Ice Dams and Chimneys

How ice dams form

University of Minnesota Extension explains that an ice dam needs two things at once: snow on the roof, and higher parts of the roof surface above freezing while lower parts are below freezing. Snow melts on the warm upper roof, the water runs down, and it refreezes when it reaches the colder roof edge. The ridge of ice grows, and water backs up behind it.

Extension staff point to heat flowing out of the house as the main reason roofs warm unevenly, much of it through air leaks into the attic. They also list chimneys among the other heat sources that can melt snow on a roof.

FEMA's snow load guide describes the same chain from the structural side: meltwater freezes at the eave, the dam grows, and it holds back more water and concentrates load at the roof edge.

Why the chimney is a trouble spot

Three things combine at a chimney:

• Drifting. FEMA notes that wind blows snow up against obstructions on a roof, and it names chimneys among the roof irregularities that collect drifts. A chimney on a long roof slope can end up with a deep pile on its uphill side.
• Pooling. Water running down the roof hits the back of the chimney. Without a way around, it slows and collects there. A building-science article from IBACOS in Pro Builder describes runoff and snowmelt settling on the uphill side of a chimney and working into the flashing.
• Heat. A chimney that serves a working furnace, boiler or fireplace can be warm. Heat that melts snow beside it supplies water to an area where it can refreeze.

Put together, the back of the chimney can become a small basin of slush and ice that sits there for days.

What flashing can and cannot handle

Chimney flashing works by overlapping. Step flashing is woven into the shingle courses and turned up against the brick, and counterflashing set into the masonry laps down over it. Mason Contractors Association of America guidance describes counterflashing lapped over the base flashing by at least three inches. Water flowing down over these layers is directed onto the roof.

Standing water is a harder test. If ice and slush hold water against the chimney deeper than the height of the flashing's upturned edge, or wind and freezing push it sideways between layers, it can get behind them. Damaged or sealant-only flashing gives way first. See chimney flashing explained for the parts.

The cricket: a small roof behind the chimney

A cricket, also called a saddle, is a small peaked structure built on the uphill side of a chimney. The Pro Builder article describes crickets as ridge-shaped structures that divert water around chimneys and other roof penetrations. Instead of meeting a flat wall of brick, water and snow on the slope above are split to either side and carried down past the chimney.

Whether a cricket is required on a particular house is a question for your local construction office. Either way, a wide chimney low on a long slope collects more water behind it than a narrow one near the ridge. The cricket also needs its own flashing tied into the chimney, so it is best built or rebuilt when the roof is replaced. See replacing your roof? plan the chimney too.

During a storm: what to do and not do

• Stay off the roof. FEMA's guide recommends that roof snow removal be done by an experienced professional roofing contractor, and warns about the fall hazard.
• From the ground only. FEMA suggests a non-metallic snow rake for steep roofs where the work can be done from the ground. Clearing the lower few feet of the roof removes snow that feeds a dam. Do not rake near power lines.
• Do not chip at ice around the chimney or flashing. Hammers and scrapers can crack shingles, bend flashing and break off pieces of the crown.
• Keep vents clear. If the chimney or a nearby vent serves a furnace, boiler or water heater, snow and ice blocking it can push exhaust back into the house. Keep CO alarms working; if one sounds, leave and call 911.

UMN Extension also describes making channels through an ice dam with tap water on a warm day, working up from the lower edge, as an emergency way to drain water. That is ground-level or ladder work that should be left to someone equipped for it.

After the thaw: what to check

• The attic beside the chimney. If you can reach it safely, look at the framing and roof sheathing around the chimney with a flashlight. Dark, wet wood or water stains on the uphill side are the clue to note.
• Ceilings and walls below. Look for new stains, bubbling paint or damp drywall near the chimney breast. Mark the edges with pencil and photograph them.
• The firebox. Water running down inside the chimney can show up as dampness or rust on the damper.
• From the ground. With binoculars, check for lifted or bent flashing, a missing cap, or debris left on the roof behind the chimney.

A stain that appeared only during a snowmelt, and never in rain, points toward the ice-and-snow path rather than ordinary rain entry. That detail is worth passing along when you book an inspection.

Snow at the top of the chimney

Heavy snow does not only collect on the roof around the chimney. It can pile on the crown and cap, and a drift can bury a short chimney or a low vent termination near the roof. NFPA advises keeping the vents of the dryer, furnace, stove and fireplace clear of snow during and after a storm. For a chimney top, that is a job for someone with roof equipment, but you can look from the ground to see whether the cap is buried and whether exhaust is visibly leaving the flue on a cold morning. If a furnace or water heater flue appears blocked, stop using the appliance and have it checked.

Fixing the causes, not just the leak

A chimney that leaked in an ice-dam event usually has two problems to solve: the roof conditions that made the ice dam, and the chimney detail that let water in.

• The roof side. UMN Extension's first long-term step is making the ceiling airtight so warm, moist air cannot flow into the attic, then adding insulation. Around a chimney, the gap between the masonry and the framing is a fire-safety detail as well as an air leak, so it should be sealed with materials suited to that location.
• The chimney side. Flashing that is loose, rusted or relying on caulk alone, a missing or undersized cricket, a cracked crown that lets meltwater into the brickwork, and open mortar joints on the uphill face. See chimney flashing repair.

When the entry point is unclear, we use controlled water testing to separate a flashing leak from a masonry or roof leak, and we check the attic side when it is safe and accessible.

Questions homeowners ask

Sources

1. NFPA: Carbon Monoxide Safety tip sheet (copy hosted by the Village of Penn Yan, NY)
2. University of Minnesota Extension: Dealing with and preventing ice dams
3. FEMA P-957: Snow Load Safety Guide (2013)
4. Pro Builder (IBACOS): The Ins, Outs, and Arounds of Proper Crickets and Saddles
5. Mason Contractors Association of America: Brick Brief, Flashing Chimneys

This guide is general information. It can't diagnose a specific chimney; an inspection can.