Masonry Magazine June 1993 Page. 20
Flashing
Flashings are recommended for any location where the potential exists for water penetration. This includes tops of walls at copings, over openings, beneath sills, over bond beams, at shelf angles, tops of foundations, and in walls at ground level to serve as a water stop.
Flashings are intended to intercept water movements. Therefore they must be designed to prevent the movement of water and cause it to be redirected to a location where it will no longer affect the building, usually the exterior.
When selecting the flashing material for a particular application, service conditions, projected life of structure, and past performance characteristics of the flashing materials should be reviewed. Flashings should be designed to perform satisfactorily for the life of the building, since repair or replacement is very labor intensive and expensive.
FLASHING MATERIALS
Flashing may be composed of a wide variety of materials including stainless steel, copper, a combination of copper and lead, polyvinyl chloride (PVC) and other plastics. When making a choice, consideration should be given to the selection of the material which provides the best:
o resistance to puncture during installation
o resistance to degradation by ultraviolet light
o performance at elevated temperatures
o compatibility with joint sealants.
Flashing boots for inside and outside corners may be available. These boots eliminate the necessity for cutting, folding, or tucking the flashing material at both locations.
Stainless Steel
Stainless steel is technically any of a large and complex group of corrosion resistant iron chromium alloys possessing excellent weather and chemical resistant properties. Preformed sections must be properly sized so that modification on the site is minimal. Stainless steel flashing with conventional annealed finish should comply with ASTM Specification A167. Generally, Type 304 stainless steel with a minimum thickness of 0.010 inches is satisfactory. Lap sections require solder conforming to ASTM Specification B32 (60% tin and 40% lead).
Copper
Copper is a nonferrous metal possessing good ductility and malleability characteristics. Like stainless steel, it also possesses excellent weather and chemical resistant properties. Preformed sections or sheet materials are easily modified to conform to site requirements. However, copper flashings can impart an objectionable green stain to masonry surfaces.
Plastics
Plastics are categorized as polymeric materials of large molecular weight, usually polyvinyl chloride, polyethylene, or urea-formaldehyde. Dependent upon their composition, plastics may perform very well in alkaline environments common to masonry.
Manufacturers of plastic flashings should be consulted for documentation establishing the longevity of the plastic in a caustic environment (pH = 12.5 to 13.5), the composition of the plastic, and ease of working at temperatures ranging from 20° F to 100°F.
Composite Flashing
Combinations of metals and plastics are supplied by some dealers. The composition and service records of these combined materials should be determined before use. Aluminum flashings should be avoided, as an adverse chemical reaction occurs when caustic solutions contact aluminum.
CONSTRUCTION PRACTICES
To perform satisfactorily, flashings must be installed properly or they may aggravate rather than eliminate water problems. Flashings should be longitudinally continuous or terminated with an end dam. Longitudinally continuous implies that lap joints must be overlapped a sufficient distance to prevent moisture from entering between the joints and bonded together with adhesive to prevent water movement through the lap area.
Flashings should be secured at their upper level by embedment into an adjacent wythe, a reglet, or adhesively attached so that water cannot infiltrate or move behind the attachment. The flashing should then project downward along the outer surface of the inner wythe and then project outward at the masonry joint, shelf angle, or lintel where it is to discharge the water. Every effort should be made to have the flashing slope to the exterior. Effectively placed mortar bed or sealant material can help to promote this drainage. The flashing should terminate beyond the exterior face of the masonry. Although considered by some to be aesthetically objectionable, the flashing should terminate with a sloped drip ledge.
The installed flashing should be maintained free of debris or obstructions in order to perform its intended function. By omitting every third masonry unit in the course seated on the flashing, openings are formed which serve as cleanout locations which are later filled with units after the masonry work has been completed.
Other recommended practices involve the use of concave mortar joints to reduce intrusion of mortar into collar joints or cavities. Masons should be careful to ensure that mortar droppings to the flashing reservoir are minimized. In addition, gravel beds and screens are often used to prevent mortar droppings from collecting on the flashing to form dams and block weepholes. However, this practice may create moisture problems in the courses above.
Weepholes, the inseparable companion to flashings, should provide free movement of water from the concrete masonry cores, collar joints, or cavities. Any construction practice which allows forming the weepholes without inhibiting water flow may be used.
Rope wicks or plastic tubing, with or without screens, are considered less desirable than an unobstructed opening. Insects can be thwarted by inserting stainless steel wool into the openings.