Untitled - National Roofing Contractors Association

. Dead loads, such as topside mechanical equipment
and the deck itself
. Deck strength
. Deflections
. Drainage
Placement of expansion joints and area dividers
. Curb details
. Attachment provisions for the deck
. Rolling construction loads of 300 pounds
The following chart shows the typical allowable deck
deflections for various spans under a concentrated load
of 300 pounds. Deflections greater than those listed in
the table may lead to roof problems.
~
4 feet
5 feet
6 feet
Deck Deflection
0.20 inches
0.25 inches
0.30 inches
Before applying roofing materials, the roofing contractor
should make a visual inspection of the deck surface to
see that it is ready for application. The deck must be
clean, smooth, free of wids or depressions and rigid, and
it must not deflect excessively under live loads.
If the roofing contractor discovers defects in the surface
of the deck during this inspection, a second inspection
should be made prior to roofing by the roofing contractor,
the general contractor, the deck contractor and the
owner's representative. All defects in the deck at the time
of this inspection should be noted, and corrections
should be made before beginning work. The trade contractor
who is responsible for any defects or damage to
the deck should be responsible for making repairs. Job
specifications should clearly define this responsibility.
VI. SLOPE AND DRAINAGE
All roofs should be designed and built to ensure positive,
thorough drainage. (See Positive Drainage in the
Glossary.) Ponding water can be detrimental to roof membranes
and can result in:
. Deterioration of the surface and membrane
. Debris accumulation and vegetation growth
. Deck deflections (sometimes resulting in structural
problems)
. Tensile splitting of water-weakened felts
. Difficulties in repair should leaks occur
. Ice formation and resulting membrane damage
Because every roof has its own specific set of drainage
requirements, either the architect or the structural engineer
is responsible for including proper drainage pr0-
visions in the roof design. The designer should not simply
specify a standard 1/8 inch or 1/4 inch of slope per
foot but should make provisions in his design for positive
drainage. In order to achieve the necessary slope
in his design, he must consider the structural framing
of the roof, the deck type, the roof membrane specification,
roof deflections and the building la)Uut. The necessity
for brevity here prevents a complete explanation of
each of these considerations. The following brief examples,
however, are typical slope and drainage computations.
Deflection Slopes
Roof deflections are critically important in providing roof
drainage. They should be limited to no more than 1/240
of the roof span in order to accommodate the stresses
of either concentrated or uniform loading. Drains should
be located at points of maximum deflection in the deck
(i.e., midspan) if possible and not at points of minimum
deflection (i.e., columns or bearing walls). For example,
in Figure 1 a deck span of 50 feet should deflect not more
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