Ice dams, attic ventilation and flashing: why new roofs still leak in Canada

Ice dams form when heat escaping from the house melts snow on the upper roof, the water runs down to the cold eaves and refreezes, and the growing ridge of ice backs water up under the shingles. New shingles do not stop that; the fixes are air sealing the ceiling below the attic, enough insulation, balanced ventilation from soffit to ridge, and eave protection (ice and water membrane) at the bottom of the roof. Many other "new roof" leaks come from flashing: reused or missing step flashing, poor chimney flashing, missing kick-out flashing where a roof meets a wall, and badly sealed vents and skylights. Because these are installation and building-envelope issues, they are usually excluded from manufacturer warranties and home insurance, and fall under the contractor's workmanship warranty, if anyone's.

This guide is part of roofing problems and official sources. Prices are illustrative.

How an ice dam forms

Warm, moist air leaks from the house into the attic through gaps around pot lights, attic hatches, plumbing stacks, bathroom fans and top plates.
The roof deck above warms and melts the snow.
Meltwater flows down to the eaves, which overhang the walls and stay cold.
It refreezes into a dam; more meltwater pools behind it.
Water works up under the shingles, past the nails and into the walls or ceiling.

Signs: icicles along the eaves, ice ridges, water stains at the top of exterior walls, frost or mould on the underside of the deck in the attic.

The three fixes, in order

Fix	What it does	Who does it	Illustrative cost
Air sealing	Stops warm moist air reaching the attic	Insulation contractor or energy retrofit trade	$500–$2,000
Insulation top-up	Reduces heat loss through the ceiling	Insulation contractor	$1,500–$4,000 for an attic
Ventilation balance	Keeps the deck cold and removes moisture	Roofer	$300–$1,500 for vents and baffles
Eave protection	Membrane under shingles at eaves resists backed-up water	Roofer, during re-roofing	Often included; $500–$1,200 if extra

Air sealing and insulation reduce the cause; ventilation and membranes manage the symptoms. Doing only the membrane hides a problem that can still rot the deck and cause attic mould. Insulation done at the same time as a roof job may qualify for a rebate; see rebates and tax credits in 2026.

Ventilation basics

The National Building Code of Canada sets minimum attic ventilation, expressed as a ratio of unobstructed vent area to insulated ceiling area, 1/300 for typical attics (1/150 for roofs under 1 in 6 slope or built with roof joists), with vents spread on opposite sides and at least 25% of the required area at the top of the space and 25% at the bottom: intake low at the soffits and exhaust high near the ridge. Check the code adopted in your province and the shingle manufacturer's requirements, which some warranties make a condition of coverage.

Common mistakes:

Blocked soffits: insulation pushed into the eaves, or soffit replaced with unvented panels during siding work.
Mixing exhaust types: combining ridge vents with powered or gable vents can short-circuit airflow. Manufacturers often advise against it.
Too little exhaust: one or two box vents on a large roof.
Bath fans venting into the attic: moisture source; they must vent outside.

Worked ratio example: an attic of 1,200 sq ft insulated ceiling area at 1/300 needs 1,200 ÷ 300 = 4 sq ft of net free vent area, which is 576 sq in, split roughly half intake and half exhaust: about 288 sq in each. A ridge vent rated at an illustrative 18 sq in per linear foot needs 288 ÷ 18 = 16 ft of ridge. Use the rated figure on the actual product.

Eave protection

The building codes based on the National Building Code require eave protection on shingle roofs in most of Canada, extending from the edge of the roof at least 900 mm up the slope and to at least 300 mm inside the inner face of the exterior wall, with exceptions for mild climates (3,500 degree-days or fewer), unheated garages, carports and porches, very steep roofs (1 in 1.5 or more) and low-slope shingle applications, which have their own underlayment rules. Ask your roofer how far up the membrane will go, especially on low slopes and wide overhangs, and whether valleys and around chimneys and skylights get membrane too.

Flashing failures on new roofs

Location	Typical mistake	Result
Roof-to-wall (sidewall)	Old step flashing reused or face-nailed; no step flashing at all	Leaks into wall cavity
Bottom of roof-to-wall	Missing kick-out flashing	Water runs into the wall behind siding, rotting sheathing
Chimney	Counter flashing not let into mortar joints, relying on caulk	Leaks after a winter or two
Skylights	Old skylight left in place under new shingles, or no kit	Leaks around the curb
Plumbing stacks	Cracked rubber boot reused	Ceiling stains
Valleys	Shingles cut too close or no valley membrane	Leaks in heavy rain
Drip edge	Missing at eaves or rakes	Water wicks into fascia and deck edge

Kick-out flashing ties roofing and siding together; if separate contractors are involved, agree who installs it. See siding replacement costs.

Fixing a leak on a newly replaced roof

Note when it leaks: in rain only (likely flashing), in thaw after snow (likely ice dam), or in cold weather without precipitation (likely condensation from air leaks).
Photograph the attic and the stains, with dates.
Notify your contractor in writing under the workmanship warranty.
If the contractor blames ventilation or the house, ask for that in writing with recommendations.
Get an independent opinion if needed.

See manufacturer versus workmanship warranties and how to complain about a roofer.

Safe ice dam removal

Do not chip ice with axes or climb onto icy roofs. Roof rakes from the ground can remove snow near the eaves. For active leaks, ice dam removal services usually use low-pressure steam; ask about insurance and how they will protect shingles. Salt or chemicals can damage plants, metals and some roof materials.

FAQ

Will heating cables solve ice dams?

They can keep channels open in problem spots but use electricity and do not address the cause. Air sealing and insulation are the long-term fix.

Is condensation in the attic a roof problem?

Usually it is an air-leakage and ventilation problem. Fixing air leaks from the house is the priority.

Does insurance cover ice dam leaks?

It depends on the policy wording. See home insurance roof claims.

Sources

National Research Council Canada — National Building Code of Canada, eave protection and attic venting
Canada Mortgage and Housing Corporation — ice dams and attic moisture guidance
Natural Resources Canada — air sealing and insulation guidance
Insurance Bureau of Canada — winter weather and water damage