Snow Loads Simplified

I'd like to address a common misconception I see in calculating the dead weight of a solar system. Ground Snow load does not equal load on the Solar Modules!

ASCE 7-05 addresses this pretty clearly, although I don't have my book in front of me, I'll summarize, then bring some math home tomorrow.

Snow load on flat roofs =.7*ground snow*importance factor*exposure factor*thermal factor

Where:
ground snow= ground snow load at your location

Importance factor=most often 1 unless its an area where many people often congregate, then it can be more like 1.1 or 1.2, .8 for Cow barns, unoccupied buildings.

Thermal factor= I mostly use 1.2 as the Solar modules aren't exactly heated but the buildings usually are.
1.2 when the structure is totally unheated
1.1 when the roof is cold ventilated, and with R value between heated and unheated space exceeds 25
.85 for heated greenhouses with r value less than 2



Exposure factor= Depends on whether your installation is fully exposed, partially exposed, or sheltered, also what exposure category you have, usually b, c or d

Fully Exposed: Roofs exposed on all sides with no shelter. Shelter is defined as trees, buildings or obstacles within 10 times the difference between building height and obstruction height, trees that drop their leaves in winter do not count.

Sheltered: Roofs located tight among Evergreen trees or other shelter
Partially Exposed: All other roofs.



Category--Fully Exposed --Partially Exposed --Sheltered
b

---

.9

---

1.0

---

1.2
c

---

.9

---

1.0

---

1.1
d

---

.8

---

.9

---

1.0

Note: Monoslope roofs of less than 15 deg, Hip/gable roofs with the slope less than either 2.38 deg or (70/width of roof from ridge to gutter+.5) may not be reduced below 20 psf.

Also note:Rain on snow surcharge load applies to locations where ground snow is 20 psf or less, and the roof slope is less than (gutter to ridge length in ft/50) shall have a 5 psf added load.

Next, if you have a sloped roof, multiply the roof snow load by the slope factor to get your adjusted snow load on the modules.


slope factor=Most conservatively (thermal factor 1.2)on a slippery surface this is a range between 1.0 @ 15 deg and 0 at 70 degrees with linear interpolation between these 2 values. Note: slope factor only applies to installations where the modules are at the lower edge of the roof, or over a slippery roof surface.

So for slippery surfaces approximately
4 on 12=.93
6 on 12=.8
8 on 12=.68
12 on 12= .45

shingles, composite roof surface ect:
12 on 12 =1
45 degrees=.8
60 degrees=.4
70 degrees=0

Wow, I thought I could make this quite simple, but after rereading, it might be more difficult for someone who hasn't done this before to get it.