Your stone chimney, repaired and repointed in Holden Heights

Three ways a chimney ends up stone

A stone chimney can be built of solid stone masonry, with thick units that make up the wall itself. It can be natural stone veneer, thinner stone set against a masonry or framed backing. Or it can be manufactured stone veneer, a lightweight cast product adhered over a layered backing.

From the street, all three can look alike. Their repairs do not. We identify the construction first, because the right mortar, the right method and the right questions about water all depend on it.

• Solid stone masonry
• Natural stone veneer on a backing
• Manufactured stone over lath and barrier

Finding hollow stones by tapping

A stone with voids behind it sounds different when tapped. On a veneer chimney, we tap every stone within reach and mark the ones that sound hollow or move.

One hollow stone in a sound wall may just be reset. A cluster of hollow stones points to a wider bond problem in the setting bed or scratch coat, and the plan changes accordingly. The findings show where the hollow areas are, so you can see why a repair covers the area it does.

Working in hot or dry weather

The same instructions say that in hot or dry weather, the back of each stone should be moistened with a fine spray or wet brush, and a concrete, masonry or scratch coat backing dampened before mortar goes on. Surfaces should look damp but carry no standing water.

Skipping that step lets the stone and backing pull water from the fresh mortar too quickly, weakening the bond. We plan the work and the dampening around the conditions on the day.

White bloom on stone veneer

Efflorescence is a water-soluble deposit left on masonry as water evaporates from the wall. For manufactured stone, one maker advises letting the stone dry thoroughly, then scrubbing with a stiff bristle brush and clean water, and for stubborn cases a solution of one part white household vinegar to five parts water, rinsed thoroughly.

Cleaning treats the symptom. The water that carried the deposits out is the real finding, and we trace it to the cap, the flashing or the base before calling the job done.

Why the stone itself is not the waterproofing

One maker says plainly that its manufactured stone veneer should not be considered a waterproof material. Water passes through stone and joints to some degree, and the assembly behind them is what keeps the structure dry.

That is why a leak near a stone chimney is rarely fixed by the stone alone. We look at the barrier, the flashing, the cap and the base when water shows up, and we explain which part failed.