If anybody is worried about their roofs, please use at least 320 kg/m³ for wet
snow on sloped roofs. If you have a flat roof, determine whether the drain is
working, it will be the difference between 320 kg/m³ wet snow and 960 kg/m³
slush.
I believe that those figures assume an accumulation of 80 cm of "snow",
whether it be light and fluffy, wet and dense, slushy, or icy. If 80 cm
of snow falls, then, due to insolation (not insulation!) and warming by
air or by conduction from below, the depth will no longer be 80 cm.
And of course, not all snow accumulations are 80 cm in depth.
Rather than recommending a load estimate figure (that is based on 80 cm
of accumulation of whatever sort -- snow, slush, ice), it might be
preferable to teach the method, which then can be adapted to any given
precipitation amount.
I collect and report daily precipitation data for CoCoRaHS
http://www.cocorahs.org
including snowfalls. For snow I measure the depth of the accumulation,
collect snow from an area of known size, and weigh it to determine the
"rainfall equivalent". Working backwards for this winter for some of our
snowfalls, I observed that the snow:rain ratio might be better stated as
1 cm:0.7 mm
1 cm:0.75 mm
1 cm:1.2 mm (followed rain and freezing rain)
1 cm:0.26 mm (notes indicate unusually light & fluffy snow
with large "flakes")
As you can see, there is quite a bit of variation in those ratios for my
location. Generally, I would tend to characterize our snows as averaging
0.7 mm to 0.8 mm rainfall equivalent in 1 cm of snow from what I have
seen the last 4 years. As John suggests, other areas might typically see
snow of a different average density.
This variation is exactly why meteorologists melt (or weigh) fallen and
accumulated snow to determine its actual water content. If one is
concerned and capable enough to estimate roof loading, they probably
should do likewise.
By the way, some architects might show maximum snow loadings on the
plans for the structures built from those plans.
Jim
On 2011-02-09 1208, John M. Steele wrote:
I just received Metrication Matters 93, and saw the aircraft hanger
example again. As it is snow season in the US, and people need to worry
about their roofs, I have to point out two huge errors in the example as
I don't believe anyone should rely on that example.
80 cm of snow != 8 mm of rain
Even if the 10% rule were true, it would imply 80 cm of snow is 80 mm of
rain. Doing some Googling on snow load and roof designs, I find the
density of wet, heavy snow is more like 32-33% water density, 320 - 330
kg/m³. The Washington (DC) area is not noted for light, fluffy powder,
and light fluffy powder isn't what collapses roofs. Using the 320 kg/m³
x 0.8 m, the actual roof load was more like 256 kg/m³ if the drainage
system was still working, not the 8 kg/m² of the worked example.
Flat roofs are a particular problem as snow tends to clog drainage and
then you get slush, a mixture of ice and water. Not surprisingly, the
density of slush lies between 920 kg/m³ (ice) and 1000 kg/m³ (water).
The figure above of 320 kg/m³ is for drained (but wet) snow - imagine
snow on a screen so any water melt can drip out.
I can't find the spec for flat roofs, a lot of local codes in the
northern US are 35-40 lb/ft² for sloped roofs. That converts to 170
kg/m². Some extreme snow areas are higher, and I would expect flat roofs
to be higher.
If anybody is worried about their roofs, please use at least 320 kg/m³
for wet snow on sloped roofs. If you have a flat roof, determine whether
the drain is working, it will be the difference between 320 kg/m³ wet
snow and 960 kg/m³ slush.
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
