Untitled - National Roofing Contractors Association

than 2-1/2 inches (1/240 of 50 feet) and should have
drains located at midspan in order to provide drainage
under maximum loading conditions.
Figure 1
If drains are required to be placed at columns or bearing
walls, the slope of the roof must be increased in order
to compensate for the minimum deflections that exist
at these locations. The allowable deflection will still
be 2-1/2 inches, but the designer must provide for 5
inches of additional slope at the column or bearing wall
location in order to keep this deck level at midspan under
maximum loading conditions. (See Figure 2.)
Positive Slopes
After the drain locations have been selected and the
deflection slopes computed, the designer must provide
additional slope to ensure positive drainage. Because
drainage must occur under both minimum and maximum
loading conditions, an additional minimum slope of 1/8
inch per foot should be added to the deflection computation
in order to attain a slope that will positively drain
the roof. Thus, to drain the 5O-foot span in Figure 1, where
the drain is placed at the point of maximum deflection,
the designer should provide at least 3.125 inches of
differential slope for positive drainage under all loading
conditions. For Figure 2, where the drain is located at
a point of minimum deflection, the positive slope required
is 6.25 inches, which, when added to the 5 inches of
deflection slope, yields a total required slope of 11.25
inches under all loading conditions.
Certain decks, such as precast concrete decks or Iongspan
prestressed concrete decks, incorporate camber in
anticipation of future loading conditions. The camber
must be considered in the design of the drainage slope
system. Depending on the structural design of the roof
and the placement of drains, the camber may assist or
restrict drainage. (See Figure 3.)
Maintaining a roughly level line between one support and
a 2-1/2 inch midspan deflection requires raising the other
support 5 inches.
-Note: Figures 1,2, and 3 have been exaggerated to help
illustrate drainage conditions more clearly.
These examples illustrate that the computations for roof
slope should be determined by the deflections expected
in each particular roof deck and that the commonly
specified 1/8 inch or 1/4 inch of slope per foot is inadequate
as an absolute specification to attain proper drainage
on all roofs.
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