Weather · Snow & ice

Snow load & ice dam damage — what to know

Most residential roofs are engineered for 20–40 lbs/sq ft of snow load. Ice dams — not weight — cause the majority of winter roof leaks by forcing melt water back up under the shingles.

Prevention basics

• • Attic insulation of R-49+ keeps the roof deck cold and melt-free.
• • Balanced intake + ridge ventilation flushes warm air out.
• • Ice & water shield membrane at the eaves for 24" past the interior wall.
• • A roof rake clears the first 3–4 ft of snow after big storms.

How an ice dam forms

An ice dam is an attic problem that shows up on the roof. Heat escaping into the attic warms the underside of the deck, melting the bottom layer of snow. That melt water runs down the slope until it reaches the eave, which overhangs unheated space and stays at outdoor temperature, where it refreezes into a ridge of ice. Each cycle grows the dam until water backs up several inches deep and finds its way under the shingles — shingles shed water, they do not seal against standing water.

The tell-tale signs are thick icicles along the gutter line, uneven melt patterns on the roof, and brown staining on the ceiling near exterior walls. Rooms below cathedral ceilings, over garages, and near recessed light fixtures are the most common leak locations because those areas have the least insulation.

Fixing the cause, not the symptom

Chipping ice or throwing salt pucks on the roof is triage. The permanent fix is air sealing every penetration between conditioned space and the attic — can lights, bath fans, plumbing stacks, the attic hatch — then bringing insulation up to R-49 or better, then confirming intake ventilation at the soffit is not blocked by that new insulation. A cold, well-vented deck cannot melt snow from below.

During a re-roof, insist on an ice-and-water membrane running from the eave edge to at least 24 inches inside the exterior wall line, which is code in most northern jurisdictions. On low-slope sections and in valleys, extend it further.

Snow load: when weight is the real risk

Fresh powder weighs roughly 5–7 lbs per cubic foot; wet packed snow can reach 20 lbs, and an ice layer 57 lbs. A two-foot wet snowpack on a 1,500 sq ft roof can add 15 tons. Most residential framing is designed for 20–40 lbs per square foot depending on your snow zone, so warning signs matter: sagging ceilings, sticking interior doors, new cracks in drywall, or creaking under load mean you should leave the house and call a structural professional.

Clear snow with a roof rake from the ground, working the bottom three to four feet of the slope only. Never climb onto a snow-covered roof and never use a metal shovel on shingles.

Frequently asked questions