Various numbers of plies can be used in the construction of a BUR sys-tem. Figure 5-11 illustrates the use of three-, four-, or five-ply mem-branes over a structural deck (without insulation) that can receive and adequately retain nails or other types of mechanical fasteners, as rec-ommended by the deck manufacturer. Examples of such decks are wooden plank and plywood. This procedure is not for use directly over gypsum, lightweight insulating concrete decks either poured or precast, or over fill made of lightweight insulating concrete.
Nail
Min. 31/2"
nailer
3/4"
Insulation
81/2"
6" lap
2" lap Mineral
surfaced cap sheet
Felts
D
F I G U R E 5 - 1 0 At points of termination, locate five nails at each nailer.
Nail
Min. 31/2"
nailer
3/4"
81/2"
36"
27"
18"
F I G U R E 5 - 9 Pattern for nailer and spacing with four-ply system.
Figure 5-12 shows a four-ply gravel-surfaced fiberglass mat BUR application that can be used over any type of structural deck that is not nailable and that offers a suitable surface to receive the roof. Prime poured and precast concrete decks.
Flashing
Flashings prevent water intrusion at any roof area where the mem-brane is interrupted, is terminated, or joins an area or projection having
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CHAPTER FIVETABLE 5-4 Nailer Spacing for Various Roof Types Incline (in.) Nailer spacing (D) Asphalt type
Asphalt/cap sheet roofs
0–1⁄2 Not required II
1⁄2–1 Not required III
1–2 20' face to face III
2–3 10' face to face III
3–6 4' face to face IV
Asphalt/smooth-surfaced roofs
0–1⁄2 Not required II
1⁄2–1 Not required II
1–2 Not required III
2–3 20' face to face III
3–4 10' face to face IV
4–6 4' face to face IV
Asphalt/gravel-surfaced roof
0–1⁄2 Not required II
1⁄2–1 Not required III
1–2 20' face to face III
2–3 10' face to face III
81/2"
a marked change in slope or direction. This condition can occur at gravel stops, curbs, vents, parapets, walls, expansion joints, skylights, drains, and built-in gutters.
Flashing is generally divided into two categories, base flashings and counterflashings. Base flashings are, in a sense, a continuation of the membrane that is turned up on a surface and installed as a separate operation. Base flashings are usually of a nonmetallic material, such as a plastic, or an asphalt-impregnated product. Counterflashings or cap flashings can be made of metal, plastic, or impregnated felt. They shield or seal the exposed edges of the base flashing.
Combining nonmetallic materials and metal brings out the best in each material. Since the bituminous-based flashings have the same movement capabilities as the roof membrane, they work together as a unit and are recommended as the only type of material for base flash-ings. Because of the rigidity of metal and its extreme movement with temperature changes, its use for base flashing is not recommended.
Metal cap or counterflashings, installed where there is no possibility of standing water, are acceptable only after composition base flashing is properly sealed.
Flashing can be applied by hot or cold application methods.
Asphalt-coated fiberglass can be applied in a hot mopping of bitumen.
The advantages of hot-applied flashing are speed, economy, and con-venience. The proper grade of asphalt must be used, however. Do not use coal-tar pitch or low-softening-point asphalt because flashings embedded in these bitumens sag. Secure hot-applied flashings at the top edge with mechanical fasteners spaced 4 to 8 inches on center or by some other positive means to prevent sliding or sagging.
Use reinforced base flashing in conjunction with a ply of fiberglass-ply felt as a backer felt and set in a mopping of hot asphalt to the primed masonry surface. The desired guarantee period to be offered for the roof’s life determines the need for a backer felt.
The cold-application system consists of alternate layers of ply felt set in and covered with trowelings of bestile flashing cement. The thinner ply felt conforms readily to the angle between deck and para-pet. Bestile flashing cement provides adhesion to the primed wall and to each ply, enhances interply waterproofing, and provides more time for careful application.
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CHAPTER FIVEFlashing systems of this type set up quite hard, so they do not have the tendency to sag or run under hot temperatures. They are especially prac-tical when no nailing facilities are available. A major benefit to the bestile flashing system is its compatibility with any bituminous roofing system.
There are two main types of bituminous flashing cements for cold application. Industrial roof cement is a nonasbestos, fiber-reinforced, solvent-cutback, all-purpose plastic cement. This cement remains in a somewhat plastic state and is used where some movement is expected, such as under a metal roof edge, where the differential movement between metal and felt is absorbed in the plastic cement. It also is used to repair blisters and to accomplish general roof maintenance by itself and in conjunction with fiberglass-ply felt on horizontal surfaces.
Bestile flashing cement is basically the same as industrial roofing cement, but it contains certain nonasbestos reinforcing fibers. This material sets up quite hard instead of remaining plastic. Bestile is used almost exclusively for flashing work on vertical surfaces where differ-ential movement is not expected. It should not be used with metal flashing.
The solid fastening of all flashing accessories to wooden nailers, or the substrate, is mandatory. Unequal movement due to poorly secured flashing results in splits in the felt stripping and subsequent leaks.