What I Found at the Peak of a Brookline Slate Roof

I was on the roof of a large early-1900s house in Brookline, standing on a flat section of rubber membrane. The house carries three different roofing materials. Beyond the parapet edge in front of me, the slate ran down and away.

In the middle of that flat section, a small square of membrane sat over an opening like a lid. Under it was a hole that ran straight through the roof deck and the ceiling below it. Not a vent. Not a skylight curb. An opening from the living space to the outside with a piece of rubber laid over the top.

The cap lifted back. The opening under it runs through the roof deck and the ceiling below, straight from the living space to the outside.

There was no snow on the roof the day I was there, so I did not see a dam. I saw what they leave behind. Staining in the eaves from previous ice damming. The membrane at the very top gone wavy and loose from moisture. This house has a clear history of ice dams.

Here is how an opening like that produces them. Warm air inside the house rises and leaves through the hole. It warms the deck around it. Snow melts there while the rest of the roof stays cold, the meltwater runs down to the eave, and it refreezes where the roof never warmed up. Then the ice backs water up under the roofing.

Where the heat leaves and where the ice forms at the eave below. Drawn for this roof.

The slate had nothing to do with it. That slate is about a hundred years old and most of it is in fair shape. I see the same pattern anywhere the shape of a roof gets interrupted: a cupola, a turret, a flat insert dropped into a pitched roof. The break in the shape is where the heat gets out.

So when I am looking at a roof like this one and the staining is concentrated near one feature instead of spread along the whole eave, I do not start with the roofing. I get into the space under that flat section and I look at what is sealed and what is insulated. That is where it gets fixed.

Here is my opinion, and it is why I bothered writing this down. Most people who get an ice dam are told they need a new roof or some kind of exterior "correction"... some people are even told the gutters are the problem and just remove them. On a house shaped like this one, a new roof would not have changed anything. The opening is still there, and every winter it runs the whole show. Look at the shape of a roof before you spend money on what is covering it.

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