After the Nor'easter: How Ice Dams Start Mold in Boston Homes

During the storm: what wind-driven rain finds

Rain arriving horizontally behaves nothing like rain arriving from above. It gets under clapboard laps, past the head of a window, into the mortar joints on the exposed face of a brick party wall and behind the counter-flashing at a chimney. A nor'easter drives it in off the water for hours at a stretch, out of the northeast, at the one exposure most Boston buildings have the least protection on. In East Boston and along the harbor edge of South Boston there is nothing standing between that wind and the wall.

The rear roof is the usual entry point in older housing. Three-deckers and rowhouses commonly carry a low-slope or flat rubber roof over the back half of the building, drained by a single internal drain or a scupper through the parapet. Fill that drain with wet leaves or let it freeze and the roof becomes a shallow pan. On a South Boston or Charlestown rowhouse the joint where the roof meets the party wall parapet is the seam that gets tested in every storm.

The Victorians of Newton, Brookline and Roslindale fail in a different vocabulary. Slate valleys funnel an enormous volume of water into one narrow seam. Dormer cheeks meet the main roof at a joint that depends entirely on flashing. A mansard puts a nearly vertical slate face directly above the top-floor ceiling. None of those are defects. They are details that have to be intact. A coastal storm is the test.

Days later: the ice dam at the eave

An ice dam is a heat-loss problem before it is a weather problem. Snow on a well-insulated roof stays where it landed. Snow on a roof leaking warm air melts from underneath. That meltwater freezes again the moment it reaches the overhang, which has nothing warm beneath it.

In an older Boston house the heat is getting out through the hatch to the knee-wall, around recessed lights punched into a top-floor ceiling, up an open stair or through the chase where the plumbing runs. The icicle line along the front gutter is the visible symptom. The pool of water standing behind the ice, under the shingles, is the actual event.

Where it shows up inside is fairly predictable: the ceiling along the top-floor perimeter, the head of the stairwell where warm air collects, the tops of window casings, the corner of a knee-wall room and the inside of an exterior-wall closet. Insulation at the eave takes on water and holds it long after the ceiling looks dry, because nothing moves air through that space.

The areaway, the bulkhead and the snow nobody moved

Snow gets shoveled off the stoop and plowed off the street and a lot of it ends up in the areaway, the sunken well in front of a garden-level door or window on a South End or Dorchester block. The drain at the bottom of that well is small, usually original and usually full of grit and leaves. A week later the pile melts into a space with a foundation wall on one side and a blocked drain underneath.

The bulkhead is the other funnel. Sloped doors over the cellar stairs rust at the frame and the seal at the sill is rarely tight. Snow banked against them holds meltwater at exactly the point where the door meets the masonry. The bottom of those stairs is a wet spot on a great many Boston properties and the water stands there against the sill of the house.

Downspouts finish the job. A downspout that discharges beside the foundation or empties into the areaway itself delivers a whole roof to a below-grade wall during a mid-winter thaw. This is the cheapest problem on the entire list to fix and the one most often left alone.

Weeks later: what shows up and where

A stain is the end of the story rather than the beginning. Water that entered at the eave in February can take weeks to work through insulation, plaster and paint before it marks anything. In many houses the smell arrives first: a damp, earthy note on the top floor that gets stronger when the heat runs and the air moves.

The material that stays wet is the material that grows something. Wet blown insulation at the eave, the paper facing on batts, the back of plaster and the top plate of an exterior wall all sit in a pocket with no ventilation and no light. Growth there does not need standing water. It needs a surface that has been damp for a while.

Spring humidity accelerates it. A cavity that stayed quietly damp through the heating season starts to smell in April. That is often when the phone call happens. Checking the same places in March, while the evidence is fresh and an insurer can still be shown ice on the roof, is a far better plan.

The week after a storm: a short list

Get water out of the building before anything else. Pull wet insulation rather than letting it drain, open plaster that has gone soft, get air moving on the cavity and keep it moving longer than seems necessary. Painting over a stain is the one action guaranteed to make the next inspection harder.

Take an ice dam apart with steam or with heated cable rather than a hammer or a shovel, because chipping ice off a roof takes shingles and slates with it and buys a worse leak next winter. If the dam is out of reach, the safer move is to clear a channel for the water and manage it until the weather turns.

Photograph everything while it exists. The icicles, the snow depth against the wall, the wet ceiling, the date. That evidence has a shelf life of about a week and it is the difference between a storm claim and an argument about long-term seepage.

Then decide about testing. If the cavity dried within a few days and nothing smells, you may be finished. If it stayed wet, if the smell persists into spring or if you are going to ask an insurer or a condo board for anything, documentation is worth having. That is the point where we can put you in front of somebody who does that work independently.