Ice Dams and Attic Mold in Cleveland's Snow Belt

How does a Cleveland roof make its own leak?

An ice dam is heat loss with a weather forecast attached. It builds in a sequence.

Snow falls and stays, because the temperature holds below freezing for a stretch. That is an ordinary December through February around here.

Heat leaks out of the house into the attic. A pre-war attic with a few inches of tired insulation between the joists loses a great deal of it. So does a bungalow with knee wall doors opening straight into an unconditioned triangle. So does a Cape Cod where the slope is insulated and the flat above it never was. Uninsulated ducts, recessed lights, an open chimney chase and a bath fan venting into the attic all add to it.

The deck warms and the snow touching it melts from below. That meltwater runs downhill under the snow blanket.

At the eave the roof runs out of heated space. The overhang is as cold as the outside air, the water refreezes there and a ridge of ice builds along the edge and in the gutter.

The next day's meltwater backs up behind that ridge and stands on the shingles. Shingles shed running water. They are not a membrane and they cannot hold a pond, so the water finds the laps and travels under them, onto the sheathing, into the top plate and sometimes down inside the wall cavity.

Valleys and the north side make it worse. Two roof planes meeting in a valley funnel snow and meltwater into one line, so a dormer or an addition that creates a low valley over a heated room is where dams form first. So is any stretch of eave over a room with a poorly sealed ceiling, which is why a bathroom under the eave is such a reliable trouble spot.

Why nobody notices until a stain shows

The wet part of this assembly sits outside the part of the house you breathe. A vented attic is connected to the outdoors, so the smell of damp sheathing leaves through the ridge instead of drifting into the hallway. Nobody wakes up to a musty attic.

That is why the evidence is visual and late. A brown ring on the ceiling a foot or two in from an exterior wall. Paint blistering or a crack opening along the top of a wall. Nail heads rusting through drywall in an upstairs bedroom. A damp patch above a window on the north side. Water dripping from a light fixture during a February thaw, which people report as a roof leak even though the roof is intact.

Plenty of snow belt houses carry attic staining and have never marked a ceiling at all. The wet stayed in the top plate and the sheathing, dried out in April and came back the next winter.

What the sheathing looks like when you open the hatch

Go up on a cold clear day with a flashlight and look at the north facing slope first. North slopes take no winter sun, stay cold longest and dry slowest, so they show it before the south side does.

What to look for: gray to black mottling on the underside of the plank sheathing, usually darkest between the rafters and along the nail lines. A distinct band of staining across the bottom of the slope over the eave, which is the ice dam signature. Frost or ice crystals on the nail points on a very cold morning, which means moist indoor air is reaching the deck. Rust rings around the nails. Insulation at the eave that is matted, dark or crusted. A stain circle around a bath fan duct that ends in the attic instead of at the roof.

Most attic staining is dry and historic. That matters more than the color of it. A meter on the sheathing settles whether the assembly is still getting wet, which is the question that decides whether you spend money now or keep watching.

An inspector adds what a flashlight cannot. Moisture readings in the sheathing and the top plate, a check on whether insulation at the eave is blocking the soffit path, an infrared pass from inside the top floor showing where warm air is leaving the ceiling, plus photographs of the affected bays. That is what tells you which part of the attic floor to seal instead of sealing all of it and hoping.

The fix is the attic floor, not the roof deck

Cleaning stained sheathing is the last step and the least important one. Cleaning it without correcting the heat loss buys the same stain next winter plus a bill.

The order that works:

Air seal the attic floor. Every hole between the house and the attic: the hatch itself with weatherstripping, the top plates, plumbing and wiring penetrations, the chimney chase, recessed light housings, the knee wall doors and the back side of the knee walls in a bungalow or a Cape Cod. Air leakage carries far more heat and moisture upward than conduction ever will.

Then insulate to a modern depth over the sealed floor.

Then check the ventilation. Soffit vents stuffed with insulation and a ridge vent doing nothing are common in older retrofits. A cold well vented deck is what stops the snow melting in the first place.

Then move the bath fan duct and the dryer duct to the outdoors if either one currently ends up in the attic.

Roof rakes and heat cables manage the symptom for one winter. They do nothing about the melting. If a March look at the hatch finds wet sheathing at the eave, the money belongs on the attic floor.