Spalling Brick: Why Chimney Faces Flake Off in Connecticut

Why one side always goes first

If you look at a spalling chimney in this state, the damage is rarely even. The north and west faces are worse, sometimes dramatically so, and the south face can look almost untouched.

The mechanism is simple. Those faces take the prevailing weather and they get the least sun, so water that soaks in is still there when the temperature drops. Connecticut runs through dozens of freeze cycles in a season. Each one expands the water in the brick by roughly nine percent, and the face is what gives.

The practical consequence is that a chimney can look perfectly sound from the street and be failing on the side you never see. Any assessment made from the ground on one elevation is incomplete.

Two things make it faster

A failed crown above, admitting water into the structure continuously. And a flue that has been condensing — a modern gas appliance in an oversized chimney puts acidic moisture into the masonry from the inside, which softens it before the frost ever gets to it.

Repointing, rebuilding, and the test between them

These are different jobs at very different prices, and you can tell which you need with a key.

Repointing

Mortar has failed but the brick is intact. Scrape a joint: if mortar comes away as powder or falls out in pieces, it has gone. Raking out to a sound depth and replacing it is repointing, and it works while the brick still has faces.

Rebuilding

Faces have flaked through, a brick can be rocked by hand, or the stack leans. New mortar has nothing sound to bond to. The stack comes down to a sound course and goes back up, usually with a new crown and cap as part of the job.

Where it stops

Most Connecticut rebuilds are confined to the section above the roofline. That is where the weather does its work and where the masonry dries slowest. Below the roofline it is protected and usually still sound.

The mortar question, and it is a real one

On anything built before roughly 1900 — and Connecticut has plenty, particularly along the shoreline in Mystic, Stonington and Old Lyme — the original mortar is lime, deliberately softer than the brick so the mortar fails first and can be replaced.

Repoint that with modern hard Portland mortar and the relationship inverts. The mortar is now harder than the brick, so the brick becomes the sacrificial part, and it starts spalling. It is the most damaging thing that can be done to old masonry and it is always done with good intentions. Mortar should be matched by analysis, not by eye.