Spalling brick repair for chimneys in Hyde Park

Finding the water before replacing the bricks

Bricks spall when water gets into them and something drives the face off, such as salts crystallizing inside the brick or a mortar that will not let the masonry dry. Replacing the damaged bricks without finding that water leaves the new units exposed to the same conditions.

The Brick Industry Association advises investigating moisture sources before repairs. On a chimney we look at the crown or cap, the flashing, the joints, any coating and the way water runs off the roof around it.

The estimate names the water path we find alongside the brick replacement, so the repair fixes the cause as well as the result.

Setting the replacement brick

BIA's maintenance guidance lays out the method. The surrounding bricks are dampened but allowed to absorb the surface water, the replacement brick and the opening are fully buttered with mortar, and the brick is centered and pressed into place.

Excess mortar is struck off, the joints around the new brick are pointed to make sure they are full, and they are tooled once the mortar is thumbprint hard to match the profile of the existing joints.

Full joints around the new unit matter as much as the brick itself, because gaps there would let water straight back into the repair.

Testing the existing mortar

Matching the original mortar is easier when you know what it is. The Brick Industry Association notes that a laboratory can analyze a sample of the original mortar to estimate what it contains, in what proportions and how strong it is, generally following the methods of ASTM C1324.

Most chimney brick repairs do not need a laboratory report, but on older or unusual masonry it can be worth the time. We explain when testing would add value and when a careful comparison on site is enough.

Hairline cracks and open cracks

Not every crack means a brick must come out. A fine hairline crack at the surface behaves differently from an open crack that runs through the face of the brick. The Brick Industry Association notes that a water repellent cannot keep water out of cracks wider than 0.02 inch.

Cracks extending through the face shell are one of the conditions BIA says call for removing and replacing the brick. We measure and describe the cracks we find, so the recommendation rests on what is actually there.