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Ice Dams, Snow and Chimney Leaks
Leaks near a chimney often appear in winter, when snow and ice put the roof junction under the most stress. The tricky part is that the water may come from the chimney, the roof, or warm air leaking into the attic. This guide helps you sort those apart.
The short answer: A winter leak near a chimney may be failed flashing or masonry, or it may be an ice dam caused by heat loss into the attic. The location, timing and shape of the stain give clues, and an inspection confirms the source.
Why leaks show up in winter
Water gets in during every rain, but winter changes the way it travels. Snow sits on the roof for days and melts slowly. Meltwater runs, refreezes, and can be forced sideways or uphill. Freezing and thawing also work on brick, mortar and sealants all season.
Two different problems are easy to confuse:
- A flashing or masonry leak. The seam between roof and chimney, or the chimney itself, lets water in. It may leak in any heavy rain, not only in winter.
- An ice dam leak. Water backs up behind ice at the roof edge and finds a way under shingles. The chimney may be innocent.
For the full list of chimney leak sources, see why a chimney may leak.
How an ice dam forms
The University of Minnesota Extension explains it as a three-part condition: snow on the roof, upper roof surfaces above freezing, and lower edges below freezing. Snow melts on the warmer upper part, runs down to the cold edge and refreezes into a ridge of ice. Later meltwater pools behind that ridge, and it can seep through small gaps in the roofing into the attic, walls and ceilings.
The heat that melts the snow usually comes from the house. Extension researchers say air leaking through ceiling openings, such as light fixtures, is often the biggest contributor. Chimneys, leaky ductwork and bath or kitchen exhausts near the roof can also warm the roof locally.
This matters because a chimney is a warm column running through the roof. Snow around it can melt faster and refreeze at the edge below. The chimney is a place where ice dams are noticed, but the cause may be elsewhere.
Chimney, roof or attic: a quick way to sort the clues
| What you notice | It may point to | Next step |
|---|---|---|
| Stain on the ceiling or wall right beside the chimney, after rain and melt | Flashing, counterflashing or chimney masonry | Inspection of the roof junction and masonry |
| Stains along an outside wall or at the eaves, with icicles hanging at that spot | An ice dam at the roof edge | Roofing and attic air sealing review |
| Leak only in wind-driven rain or only during thaw | Flashing gaps, a cracked crown, or masonry that absorbs water | Chimney inspection |
| Water inside the fireplace or on the damper | Open or damaged cap or crown, or cracked flue | Chimney inspection and cap check |
| Frost or damp insulation in the attic around the chimney | Warm air or moisture reaching the attic, or a flashing leak | Attic review plus chimney check |
This table gives possibilities, not a diagnosis. Several causes can happen at once.
Flashing and the cricket
Flashing is the metal that seals the roof to the chimney. The Brick Industry Association describes two parts: base flashing laid on the roof and turned up the chimney face, and counterflashing set into the masonry that laps over it. The lap and joints must be sealed, and flashing that does not properly reach into the wall gives little protection. The same technical note recommends copper or stainless steel and advises against galvanized steel and aluminum.
Snow and ice add load where the chimney meets the slope. If your chimney is wide and sits on the slope, a cricket may be present. A cricket (sometimes called a saddle) is a small peaked roof behind the chimney that splits water around it instead of letting it dam against the uphill side. The BIA flashing note we read does not cover crickets, so how to size or build one is a question for a roofer or mason and the local code official. Learn the layers of flashing in chimney flashing repair and roof leaks.
Flashing repair can involve the roof and the chimney, so it is not a DIY job from a ladder in winter.
Masonry, the crown and freeze damage
Brick, mortar and the crown absorb water. In winter that water freezes and expands, which can flake brick faces and open mortar joints. Those openings let in more water, and the cycle repeats. If you see flaking brick or crumbling joints, read about spalling and freeze damage.
The crown is the slab at the top. A cracked crown sends water into the chimney. See chimney crown repair and replacement and waterproofing water-damaged masonry. A missing or damaged cap lets rain and snow straight down the flue. See chimney caps and chase covers.
The attic side: air sealing, insulation and ventilation
The University of Minnesota Extension recommends fixing ice dams from the inside. First make the ceiling airtight so warm, moist air cannot enter the attic. Then add insulation. Roof and attic ventilation still matters for drying, but if heat loss is controlled only small amounts are needed. Their guidance also cautions against adding mechanical attic ventilation as an ice dam fix.
The same guide warns not to tighten the house without making sure combustion appliances still have enough air. That applies to anyone with a fireplace, wood stove or gas appliance. Air sealing around a chimney in the attic uses fire-rated materials, which is not a place for ordinary foam. Read air sealing around a chimney safely before anyone seals that gap.
An insulator or roofer can address attic air sealing and roof ventilation. AM can inspect the chimney, flashing area and masonry, but does not provide insulation work.
What to do when you see a leak in winter
- Protect the interior: move belongings, place a container under drips, and note the date and weather.
- Photograph the stain and where it sits relative to the chimney.
- Stay off the roof. The Minnesota guide specifically advises against going on the roof in winter or working on it without professional help.
- Dry wet areas quickly, because lingering moisture leads to mold.
- Call for an inspection of the chimney and roof junction once it is safe to do so.
Water in the fireplace, or a wet smell around a gas appliance, is a reason to stop using the appliance until it is checked. A leak can also damage a liner or damper over time.
This guide cannot tell whether your leak is from the chimney, the roof or the attic. Only an on-site look can. If you find sagging ceilings or heavy snow loads, contact a qualified professional or your building office.
Frequently asked questions
Can a chimney cause an ice dam?
A chimney can warm the roof around it, and extension researchers list chimneys among attic heat sources. Most ice dams trace mainly to heat and air leaking from the house into the attic.
Is a leak next to the chimney always flashing?
No. It could be flashing, the crown, cracked masonry, or an ice dam. An inspection tells these apart.
Should I chip ice off the roof?
Not yourself. Working on a winter roof risks injury and can damage roofing. Ask a qualified professional.
What is a chimney cricket?
A small peaked section of roof behind the chimney that diverts water and snow around it. Whether yours needs one is a roofing question.
Related reading
- Why is my chimney leaking?
- Chimney flashing repair and roof leaks
- Air sealing around a chimney safely
- Chimney repair
- Chimney crown repair
Sources
- University of Minnesota Extension, ice dams: how ice dams form, heat loss causes, air sealing first, cautions
- Brick Industry Association, Flashing Chimneys: base flashing, counterflashing, sealing and metal choices
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