Flashing
No cladding is perfectly waterproof. Wind driven rain will find its way behind stone panels where pointed cement mortar or caulking separates the stones. Porosity, which is the volume of voids related to the apparent volume of the stone, under pressure of wind driven rain could let water seep through. Condensation can also produce moisture on the back side of the stone panels. Therefore, a second line of defense is necessary to collect and let the water out of the cavity behind the stone panel, and ventilate the cavity to keep the stone and the back-up masonry dry.
Properly designed weep holes, weep slots, and flashing serve this purpose. Flashing is a flexible material installed at one end, higher up against the structure, and turned at the other end, into the stone joint. Waterproof, rubberized fabric, polyethylene, or soft neoprene sheets, or soft thin-gauge stainless steel flashings are the most widely used.
-
An experienced, qualified person, with a thorough understanding of the cladding system, including the windows, is needed to design the flashing and the components of the secondary water defense. Leaving it only to the person installing the flashing is NOT recommended.
Prefabricated systems, such as stone on trusses, or stone on unitized curtainwall panels require a galvanized sheet metal water defense designed behind the stone slabs, including gutters and weep-tubes to collect and discharge water from the cavity.
-
source: Building stone Institute