Metal chimney caps have become ever more common. They sit atop a chimney with the flues for the fireplace or furnace sticking out, and they provide a useful function.
They keep water from getting into the core of the chimney, and with their two inch minimum overhang they direct water away from the sides of the chimney, as bricks don’t really like getting soaked.
But there are times when it’s not appropriate to use a metal cap.
A metal cap has the flues sticking out, through holes cut in the cap. Sealant is used around these flues to keep water from seeping in through these holes.
The problem with sealant is that it weathers, and the seal can break. So it needs to be checked, regularly. And here is the deciding factor as I put it to clients – will you be likely to get up there and check the sealant? If not, perhaps the chimney is too high, perhaps they are too old, then you shouldn’t get a cap that requires maintenance. A cement cap can be made to be maintenance free.
Part of the problem with a cap’s sealant is that is exposed to the weather like no other sealant on your home. It’s up top, exposed to the sun all day long, and exposed to the worst of the weather. Whereas the rest of the sealant uses that you commonly see are around doors and windows, and they’re sheltered.
And with a larger cap, the flat surface of the top of the cap can collect snow which can become ice, which can accumulate to the point where it’s heavy and pushing down on the cap, stretching the bond of the sealant. I’ve found caps that have this sunken middle, where water has been funneled into the chimney. The cap pictured here had sagged, accumulated water, rusted, and then leaked into the chimney.
So, if you’re able to check it, great, it won’t crack. But if you are unlikely to check it then don’t pick one. If you’re buying a house with one, you might want to consider having it replaced with a concrete cap, or hiring someone. But people tend to forget about their chimneys until there is a problem, understandaby.