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Old 08-24-2011, 09:47 PM   #1
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maximum simple 2x4 truss span


How much deflection of roof will there be with a pent rafter (basic three member rafter without web bracing or bottom chord) 2x4 top chord placed 2' OC spanning 12' on a 4/12 pitched roof with 3/8" osb and asphalt shingles?

There is a reasonable snow load at times in these here mountains.

Too much?

If it is too much can one make the 2x4's 1' OC; or upgrade to 2x6 top chord's; or 2x6 top chords 1' OC?


The proposed building will actually be 12'Wx22'L with the roof being 16'x24' (1 foot gable overhang 2' eave and ridge overhang)

This is to keep an old rv trailer roof from leaking like siv. The rv is beyond repair but still useable for those long cold nights.

A simple design with being able to tow the trailer out eventually is in mind, thus the clearance with no bottom chord.

The poles and the holes for this structure have also already been invested in .

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Old 08-24-2011, 10:12 PM   #2
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maximum simple 2x4 truss span


ok number 1 the osb or plywood you want to use is way to thin especaily in the mountains , use 1/2 osb or plywood , do you now how much a square of shingles weigh average 220pounds per square , i see disaster ahead if you plan to do this the way you want to

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Old 08-25-2011, 12:02 AM   #3
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maximum simple 2x4 truss span


"Reasonable snow load?"

What's the actual anticipated snow-load?
Or annual average snowfall in that area?
Is 4:12 suitable there?
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Old 08-25-2011, 12:13 PM   #4
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maximum simple 2x4 truss span


Fair enough.

3/8 or 7/16 has been the predominant thickness of roof sheathing here for as long as I have been in the trades, which is about 20 years. I know there wont be an issue there.

I think OSB is made differently here in Canada, or something...

A bit more thought into the situation has led me to concur with the disaster statement.

With regard to snow load, the location is about 10 km from the nearest measurable centre at 150 cms.

I also dug this information up from jabacus.com;

<H3>Specified Snow Load

S = Is[Ss(CbCwCsCa)+Sr] [4.1.6.2]

Factors

Location: Calgary, Alberta
Ss = 1.1 kPa / Sr = 0.1 kPa
Importance Factor, ULS: Is = 1.15 / SLS: Is = 0.9
Roof slope = 26.6 degrees
Slope Factor
For non-slippery roof:
Slope <= 30 degrees.
Cs = 1



ULS:

S = 1.15[1.1(0.8*1.0*1*1.0)+0.1] = 1.13kPa

S = 1.13 kPa

S = 23.5 psf



SLS:

S = 0.9[1.1(0.8*1.0*1*1.0)+0.1] = 0.88kPa

S = 0.88 kPa

S = 18.4 psf





</H3>

I do not know how to interpret these figures however .

The snow load here I have discovered to be surprisingly light compared to many places not so close to the mountains, specifically eastern Canada.

Looking into things more I think it might be best to construct either a Kingpost or a Howe set of trusses at a 6 or 7/12 pitch (as suggested by the info above)

If the trailer needs to come out there is always the chainsaw...

There are actually a lot of 4/12 shingled structures in the area. The biggest concern as far as loads go would be trees snapping off in a wind storm. There are lots of tall spindly black poplars around which tend to snap off about halfway.
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Old 08-25-2011, 09:43 PM   #5
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Your from CANADA that explains everything ! LOL just kidding friend, alot of OSB is made in CANADA almost all of it as far as i now , if it were me i would go with 1/2" osb with H-CLIPS it would make for a more secure roof
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Old 08-26-2011, 05:57 PM   #6
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Quote:
Originally Posted by fulton 22175 View Post
Your from CANADA that explains everything !
LOL
I agree with what fulton says, although I still prefer 5/8" cdx in snow country

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