Snow is a reservoir, not weather
Rain leaves. Snow stays. A foot of settled snow holds roughly an inch of water, and it sits on the roof for as long as the weather keeps it there. The leak is not caused by precipitation on the day it appears; it is caused by whatever melts that stored water and whatever stops it draining.
That is why ice dam leaks appear on clear, cold, sunny days and during the first mild afternoon after a freeze — exactly the days a homeowner is least likely to suspect the roof.
What melts it from underneath
Heat from the house. Recessed lights, an open attic hatch, gaps around pipework and wiring, a bathroom fan venting into the attic, thin or compressed insulation over the top of the walls, and ductwork running through a cold attic all push warm air up against the underside of the deck.
The deck warms, the snow touching it melts, and the water runs down to an eave that is still at outside temperature. There it stops and freezes.
Why the first warm day is the worst day
A mild afternoon melts the pack from above as well as below, while the eave is still frozen from the night. Supply goes up, drainage stays at zero, and the water behind the dam gets deep enough to go over the top of the underlay and under the shingles. If your ceiling is going to leak, that is the day it does it.
How to tell which one you have
It is probably an ice dam
Snow on the roof, icicles or a ridge of ice at the gutter line, leak follows sunshine and thaws rather than rainfall, top floor, nearer the outside wall.
It is probably plumbing
No snow on the roof at all, or the leak tracks fixture use rather than the weather, or it happens at a constant rate day and night in any conditions.
It is probably a frozen or burst pipe
A sudden heavy flow rather than a slow stain, often with a drop in water pressure, after a hard freeze. Turn the water off at the stopcock and call a plumber, not us.
What to do in the next ten minutes
- Check the stopcock test: turn the mains water off for an hour. If water keeps coming, it is not a pipe.
- Look at the eaves from the ground. A ridge of ice at the gutter line with icicles over it is a dam.
- Bucket and photographs as usual, and keep the photographs dated.
- Do not throw salt or de-icer onto the roof. It corrodes flashing, fixings and gutters and it does not clear a dam.
- Get the pack and the dam off before the next mild day.
Questions people ask
Icicles are the symptom, not the dam. The ice that matters is the ridge sitting on the shingles behind them, and hitting that with anything hard in freezing weather cracks shingles and punctures decking. Low-pressure steam melts it off without touching the roof, which is why it is the only method we use.
A roof rake used from the ground on a single-story eave is reasonable if you are steady on your feet and you keep away from the power line. Do not go up a ladder onto an icy roof. Falls on ice are the common serious injury in this trade and they happen to experienced people.
Same day for an active interior leak where a crew is in range, usually 24 to 48 hours elsewhere in the 38 states we cover. Put your ZIP code in on this page and you will get a yes or no for your address right away.