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ROOF DECKS |
4.2.2 Nailing strips and wood edging or curbs should be of #2 or better lumber, treated by the pressure process with a waterborne salt, as
approved by the American Wood Preservers’ Institute Standard AWPB LP-2 for treatment of wood used above ground. Oil base
preservatives such as creosote are not acceptable, as they are not compatible with asphaltic built-up or modified bitumen roofing materials.
4.2.3 Nailers may not be required at perimeters of roof areas that include parapet walls, if an approved securement detail is used to
adequately attach the membrane to the wall.
4.2.4 The use of tapered edging strips is recommended to direct water away from roof edges. Wood nailers should be equal in thickness to
tapered edge strips and insulation, and wider than the flange of metal edge strips or gravel stops, to provide adequate nailing. If interior drains
are not practical, scuppers should be provided to drain water from the roof.
4.2.5 On non-nailable decks where the incline is such that nailing of built up or modified bitumen roofing membranes is required, wood nailing
strips should be provided at the ridge and at intermediate points not exceeding 36'-0" (11.0 m) inside face to inside face. Nailing strips are
installed perpendicular to the slope of the roof deck. (See the appropriate membrane application section for specific dimensions.) These
nailing strips act as a stop for roof insulation as well as a nailing facility for the built-up roofing or modified bitumen felts. Felts are to run
parallel to the incline. On nailable decks, where nailing is required, and insulation is not used, felts may be run perpendicular or parallel to the
incline.
4.3 Roof Drainage
4.3.1 Roof decks should be constructed to have adequate slope, to drain all water to appropriate outlets. Drains or other outlets should be set
below the plane of the roof surface, at low points in the roof. Lower level roofs should be protected from water running off or down from higher
points.
4.3.2 Water standing for more than 24 hours on roof areas is hazardous in many ways:
A. |
Possible seepage of moisture through the membrane into the roof system, as no multiple or single ply
membrane can be applied over a large area by practical roofing methods with complete perfection. |
B. |
Damage to the roof from freeze-thaw cycles during the winter. |
C. |
Deformation of the deck structure. |
D. |
Major interior damage from minor punctures. |
E. |
Relatively small amounts of moisture, beneath the roof membrane, reduce the thermal efficiency of the
insulation and cause serious damage to both the insulation and membrane through repetitive cycles of
evaporation, condensation, freezing and thawing. |
F. |
Ponding of water on roofs, in areas subject to winter freeze and thaw, can create further hazards. Ice
formations move constantly with temperature changes. This movement can "scrub" the roof surface to
such an extent that considerable physical damage to the membrane can result. |
G. |
Insufficient number of drains or inadequate sized drain leaders may restrict immediate run off of surface
water. In such cases, water may back up under the drain flanges or accumulate on the roof surface to a
depth which exceeds the height of the base flashings or other perimeter flashings. Overflow through wall
scuppers should be considered in areas of excessively heavy rainfall or where delayed drainage is
required. Internal drain systems, which provide drainage from the body of the roof, should be planned to
ensure that drains are located at low areas to facilitate water run off. All roof decks should be sloped to
the drains, or crickets installed to ensure proper drainage. The JM Tapered Systems Group can provide
full service manufacturing and roof drainage design capability on tapered insulation systems. These
designs utilize Tapered Fesco Board, Tapered Fesco Foam or Tapered E’NRG’Y 2 to obtain roof
slope and are particularly useful in reroofing applications where ponded water is a problem. (See
Section 9 for details.) |
4.3.3 The effects of ponded water have been studied by members of the roofing industry for many years. The Built-Up Roofing Committee of
the Asphalt Roofing Manufacturers Association unanimously recommends that roof design provide slope for roofs so that the roof drains freely
throughout the life of the building.
4.3.4 This recommendation has also received the approval and concurrence of not only JM, but the National Roofing Contractors Association,
the Midwest Roofing Contractors Association, and the American Institute of Architects, as well as other regional roofing contractors
associations and governmental agencies.
4.3.5 JM will not be responsible for roof damage, or failure of the roofing system, due to standing water. Any standing water must evaporate
within 24 hours, or crickets or saddles must be installed using JM tapered insulation board. JM defines ponded or standing water as water
that remains on the roof for more than 24 hours.
4.3.6 One method of obtaining slope in structural decks is by varying the height of columns and girders to design positive drainage into the
deck surface. In theory, dead-level or low-slope decks should drain, but they rarely do. Elevations of columns are never exact, or deck units
may have a slight camber or curvature, which under normal deflection of the deck from the weight of roofing and rooftop equipment can cause
portions of the roof deck to sag below the level of drains, especially if the drains are located at columns. Even if the deck is designed to drain
immediately after construction, it may not do so after the building settles. A clogged drain can also allow a build-up of water which could
permanently deform the deck.
