The pocket behind the stack
Snow coming down the slope stacks up against the chimney and stays there long after the rest of the roof has shed. Underneath it, heat from the house and from the flue itself melts the base of the pack, and that water refreezes at the edges of the pocket.
On a wide chimney the builder should have fitted a cricket — a small ridged structure on the upslope face that splits the flow around the stack. A great many chimneys over 30 inches wide do not have one, and those are the ones that collect the deepest ice.
Step flashing works downhill only
The flashing up the sides of a chimney is a series of overlapping L-shaped pieces woven into the shingle courses. Water running down crosses each lap from above and is carried out. Water standing in a frozen pocket seeps behind the counter-flashing and runs down inside the masonry and along the framing instead.
That is why chimney leaks in winter tend to show some distance from the chimney itself — along a rafter, at a ceiling seam, or down the side of the breast a floor below.
Masonry holds the water
Brick and mortar are absorbent. Water that gets into a stack in freezing weather expands as it freezes, which spalls the face of the brick and opens the mortar joints, which lets in more water next time. A chimney that is allowed to do this for a few winters needs repointing or rebuilding rather than flashing.
How to tell which one you have
It is probably an ice dam or ice pocket
Snow on the roof, ice banked on the upper side of the stack, staining appears in thaws, often on the chimney breast rather than at the roof.
It is probably flashing or a missing cricket
It leaks in heavy rain as well, the same spot every time, and there is no cricket on a wide stack.
It is probably the crown or the cap
Water appears inside the flue or at the top of the breast, with a cracked crown or a missing cap visible from the ground.
What to do in the next ten minutes
- Photograph the chimney breast inside and the stack from the ground with a zoom.
- Check the room below as well — chimney water travels down the masonry.
- Do not light a fire to dry it out. Heating a wet, frozen stack accelerates spalling.
- Keep anything absorbent away from the breast.
- Get the ice pocket cleared, then have the flashing and the crown looked at properly once the roof is clear.
Questions people ask
If your stack is wide and it collects snow and ice every year, a cricket is the permanent fix and it is cheap compared with repointing. It is dry-weather work, so it is something to arrange for the spring rather than now.
No. Steam melts the ice at low pressure without impact. What damages masonry is the freeze-thaw cycle you get from leaving the ice there, and anybody attacking it with a chisel.
Sudden interior water damage is usually within scope. Repointing a stack that deteriorated over several winters usually is not, because most policies exclude gradual wear. Those are two different claims and it is worth being clear which one you are making.