Loose stones reset and joints repointed on Roosevelt Park chimneys

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

ASTM C1780 and manufactured stone

ASTM C1780 is the standard practice for installation methods for adhered manufactured stone masonry veneer. Makers publish installation instructions written to comply with it, covering the backing, the barrier, the lath, the scratch coat, the setting bed, the joints and the cleaning.

When veneer fails, comparing what we find against those steps usually shows where the installation or a later repair went wrong. That comparison goes into the findings, so you understand the cause as well as the fix.

When moisture is forced through the stone

The brief explains that masonry deterioration from deposits left by evaporating water happens when the mortar is less permeable than the masonry unit. Moisture that cannot leave through the joints leaves through the stone, and the minerals it carries can damage the stone face as they are left behind.

Flaking stone next to hard, gray patching is a pattern we look for. The repair is to remove the hard mortar, repoint with a compatible mix and replace any units too damaged to keep.

Removing caulk and sealant from joints

Caulk pressed into stone joints is a common quick fix for a leak. It seals the joint face while water still moves behind it, it stains stone as it ages and it leaves a surface that new mortar will not bond to.

We cut caulk out completely, scrape the residue off the stone edges and rake the joint to sound mortar before repointing. If the caulk was hiding a leak, the leak goes into the findings with its source.

Blending manufactured stone from several boxes

Manufactured stone comes in boxes with a range of sizes, shapes and colors. One maker advises spreading the stone out on site and mixing from different boxes to get a good balance, with small stones next to large ones and heavy textures next to smooth.

For repairs, the same idea applies on a smaller scale. We choose replacement pieces so the repaired patch blends with the pattern around it instead of forming a block of look-alike stones.