[Strawbale] Death in collapsing SB house

[Matt Chase]

Hi Sherwood

Not sure where you're coming from, but here in Canada, the national 
building code has a feature for calculating snow loads that is as 
follows:

There is a slope factor (given the symbol Cs) that is multiplied by 
the snow load to reduce it if one can count on sliding off of snow to 
occur.  

For "normal" roofs, it ranges between 1 (when the roof slope is less 
than 30 degrees, approximately a 7:12 slope) and 0 when the roof is 
steeper than 70 degrees (a very steep roof indeed!, a 33:12 slope).  
The factor Cs changes linearly with the angle, so right in the middle 
(i.e. 50 degrees) would give a Cs factor of 0.5, reducing the snow 
load by half.

For unobstructed, slippery roofs (i.e. your steel roof), the range is 
between 15 and 60 degrees (3:12 to 21:12 slope).  For your 12:12 
slope (45 degrees) the factor Cs is equal to 0.33 which means that 
the snow load is reduced by 1/3 - a considerable reduction.

Building codes where you are from probably have a similar means of 
treating the snow load.  So, you could probably build a lighter truss 
by making it steeper.  However, to make the roof steeper, you would 
also need some extra material simply to get up that high.

There is a trade-off involved...

Cheers,



Matthew Chase
MSc. Candidate
Biosystems Engineering,
University of Manitoba
Winnipeg
(204) 880-8403
(204) 237-5315
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