Flashing performance requirements
The following performance requirements should be considered when materials to be used as flashing are selected:
The flashing assembly that includes materials and joints must shed water without allowing leakage. The ability of the flashing to seal and be detailed to avoid leakage is fundamental. Creating an effective and durable joint seal is often difficult. It is good practice to provide a secondary, continuous flexible membrane flashing under jointed materials such as brick, stone or sheet metal.
The flashing must be able to accommodate differential thermal and structural movements. As a result, it must either be made of a flexible material or have joints designed to accommodate movement such as thermal expansion and contraction of all building components including the flashing itself, normal deflections under service loads, shrinkage and creep of concrete and wood; and expansion of brick after construction.
Terminations should be formed into sharp breaks and to be sufficiently rigid at points of discharge to adequately project water away from materials below.
The flashing must be tough enough to resist physical damage during construction, as well as normal wear, which may be related to the environment and building maintenance activities. Other factors to be considered include deterioration from corrosion, metal incompatibility and galvanic action, deterioration due to exposure to ultraviolet (UV) light, extreme (hot and cold) temperatures, freezing water and fatigue due to movement.
For more information on durability, consult the CSA Standard S478-95: Guidelines on Durability in Buildings. In general, the service life of flashings must be equal to or more than that of the wall system or roof system at locations where maintenance or replacement of flashings would be uneconomical.
Flashings along with their primers and sealants must all be chemically compatible with adjacent materials. Avoid contact between dissimilar metals, as this can lead to galvanic corrosion when the metals are moist. Water acts as the electrolyte, and dissimilar metals as electrodes.
Alkaline concrete and mortar aggressively attack materials such as aluminum and copper. These materials should be protected from contact with concrete or mortar by the sheathing membrane or other suitable materials. Consult the manufacturers of selected materials to determine whether adjacent materials will be compatible.
Creating a flashing detail that is easy to build will greatly increase the likelihood that the flashing will be built to perform acceptably. Ask yourself the following questions:
- Can the installation of flashings be sequenced to be separate from other construction activities, or can this be easily incorporated into other work? For example, coordination may be required between roofing and masonry activities at wall-roof junctions.
- If the installation of the flashings is to be incorporated into other work, do the workers have the required technology and skills?
- Can the flashing accommodate normal stresses?
- Is it possible to mould or modify the material to accommodate unusual conditions, or is it possible to repair damage?
Besides durability, future maintenance of the flashing must be considered. Materials or joint sealants with limited service lives should be avoided if the flashings are not accessible for replacement. Metal flashings built into concrete or masonry cannot be removed to allow for inspection or repair of the materials underneath. This should be considered when selecting materials and details for flashings. The flashings and sealants should be inspected annually, particularly in areas with the greatest exposure to water and sunlight.