WORLD FOR CIVIL

Roof slope to prevent ponding

Roof beams should have a continuous upward slope equivalent to 1/ 4 in/ft (20.8 mm/m) between a drain and the high point of a roof, in addition to minimum recommended camber to avoid ponding. When flat roofs have insufficient slope for drainage (less than 1 /4in/ft) (20.8 mm/m)

the stiffness of supporting members should be such that a 5-lb/ft² (239.4 N/mm²) load causes no more than 1 /2 -in
(12.7-mm) deflection.
Because of ponding, snow loads or water trapped by gravel stops, parapet walls, or ice dams magnify stresses and deflec- tions from existing roof loads by

C_p=1/[1-W’L³/?⁴EI

where
C_p= factor for multiplying stresses and deflections under existing loads to determine stresses and deflections under existing loads plus ponding
W =weight of 1 in (25.4 mm) of water on roof area supported by beam, lb (N)

L= span of beam, in (mm)
E= modulus of elasticity of beam material, lb/in²
(MPa)
I =moment of inertia of beam, in⁴ (mm⁴)