Because it usually lasts for more than a century, brick is the most prevalent building substance in New York City. Mortar, however, deteriorates quickly, especially in high-exposure locations along rivers. The most obvious sign of mortar trouble: loose bricks. In many cases, they fall out at comers and along the tops of walls.
The best method of spotting deteriorated mortar before the bricks fall out is to take a pen or screwdriver and scrape lightly at a small patch of mortar. If more than half an inch of it below the face of the brick crumbles, away, you need new mortar. Replacing the mortar is called pointing. In a well-executed, careful pointing job, all of the old mortar between the bricks is removed and replaced with fresh mortar.
Limestone or sandstone appears on hundreds of facades throughout the city. It can last as long as brick, so again look for deterioration in the mortar between the slabs. Stone should be repointed as often as brick. A common problem with stone in general, and limestone in particular, is algae, which usually grows on the outer surface. Algae not only turns the facade green, it also erodes the stone. The best solution is a professional cleaning with herbicides. In addition, stone slabs can develop '''stress'' cracks on the surface. These must be pinned to stop them from spreading.
Brownstone is an attractive facade — and the most difficult type of sandstone to preserve. Brownstone, which does not repel water, is brittle and tends to dissolve. And because it falls apart, cleaning it is nearly impossible. The only viable preservation methods are pointing or replacing it with another, more durable material. (The latter is not allowed in historic districts, however.)
In examining the exterior walls, also take a close look at the numerous "trouble spots" around a building, particularly where water accumulates: coping stones, flashing, lintels and window frames. A comprehensive waterproofing job should include repairs in these areas. Otherwise, repointing can be worthless.
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