Moisture is thought to be the most damaging attack on masonry construction, believed to cause up to 90% of masonry issues and failures. Water can enter the inside of a wall cavity as a result of cappilary action and thermal bridging. It can break down joints, degrade anchoring systems, and cause mold to generate. Masonry flashings are utilized (in combination with complete watershed systems) to help migrate water from behind the brick or block to the surface of the wall. Masonry wall flashing should serve as a moisture collection location and also offers a barrier to infiltrating moisture which diverts it away from the inside of the wall cavity. Flashings are commonly placed anywhere there is a horizontal interruption between the masonry veneer and the backup, i.e. base of wall, heads, sills, lintels, shelf angles, and tops of parapet walls. One of the most common locations for wall flashing is at the base of the veneer on top of the foundation wall. Here flashing is carried the length of the wall, should be lapped, and turned down past the bottom of the veneer creating a drip edge. Headers and sill flashing should be constructed with an ‘end-dam’ to keep draining water from falling behind the flashing and back into the wall cavity.
Some common types of masonry wall flashings are:
– Zinc Coated Metals – Performance: Fair – Coating will eventually corrode and flashing will fail. Ease of placement: Good. Cost Factor: Good
– Copper – Performance: Excellent – Good moisture barrier, resists alkali in mortar, more puncture resistant than plastic or synthetics. Typically laminated with with polyethylene film. Ease of placement: Fair. Available in 3oz, 5oz, and 7oz thickness. Cost Factor: Fair.
– Stainless steel – Performance Very Good – Impervious to moisture, chemical resistant. Ease of placement: Fair. Available in 10mil sheets or thicker. Coast Factor: Poor.
– Polyvinyl Chloride (PVC) – Performance: Good – Homogenous, waterproof, impermeable, crack resistant to -20 degree F, resistant to mortar alkali. Ease of placement: Good. Typical minimum thickness of 20mils. Most commonly used flashing material today. Cost Factor: Very Good
– Textile Bonded Bituminous coating and copper/lead sheeting – Performance: Good. Waterprrof, mortar-alkali/acid resistant. Ease of placement: Fair to Good – Cost Factor: Good.
– Polyester Coated Foil and Glass Fiber mesh – Performance: Good – Waterproof, cuastic chemical resistant. Ease of placement: Very Good. Available in 7 mil thicknesses or thicker. Cost Factor: Very Good.