- Q. I often see
roof loads calculated based on the horizontal run of the roof.
But isn’t it more accurate to figure the weight of snow
and roofing materials by measuring along the actual length of
the rafter? Thus, as the roof gets steeper, the rafter gets
longer, and the weight of roofing materials increases.
A. Actually, with snow, you don’t get more load
along the slope of the rafter; you get the same load as a flat
roof with the same run would get. This is because as snow falls
vertically it spreads itself further along a sloping rafter and
so accumulates less depth. The BOCA code recognizes this and
allows you to use the horizontal projection of the roof when
calculating snow loads. BOCA also allows you to reduce the snow
load for roofs with slopes greater than 30 degrees, presumably
because snow will slide or blow off steeply pitched roofs.
For dead loads, you are correct.
Technically you should use the actual rafter length when adding
up the weight of roofing materials. However, in my practice, I
typically use the horizontal run of the roof for both types of
load. To do this, I use conservative (too heavy) dead loads and
full snow loads regardless of pitch. I ignore the slope factor
altogether for snow load reduction which adds another measure
of conservatism. (Slope length cannot be ignored for wind load