Natural and cultured stone chimney repair in Tangerine

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.

The scratch coat under the stone

Over the lath goes a scratch coat, which one maker specifies at a minimum of half an inch up to three quarters of an inch thick, with enough pressure to fully encapsulate the lath so none is visible. The maker notes that encapsulation and thickness are key to the lath resisting corrosion.

A thin scratch coat with lath showing through is a weak point we look for when veneer fails. Rebuilt areas get a full scratch coat, scored and allowed to set as directed before stone is applied.

The top of a solid stone chimney

Section R1003.9.1 of the residential code tops a masonry chimney with concrete, metal or stone, edged with a drip, and seals a bond break with caulk at every flue liner per ASTM C1283. A stone chimney is covered by that rule.

A cracked cap, a missing drip edge or a failed bond break lets water into the top courses, where it works down behind the stone. We inspect the top every time and quote cap or crown work separately from the stone.

Filling deep joints a layer at a time

Where the old mortar came out more than about an inch deep, the brief has the deep spots filled first, packing mortar in layers of roughly a quarter inch and letting each reach thumbprint hardness before the next goes on. Most shrinkage happens as mortar hardens, so layering keeps it small.

Deep stone joints take several layers and more time. We price that time honestly, because a deep joint filled in one go shrinks, cracks away from the stone and lets water straight back in.

Resetting a loose stone

A loose stone comes out carefully, by hand, without prying against its neighbors. The old mortar is cleaned off the stone's back and the bed it came from, down to sound material.

Then the stone goes back with fresh mortar filling the whole bed, pressed in until mortar squeezes out around its edges, and the joints are tooled to match. Before any of that, we find out why it loosened, since a reset over the same water problem will not hold.

• Removed by hand, neighbors protected
• Old mortar cleaned off stone and bed
• Reset with full mortar coverage
• Joints tooled to the existing profile