Ice Dams in the North Penn Area: Why They Form and How to Stop Them

What is an ice dam, and why does it cause leaks?

An ice dam is a band of ice at the roof edge that blocks meltwater from draining. The water backs up beneath the shingles, finds nail holes and seams, and enters the house through ceilings and walls. The leak is from standing water behind the ice, not from the ice itself.

Shingles are built to shed water running downhill. They are not built to hold a pool of it. When a ridge of ice builds along the eaves, melting snow behind it has nowhere to go, so it pools, rises under the shingle courses and drips into the insulation or down the wall.

You may see icicles hanging from the gutter, a thick shelf of ice on the lowest few feet of the roof, or water stains on a ceiling near an exterior wall. Brown streaks on the inside of a wall below the eaves are another sign. Ice dams do not always leak the first winter. The damage often arrives as wet insulation, stained drywall or peeling paint that no one links to the roof at first.

The mechanism is simple, and that is useful, because it tells you what to fix. A cold roof edge plus a warm roof field is what builds a dam. Anything that makes the whole roof the same cold temperature stops it from forming.

Why do ice dams show up so often near Souderton?

Because the winters here swing above and below freezing. A third-party weather page gives Souderton about 20 inches of snow and about 50 inches of precipitation a year. Snow that melts by day and refreezes by night is exactly what ice dams need, and older attics leak a lot of heat.

Those climate numbers are from a weather-data page, so treat them as approximate. They are enough to show the pattern: moderate snowfall, plenty of precipitation and winters that cross the freezing line again and again. A heavy snow followed by a cold snap and a bright afternoon is the classic set-up.

The housing makes it more likely. About 30% of Souderton homes were built before 1939 and about 37% in the 1940s to 1960s, by third-party borough data. Many older houses have thin attic insulation by modern guidance, leaky ceiling penetrations and little ventilation. Those are the conditions that let heat into the roof deck. Attached rowhouses share walls but still have their own attic, so the same logic applies.

Farm-origin properties around Harleysville and Franconia often have large, complicated roofs with valleys, dormers and additions. Valleys and the places where a roof meets a wall collect snow, and they are where a dam does the most harm. This is a general pattern and not a count of local ice-dam claims, which we do not have.

A neighboring township's building documents give a ground snow load design figure of 30 pounds per square foot for Montgomery Township. That is a structural design parameter for that township, not a statement about Souderton, and it says nothing about ice dams. We mention it only because snow weight and ice are separate worries.

How does heat from the house end up creating the dam?

Warm air leaks from living space into the attic through gaps around the chimney chase, plumbing vents, light boxes and the attic hatch. That heat warms the roof deck from below, melts the snow above, and the water refreezes where the roof overhangs the cold outside wall.

Think of the roof as a lid. If the attic stays close to outdoor temperature, snow lies on the lid and melts only from the sun. If the attic is warm, the snow melts from below all winter, and the meltwater runs to the eaves, which are cold because they extend past the heated walls.

Building-science guidance from Pacific Northwest National Laboratory and from Penn State's housing research group points at the same list of shortcuts. These are the places warm air tends to leak up:

None of these is dramatic on its own. Together they can be the difference between a roof that sheds snow evenly and one that grows icicles by January.

• The chimney chase and flue, where a gap around the masonry opens into the attic
• Plumbing vent stacks and wire chases that run from the living space through the top plate
• Recessed light boxes in ceilings below the attic
• The attic hatch or pull-down stairs, which are often uninsulated
• Ductwork in the attic with leaky joints
• Gaps where interior walls meet the attic floor

What do insulation and ventilation do about ice dams?

Insulation keeps house heat out of the attic, and ventilation carries off whatever gets through. Prevention guidance points to about 12 inches of attic insulation, sealed air leaks, and vent area of 1 square foot of net free vent per 150 square feet of attic, ideally half at the ridge and half at the eaves.

Both halves matter, and the order matters. Seal first, insulate second, ventilate third. Adding ventilation to a leaky attic just pulls more warm air up through the gaps. Adding insulation over open gaps hides the leaks without stopping them.

The vent ratio of 1 square foot of net free area per 150 square feet of attic comes from the building-code guidance cited in the prevention material we reviewed. Ideally half the vent area is at the ridge and half at the eaves, so cold air enters low and warm moist air leaves high. Net free area is the open space after screens and louvers are subtracted, which is why you cannot judge it by looking at a vent cover.

A roofer can tell you whether the soffit vents are blocked by insulation, whether the ridge vent is open and whether the attic has any ventilation at all. That is a different task from insulating, which may be done by an insulation contractor. The roof ventilation page lays out what a roofer looks at, and the ice dam page covers removal and repair.

We do not give you a rating for your house or tell you what your local code requires for a specific permit. Ask the borough or township building office, since local rules can add to the state code.

What should you do, and not do, while an ice dam is on your roof?

Stay off the roof, do not chip or pick at the ice, and do not pour salt or hot water on it. If water is coming in, move belongings, put a bucket down and call a roofer. Then fix the heat loss that caused it once the weather clears.

The instinct is to knock the ice off. Resist it. Chipping with a hammer or shovel tears granules off shingles and can crack them, and a roof with ice and snow on it is slippery and dangerous even for people who work on roofs. Calling a roofer is the fastest fix and the safest one.

Inside, treat it like any leak. Move furniture and rugs away from the stain, put a bucket under the drip and photograph the damage with a date. Do not poke a hole in a bulging ceiling unless water is pooling and you are controlling where it drains. If there is any chance of water near light fixtures or outlets, keep clear and call.

Souderton Roofing puts you through to a local roofer who serves your town. No arrival time is promised, and when a roofer is free is when the visit can be arranged. A roofer can advise on removing the ice, but the long-term fix is the attic.

• Do not climb onto a snow or ice-covered roof
• Do not use hammers, axes, picks or ice scrapers on the ice
• Do not spread rock salt on shingles
• Do move belongings away from stains and put a bucket under drips
• Do photograph stains and ice with the date, for your own records
• Do call a roofer, and later check the attic for wet insulation

What are the short-term and long-term fixes?

Short term, a roofer can remove the dam carefully and repair any water entry. Long term, you reduce heat reaching the roof: seal air leaks, bring insulation up to about 12 inches, and balance the vents. A heated cable is a stopgap, not a cure.

Think in two time frames. This week, the aim is to stop water entering the house. Next fall, the aim is to make sure it does not happen again.

The table lays out how the two compare, and who usually does each job.

Do gutters and leaves make ice dams worse?

Yes. A gutter packed with leaves holds water that freezes into a solid block at the roof edge, giving the ice dam something to build on. Mature trees, such as those along Perkiomen Creek, drop leaves into gutters and valleys every fall, so clearing them before the first freeze is worthwhile.

Gutters do not cause an ice dam, but they can anchor one. Ice in a clogged gutter also pulls on the fascia and can bend a gutter off its brackets. If the fascia behind it is already damp, it can rot.

Towns along the Perkiomen Creek, such as Skippack, Schwenksville and Collegeville, have mature trees close to rooflines. That is a general pattern, and it means leaf debris builds up in gutters and valleys. See our gutter repair and installation page for what a roofer inspects. Do not lean a ladder against a frozen gutter. Wait for a thaw, or ask a roofer.