Ice dams and winter leaks

Ice dams are the reason most Kalamazoo homeowners eventually ring a roofer, and they are the reason a lot of them ring twice, because the first fix often addressed the wrong thing.

What is actually happening

Snow on a roof is an insulator. Underneath it, the roof deck is being warmed from below by heat escaping out of the house into the attic: through the hatch, through recessed light housings, around plumbing stacks, over the top plates of interior walls and through ductwork that was never sealed. The deck over the heated part of the house rises above freezing. The snow touching it melts.

That meltwater runs downhill under the snow layer until it reaches the part of the roof that hangs beyond the exterior wall. The overhang has cold air on both sides and nothing warming it, so it sits at outdoor temperature. The water stops being water there and becomes ice.

Repeat for a week and you have a ridge of ice along the eave with meltwater ponded behind it. Asphalt shingles are designed to shed water running downhill. They are not designed to hold back a pool sitting on them, so the water works its way up under the courses, through the nail holes, onto the decking and into the ceiling below. The icicles everyone photographs are a symptom, and the leak is usually two feet inside the wall where nobody sees it until the stain appears.

Why this county in particular

Lake effect means a lot of snow that arrives in quantity and stays. Southwest Michigan also swings across freezing repeatedly through a winter rather than staying hard frozen, and it is the swing that builds dams. A colder, drier climate can have more snow and fewer dams.

The housing helps too. A great deal of Kalamazoo County stock predates modern air sealing standards, has had insulation topped up without attention to ventilation, and carries additions with complicated roof geometry where a warm valley meets a cold one.

What fixes it, in order

Stop the heat getting into the attic. Air sealing is the actual cure and it is usually the cheapest part. Seal the hatch, the light housings, the plumbing penetrations and the wall top plates.

Then insulate properly. Insulating without sealing can make things worse, because the attic gets colder while the same warm moist air keeps arriving, which increases condensation.

Then ventilate. Intake at the soffits and exhaust at the ridge, in proportion, so the attic runs close to outside temperature. Blocked soffits are extremely common where insulation was topped up without baffles, and a ridge vent laid over a deck that was never slotted does nothing at all.

And when the roof is next replaced, put ice and water membrane at the eaves. It does not prevent the dam. It means that when a dam forms anyway, the water meets a sealed surface rather than a seam. Ask how far up the slope it runs, because the requirement is measured from the inside face of the exterior wall, and a shallow pitch needs it carried further.

Enquiries made here go to a single independent roofing business covering southwest Michigan. Soffit, fascia and edge rot covers what happens when dams are left alone, and tear off and reroof covers the larger job.