CIVIL ENGINEERING FORMULAS

172 CHAPTER SIX

For wood screws,

(6.44)

(6.45)

whereCegis the end-grain factor.
For lag screws,
(6.46)
(6.47)
For metal plate connectors,

(6.48)

For drift bolts and drift pins,

(6.49)

(6.50)

For spike grids,

(6.51)

ROOF SLOPE TO PREVENT PONDING

Roof beams should have a continuous upward slope equivalent to 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
insuffic-ient slope for drainage (less than in/ft) (20.8 mm/m) the stiffness
of supporting members should be such that a 5-lb/ft^2 (239.4 N/mm^2 ) load
causes no more than -in (12.7 mm) deflection.
Because of ponding, snow loads or water trapped by gravelstops, parapet
walls, or ice dams magnify stresses and deflections from existing roof loads* by

(6.52)

whereCpfactor for multiplying stresses and deflections under existing loads to
determine stresses and deflections under existing loads plus ponding
Wweight of 1 in (25.4 mm) of water on roof area supported by beam, lb (N)
Lspan of beam, in (mm)
Emodulus of elasticity of beam material, lb/in^2 (MPa)
Imoment of inertia of beam, in^4 (mm^4 )

Cp

1

1 WL^3 /

4 EI

(^1) 
2
(^1) 
4
(^1) 
4
ZZCDCMCtC
ZZCDCMCtCgCCdCeg
WWCDCMCtCeg
ZZCDCMCt
ZZCDCMCtCgCCdCeg
WWCDCMCtCeg
ZZCDCMCtCdCeg
WWCDCMCt
*Kuenzi and Bohannan, “Increases in Deflection and StressesCaused by Ponding of Water on
Roofs,” Forest Products Laboratory, Madison, Wisconsin.