Re: Snow load evaluation

[nobody@xxxxxxxxxxxx (Dave Houston)]

Lewis Gardner <lgardner@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>L. M. Rappaport wrote:
>
>> You could put stress gauges on the roof rafters, but you'd still have the
>> problem of calibration.  There are just too many variables - rafter size, rafter
>> spacing, decking, shingle material, water content of the snow, ice, etc - to be
>> able to do this in advance.
>
>I used to do something like this in my last garage.
>
>That garage lacked a BIG A** beam to connect a hoist for lifting motors
>and other big stuff. As a workaround I placed a 2" steel pipe across 5
>roof trusses and connected the chain hoist to the mid point. Using a
>tape measure I would check the deflection of the middle truss. As long
>as it was less than .5" (20' span) I didn't worry.
>
>If I were to attempt a similar snow load sensor I would measure the mid
>span deflection of a rafter in the middle of the largest lowest pitched
>section of the roof. There are no magic numbers to go by but over time
>you might be able to get a feel for "well the snow in 07 deflected the
>rafter .25" and the house didn't collapse" so this snowfall is not that
>bad (yet)!
>
>Just a thought...

A concentrated load like an engine hanging from your chain hoist is quite
different from the more evenly distributed snow load.

As a practical matter there are probably far too many variables such as the
type of roof, pitch, age, whether trusses are factory built or assembled on
site, whether gussets are metal or plywood, average weight of a cubic foot
of the snow, dead weight of the roof, attic insulation, etc. but the purpose
of the truss is to uniformly distribute the load with minimal deflection of
any of the truss members and to transfer this uniformly to the walls and
foundation.

Here's a guide to roof truss design which discusses the various types of
loads...

     http://sleekfreak.ath.cx:81/3wdev/CD3WD/CONSTRUC/H0788E/INDEX.HTM

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