The origin of flash and flashing are uncertain but these words are related to a pool of water and the word splash. Usage may come from Middle English between 1350–1400 “…flashed to sprinkle, splash [compare with] flask…” Dictionary quotes refer to lead as the material used as a flash. Counter-flashing (cover flashing, cap flashing) is when there are two parallel pieces of flashing are used together such as on a chimney where counter-flashing is built into a chimney and overlaps a replaceable piece of base flashing. Strips of lead used for flashing an edge were sometimes called an apron, the term is still used for the piece of flashing below a chimney. Flashing may be let into a groove in a wall or chimney called a reglet.
Flashing refers to thin pieces of impervious material installed to prevent the passage of water into a structure from a joint or as part of a weather resistant barrier (WRB) system.
Before the availability of sheet products for flashing carpenters used creative methods to minimize water penetration such as angling roof shingles away from the joint, placing chimneys at the ridge, and building steps into the sides of chimneys to throw off water. Birch bark was occasionally used as a flashing material. The introduction of manufactured flashing decreased water penetration at obstacles such as chimneys, vent pipes, walls which abut roofs, window and door openings, etc. thus making buildings more durable and reducing indoor mold problems. In builder’s books, by 1832 Loudons An Encyclopedia of Cottage, Farm, and Villa Architecture and Furniture… gives instruction on installing lead flashing and 1875 Notes on Building Construction gives detailed instruction and is well illustrated with methods still used today.
Membrane roofing is a type of roofing system for buildings and tanks. It is used on flat or nearly flat roofs to prevent leaks and move water off the roof. Membrane roofs are most commonly made from synthetic rubber, thermoplastic (PVC or similar material), or modified bitumen. Membrane roofs are most commonly used in commercial application, though they are becoming increasingly more common in residential application.
Synthetic Rubber (Thermoset) – This type of membrane roof is made of large, flat pieces of synthetic rubber or similar materials. These pieces are welded together at the seams to form one continuous membrane. The finished roof’s thickness is usually between 30 and 60 miles (thousandths of an inch) (.75mm to 1.5mm). Other types of related materials are CSPE, CR, and ECR.
Thermoplastic Membrane – This is also similar to synthetic rubber, but the seams do not form a continuous membrane. The ‘lap’ seams are bonded (melted or dissolved) with heat or solvents, and can be as strong as the rest of the membrane. Other related materials are CPA, CPE, EIP, NBP, PIB, and TPO.
These three application types of membrane roofing show distinct advantages over the previously more common flat roofing method of asphalt and gravel. In asphalt and gravel application, it can be very difficult to create a proper seal at all seams and connection points. This can cause many roofs to leak early in its lifespan, and require much more maintenance. When installed correctly, newer materials are either seamless, or have seams as strong as the body. This eliminates much of the leakage concerns associated with flat roofing systems. Repairs for asphalt and gravel roofs can be hard, mainly because it is difficult to locate the exact point of a leak. Newer systems can be patched relatively easily, and breaks and leaks are easier to locate. Originally asphalt roofing required a layer of gravel above it for two reasons. First, asphalt with direct exposure to sunlight degrades much faster, mainly due to the expansion and contraction throughout a day, and also the damage created by UV rays. Second, asphalt needs weight above to hold it down, because it sits on the top of a building, instead of being attached to it. Each of the three newer types of membrane roofing systems contain materials that resist expansion and contraction, as well as reflect much of the UV rays. Also, because these membranes either lack seams or have strong seams, what expansion and contraction does occur does not create leaks and breaks at these seams. These newer roofing systems are also attached directly to the top of a building, which eliminates the need for excess weight above.