What Madison Does to a Roof in February

Nobody warns you, moving into an older place, that the weather here doesn’t just get cold, it gets cold in a way that actively goes looking for the weakest point in your roof and finds it. I found out about ice dams the hard way, in the specific way that a lot of architects find things out the hard way, which is by knowing exactly what causes the problem and still somehow not catching it in my own house before it caused one.

Here’s the mechanism, since I ended up explaining it to about six people after the fact and might as well write it down once. Heat leaks out of the attic space, warms the roof deck just enough to melt the snow sitting directly above the warm spots, that meltwater runs down to the eave, which is colder because it overhangs the heated part of the house, and refreezes there into a ridge of ice. Eventually you’ve got a dam of solid ice at the edge of the roof, and every bit of meltwater behind it has nowhere to go but backward, up under the shingles, into the sheathing, and if you’re unlucky, into your ceiling.

I was unlucky. I found the stain on my bedroom ceiling at eleven at night in February, a slow brown bloom about the size of a dinner plate, spreading while I stood there with a flashlight trying to figure out if I was actually seeing it grow in real time or just panicking. Madison winters do this to a house that isn’t perfectly insulated and vented, and mine, an older duplex I bought half of specifically because it was cheap and needed work, is very much not perfectly insulated and vented yet. That’s the whole point of buying it, eventually. It just wasn’t supposed to be the point at eleven at night with a bucket under a growing stain.

I spent the next weekend up on a ladder with a roof rake, which is exactly what it sounds like, a long-handled tool built for pulling snow off a roof edge from the ground so you’re not up there in freezing wind doing it by hand. It worked, mostly, and I’ve since added better attic insulation to the very long list of things this house needs, moved up several spots ahead of where I’d originally planned to get to it.

What gets me is that I do this professionally. I know the physics of an ice dam better than most homeowners ever will, I could draw the section detail from memory, warm attic, cold eave, thermal bridge, the whole chain of cause and effect. Knowing it in the abstract and actually catching it before it costs you a ceiling patch are, it turns out, two completely different skills. Ruth, when I told her about it at work, just laughed and said that’s basically the whole profession in one sentence, we know exactly how things fail, we just don’t always catch it happening to our own stuff until it already has.