[Strawbale] Roof Slope (was Re: Death in collapsing SB house)

[Rob Tom]

On Thu, 10 Apr 2008 19:56:34 -0400, Sherwood Botsford  
<sgbotsford at gmail.com> wrote:

> With a metal roof, what is the cheapest slope to make the roof?

There's "cheapest" and there's "optimal".

"Cheapest" would be the lowest slope that is acceptable for exposed  
fastener-type, pre-formed metal roofing -- generally 2/12  (ie 2 units  
rise for every 12 units horizontal run).
That would make for the shortest lengths of material required.

If the roof is exposed to wind, the wind will tend to keep the roof swept  
clean of snow, unless of course it's a mild locale and the snow tends to  
be wet and heavy. If the roof is in a sheltered location, the low slope  
will tend to allow accumulations to occur and that could be risky (ie  
excessive deflection --> ponding --> collapse)

"Optimal" is a 7/12 slope (30 degrees) which is conveniently a nice tidy  
30 degree angle for cuts to framing.

Slopes less than 30 degrees tend to present live loading concerns WRT snow.
Slopes steeper than 30 degrees present live loading concerns WRT wind.
You pick your poison.

A 30 degree slope (7/12) is a happy medium between the two.

30 degrees is also about the maximum before which you tend to slide off  
when walking on a metal roof and you'd be well advised to start thinking  
about a safety line. That's tied off to something.

(I'm sure I've told the story here before about the freshly-minted  
B.Math/Comp Sci grad that I had working with me once who hadn't acquired  
his "Sea Legs" yet and was sitting up on the roof with me for the first  
time, hanging on tigthly to the safety line around his waist... but in his  
nervousness had neglected to tie off the other end. And he slid off off  
the plywood-sheathed, two-storey-high roof that had acquired a light  
dusting of snow in the early morning, screaming my name as he went down  
and over the edge.)

Snow doesn't stay on a 7/12 sloped metal roof for very long. Certainly not  
long enough to develop any appreciable live load. The only place snow  
might accumulate on the roof would be in sheltered (from wind) spots on  
elevations not receiving sunlight.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at chaffY a h o o  dot  c a >
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