Rain-on-Snow Surcharge Loads - Snow Loads - page 92

average intensity of 0.113 in. of rain per hour for a 12-hour period. On Janu-
ary 2, 1997, Seattle had a ground snow load of 20 lb/ft 2 and a rain event with
an average of 0.117 in. of rain per hour for an 11-hour period. Unfortunately,
the return period for this holiday storm event is unknown. For the two cities
considered, the average roof surcharge load was 4.60 lb/ft 2 .
The infl uence of roof slope and eave-to-ridge distance is shown for the
Yakima and Seattle case histories in Table G10-3. As one might expect, the roof
surcharge is an increasing function of eave-to-ridge distance, W , with the sur-
charge for W 20 ft. Similarly,

the surcharge is a decreasing function of roof slope, with the value for a ¼-on-12
(1.19°) roof slope being four to fi ve times that for a 4-on-12 (18.4°) roof slope.
The rain-on-snow provisions in Section 7.10 require that a 5 lb/ft 2 sur-
charge be added to the fl at roof snow load for roofs with slopes (in degrees)
less than W /50 ( W in feet) in regions where the ground snow load is 20 lb/ft 2 or
less. As noted in the ASCE 7 Commentary, it is assumed that rain refreezes in
ground snowpack (Scenarios 1 or 2) for sites with p g >

= 250 ft being three to fi ve times the value for W =

20 lb/ft 2 . For sites with
lower ground snow load, it is assumed that rain percolates through the roof
snowpack.
The ASCE 7-10 provisions follow the pattern in Table G10-3 in that the
rain-on-snow surcharge is required only for wide, low-sloped roofs. Also, the
prescribed magnitude of the surcharge, 5 lb/ft 2 , is reasonably consistent with
the surcharge loads given in Table G10-3. Table G10-4 presents, for the vari-
ous roof slopes, the eave-to-ridge distance that would trigger the rain-on-snow
surcharge (i.e., W [ft] roof slope [in degrees]). Similarly, the solid line in

Table G10-3 identifi es combinations of slope and eave-to-ridge distance (i.e.,
those below and to the left of the line) for which a surcharge is required when
p g ≤

≤ 50 ×

20 lb/ft 2 .

Example 10-1 Uniform Design Snow Load,

Monoslope Roof (¼ on 12)

Problem
Solve the same problem as Example 4-1, except the slope is ¼ on 12 and p g
is 15 lb/ft 2 .

Table G10-3 Average Roof Surcharge Loads for 1996-1997 Holiday Storm Events in

Yakima and Seattle

Roof Slope (degrees)

Eave-to-Ridge
Distance
W (ft)

0.60
(1/8 on 12)

1.19
(¼ on 12)

2.39
(½ on 12)

4.76
(1 on 12)

9.46
(2 on 12)

18.4
(4 on 12)

20 2.53 1.54 1.00 0.81 0.51 0.31

50 4.40 3.38 2.05 1.15 0.89 0.62

100 5.05 4.60 3.56 2.12 1.15 0.90

250 5.28 5.20 4.90 4.06 2.60 1.37

Source: O'Rourke and Downey (2001), ASCE.