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, 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!
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.
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.
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 is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
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