Rock and fieldstone chimney repair in Harlem Heights

Metal lath and how it is fastened

Lath gives the scratch coat something to grip. The instructions we follow call for lath installed to ASTM C1063, typically with corrosion-resistant fasteners every 6 inches vertically and 16 inches horizontally into framing, and self-furred so mortar can wrap behind it.

When we open failed veneer, lath tells a story. Rusted, loose or poorly fastened lath explains stones that pull away in sheets. We replace damaged lath before building the backing back up, and we note the fastening in the findings.

Tinting mortar to suit the stone

Mortar color shapes how stonework looks from a distance. One maker suggests tinting regular mortar with iron oxide pigments, for example tan mortar with earth-tone stone, to complement the stone.

For repairs, the goal is to match the existing joints as they are. We mix small test batches, let them dry, since mortar lightens as it cures, and compare them with your joints before the real batch is made.

Sealers on stone, used with care

One manufactured stone maker states that sealers are not necessary on its products. If one is used, it should be a breathable silane, siloxane or silane-siloxane blend, and the maker warns that sealers may darken the stone and slow the natural movement of moisture out of it, raising the chance of efflorescence or spalling.

For natural stone, the sealer's own data sheet decides whether it suits the stone. We recommend a sealer only when there is a clear reason, and we name the product in writing.

How deep to rake out stone joints

The National Park Service's Preservation Brief 2 sets the raking depth at 2 to 2 and a half times the joint width as a minimum, so new mortar bonds and stays put. For stone masonry with wide joints, that can mean several inches.

Loose or crumbling mortar beyond that depth also comes out. The brief shows new mortar falling out of a joint that was not prepared deeply enough. We rake to a sound depth and show you the prepared joints before filling.

• At least 2 to 2 and a half times the joint width
• Deeper where mortar is loose
• Square back so new mortar locks in

Hand tools, grinders and stone edges

The same brief notes that hand chisels and mash hammers, though slow, pose the least threat of damage to masonry units, and that power saws and grinders in unskilled hands can break edges and overcut joints. It advises that power tools never be used on vertical joints.

Stone edges broken by a grinder make joints look wider and weaker. We remove mortar by hand wherever the joint allows and use power tools only on suitable horizontal joints, with care.