Brick window sills tend to need replacing over the years. All brick sills can fall apart over time. When moisture penetrates the mortar joint it can have an avalanche affect on the entire sill (especially in a clay brick) and wall below. Moisture can completely disintegrate the brick to a point where the sill itself can become a dangerous hazard.The only way to guarantee the installation of a sill that will last is to use single or multiple precast sills.This reduces the points of penetration and provides a proper drip edge to keep moisture away from your home, therefore creating better protection for your home while improving its esthetic appearance. Precast sills are guaranteed to last. more... See more text