bathroom vents and range hood vent out this vertical wall (all electric so no flue). Leaving an unobstructed single plane shed roof over the home. Are there any issues with taking these vents out a side wall as opposed to through the roof? This design is in the South West where it only rains a few inches a year however it all comes at once.

    1. To my knowledge you can vent out the side wall, but there are certain code considerations to be aware of. I’m sure the specific requirements vary on this from place to place, so I’d recommend finding out what your jurisdiction requires rather than take generic internet advice. With that said, in general you’ll need to make sure that the vents are a sufficient distance away from any windows and doors (check code for specific distance), don’t install the vents in a way that would let gasses into the attic (if you have one), and be sure that the vents cannot freeze up with water or condensation (if applicable to your climate).

  8. My current house has the bathroom exhaust fans vented out the soffits, under the overhang. That seems better than venting through the roof. Anyone see any downside to that?

    1. It’s terrible if you have soffit vents nearby. The moisture gets sucked right back into the attics. This is unfortunately a common building defect.

      1. Well, it may get sucked into soffit vents. When I remodeled the bathroom in the condo I lived in back in 2016, I vented the bath fan right under the soffit. You can read about it in the article below:

        Installing an Exhaust Fan During a Bathroom Remodel

        Scroll down to the third photo to see what it looked like. Some commenters on the article said it could create problems with moisture, but I’m pretty sure it never has. Here’s what I wrote about it in that article:

        I know what some of you are thinking. Exhausting below the soffit is a terrible idea because moisture-laden air will be drawn back into the soffit through that vent you see. Two things. First, I really had no other choice here. It was either vent there or don’t install a fan. I chose to vent below the soffit. Second, it shouldn’t ever cause a problem. The exit velocity is high enough that the air moves the leaves on the hedges 10 feet below. Little of that air will make its way back to the soffit. It’ll be interesting to see if we get a spot of frost on the hedges on cold winter days.

      2. Cindi, I have to side with Allison on this one. Please provide a study, if you have one, showing that it’s terrible, it would be interesting to see. Under certain ideal conditions: just the right dew points outside and inside the attic, wind, static pressure, delta T, etc… maybe. And we are talking about a bath fan, 20min once or a few times per day. And we would need those ideal conditions every single of those times. As Allison mentioned, the exit velocity from the vent will probably spoil those ideal conditions most of the time. We are transferring a confined stream of moist air (some droplets, mostly water vapor) into a vast open system, aka outdoors. Water vapor diffusion, aided by the exit velocity, is probably happening quite quickly.

        1. My information comes from professionals who inspect for mold who say they find mold at those locations, but I don’t know the details of the homes they find it in.

    2. Bath fans that vent near or out a soffit is a dangerous situation. The moisture that is vented out the bathroom is allowed to enter attic through the intake vent in the soffit. This can cause wood rot and mold in attic. It is also against code.

      1. Rollo, it’s not against code. If you’re referring to “M1501.1 Outdoor Discharge”, this part of residential code refers to discharging _into_ a soffit, as in an HVAC smart aleck hooking up the duct to the bath fan and then just laying it down on the attic floor pointing the end of it into the soffit. This does not refer to discharging _through_ the soffit.

  9. Good point. I have an unvented attic (foam insulated roof deck), so I didn’t think about that issue when you have a vented attic.

    I grew up in a house where the bathroom exhaust fans just dumped into the vented attic. There wasn’t any problems with that until the second time it was re-shingled and the sheathing had to be replaced above those vents 😉

  10. I recently replaced my natural gas furnace with a heat pump. The furnace flue was sealed off at the bottom of the pipe – did you seal yours up near the roof, or just where it used to connect to the furnace? I would love to remove the roof penetration entirely, to make room for more solar panels, but maybe that should wait until it’s time to replace the whole roof?

  11. I am going to say” yes…but”. I was in my attic a couple of months ago getting Christmas decorations (and yes, I still use the attic for storage (small house, lots of stuff) and there was a large puddle on top of one of the plastic bins. Sure enough, I got a look at the roof (17 years old, architectural shingles with no wear) and the ridge shingles were completely worn through. Even the roofer who did the original roof and repairs was surprised. Totally worn out. So, you are right about the rain loads being greater on the lower sections – and I would never have expected that much leakage from the ridge, but it was there. One can never say “never” when looking at homes.

  12. I am surprised I didn’t see any mention of the spring-loaded vent caps. I had a new roof installed last summer. I personally cut the stacks low to the attic floor and patched the roof deck. The kitchen sink and laundry utility stack each alone by themselves shouldn’t require more than 2.5″ pipe. I left enough pipe showing to provide ease of access and attic space has enough volume with a 6/12 pitch.

    1. Bill, please correct me if I understand you incorrectly, but if you’re talking about cutting down the plumbing vents to the attic floor level and leaving them venting into the attic (you said “enough volume”), then this is not allowed by any code, nor common sense. Are you sure you want methane and a mix of “natural” odors to be inhabiting your attic space? I hope I am misunderstanding…

      1. Im saying when I wash dishes and do a load of laundry the volume of water is never greater than the attic space. The spring-loaded cap allows for vacuum release. The Bath vent stack is still in place at the other end of the house. And that is 4″.

        1. Bill, I see, I think you’re referring to air admittance valves? Aka, Studor (R) vent. I guess theoretically they would be allowed in the attic, pending your AHJ’s mood, and leaving at least one vent stack through the roof, per code. In practice, I would not put them in there. They rely on a rubber seal, and the temperature swings (from sub 32F to 140F+, depending where you are, and how the attic is constructed/situated) might not be so friendly to it. If you have any fibrous insulation up there, the dust would also not make the valves happy. Going into the attic once a year to verify that the valves are still working correctly would be awkward and unreliable. Glad you joined the conversation, these are interesting topics.

  13. To Kim, Cindi, and Allison: Exhaust vents under the eaves. Was Coastal South East Alaska, now South Central.
    The problem with under-the-eaves, for my old house (1978 is old on the Kenai Peninsula & before codes) is kitchen fan heats the lower edge of the roof, although not used as much. The dryer is at porch deck level, but does the same, as that stream of warm moist rises to the porch overhang, and Bath fan is above it. Causes icing at the roof edge, for about 3 feet back, and the snow can’t slide off, while it does everywhere else. Snow rake isn’t practical, the one part of the roof that isn’t true. The good part is the porch roof edge is out from the eaves of the house about 8 feet, so ice dams don’t affect the house directly. That porch roof is an addition by a previous owner, a DYI snowbird from Arizona.
    Different issue: By “winterizing” the house and leaving, his first winter, sliding snow took out every roof penetration, as it built up. Yet another reason to let snow slide off.

    1. Classic ice damming. They are ubiquitous in my climate, and everyone builds cold-roofs if they have unvented roofs.
      I live in a very snowy place, but very sunny. Joe Lstiburek recommended I insulated my eaves because so much ice damming comes from sun heating the siding (especially dark siding), the heat rising to the underside of the eave and causing ice dams. We redesigned our eave design and are insulating the S and W side. The idea is by the time the air gets to the end of the 42″ overhang where the vent comes in, it will have mixed with enough cold air to keep the “cold roof” intact.

  14. Very helpful article, Allison; I never thought about roof location of pipe penetrations.
    Allison, do you have any reference material about proper flashing of pipes going through the roof ?

  15. Allison, do you have open cell or closed cell foam in your attic? Would either application change the end result here?

    1. Grant: I have open-cell spray foam in my attic. It makes absolutely no difference about what happened here. With closed-cell, water from leaks can travel farther down the slope, but this was at the eave, where the foam ends, so it would have done about the same thing, I believe.

  16. Hi Dr. Bailes,
    I just wanted to emphasis the comment from Leif in Alaska. Snow slide in our climate (Maine) would make a nightmare of those low pipes. Even ones half way up or more are susceptible. I’ve seen snow push chimneys over. Though I must add, putting a bunch of solar collectors right around those pipes will likely lessen the push. In fact, just today I was up on the metal roof clearing the snow off the panels. Yeah, the downside of being this far north.

  17. Just remembered an article I read about roofs in Germany, can’t recall what the source was. The gist of it: builders in the USofA look at the choice of roofing materials and try to decide between 25, 30, or maybe the so-called lifetime warranty. Builders in Germany have a dilemma: their choices are 200 or 300-year warranty. I am paraphrasing the article, but this pretty much sums up what historically have been the practices on the opposing sides of the big pond.

  18. One thing my roofer did when we re-roofed our house 2 years ago was add an additional rubber seal on the top of the rubber seal on the cast iron vent stack flashing. The roofers add the extra seal over the top of the primary flashing seal to protect the primary rubber seal from UV damage. It is much easier to replace the secondary seal too when it does eventually fail. He said that he has had great success with this method and greatly reduces leaks from these type of roof penetrations. It was one of the reasons I chose them because they cared about having practical redundancies. Ice & Water shield is not installed in valleys much in my area but he has learned that it just takes 1 event to make it worth it. Not even a year later it passed with flying colors during the historic Texas freeze. Other neighbors with similar roofs had some ice damming that resulted in some drips inside the living space but my house’s valleys stayed dry.

  19. Thanks, Allison. Excellent.
    Kurt: Thanks. Your ideas/comments appreciated.
    Unable to snow-rake due age & disability. Don’t bother to suggest high school students! This is rural.
    Three comments
    One. At short overhang (16″) my intention is to use foam-board between the joists, on south side, and selected places on North side.
    Two. The clothes dryer and the bath exhaust fan causes ice to form up the N. roof to 6-8 feet up. Since the clothes dryer exhaust is at porch level and the snow that slides off reaches 4-6 feet above the underside of the porch, the only choice is to rout the exhaust up to the porch cover, then out. That means two 90-degree angles. Not ideal. Will insulate the pipe, too.
    Three. The wood-stove chimney (principle heat, S side) backs up the snow (not as a heat source, but as a physical obstacle, even with a cricket. This year, the weight of the snow bent the chimney supports (attached to the ridge.) Have to invent a better way to divert the snow at the chimney (4 feet up from the roof line). EV panels not practical
    Bizzare winter with rain on the snow, worse break-up.

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