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R317.1.3 2012

darcar

darcar

Silver Member
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Nov 12, 2009
Messages
129
I cant seem to get a photo downsized enough to post it here but i will try to describe what I have...

A homeowner built a deck addition to the rear of an existing residential dwelling. Over the deck he built a roof that is supported by 6x6 posts and ties into the existing roof.

The decking materials are treated wood and the trusses and OSB are regular non treated type. The owner has left the underside of the trusses and OSB open and not protected by soffit or similar materials.

Section R317.1.3 of the 2012 IRC addresses protection of wood against decay in geographical areas .... exposed to weather without adequate protection from a roof, eave, overhang or other covering that would prevent moisture or water accumulation on the surface or at joints...

In Iowa we experience ALL types of weather throughout the year including moisture in winter and summer months.

The in-house argument is if the trusses and OSB need to be covered/protected.

The trusses will gray and the OSB will flake when subjected to the harsh Iowa seasons over the years.

Is there an APA reference manual that addresses OSB exposure to moisture ON THE UNDERSIDE?

Thanks for your help and I'll see if I can dupload these pictures

Thanks
 
Structural sheathing has exposure ratings.

They are established and published by the APA.

Those would apply.

They are the same for OSB and Plywood.
 
Thanks brudgers.

You wouldn't happen to have a link would ya?
 
WOW, are you guys using the 2012 already. We won't get there for another 11 months. Anyway - int he 2009 IBC, the definition for weather exposed surfaces is in Chapter 25. Section 2502.1. Seems like a strange place to me but that's where I found it. Where there is wind driven rain and the like, I have to believe at least the trusses are exposed to the weather.
 
a key portion of that code wording is "In geographical areas where experience has demonstrated a specific need,"...

Have you noticed similar structures that have sat for a period of years that has shown rot, decay, delamination, or other ways affected the underside of OSB, trusses etc that do not have direct covering?

Thanks
 
The key portion is "protected by a roof". I've seen the argument successfully made that the porch floor does not then need to be durable. Check the moisture content... this doesn't "hold water" :D

No delams, decay problems on any that didn't have a leak. My barn porches are still bright and the stamps on the osb are quite legible, really no different than inside ~20 yrs old. Mountains, wet, geographically in a SWR although Iowa was consistently windier.

Come to think of it, USFPL's "Equilibrium Moisture Content of Wood in Outdoor Locations Worldwide" You'll find the emc's for wood in selected locations by month, under a roof but exposed in the same sense you are describing. Wood is under decay moisture content in air worldwide.
 
I think DRP got it on the "roof" part...always a case by case basis, and as long as you are consistent in your jurisdiction, it shouldn't be an issue... It also talks about accumalation of water....which I don't believe would occur in the situation you describe....
 
steveray said:
Sorry....and uplift connectors.....JEEZ... :) ........don't the sheathing nails hold the sheathing on?
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you said you couldn't require anything on the bottom of the truss, so if the bottom is opened and exposed on a porch can the wind enter and lift/push the sheathing from the trusses from below through uplift and nails in withdrawl?
 
Same way it would push on the bottom chord of the truss if it had a covering(for uplift), or if the windows were open or blown in by a storm(for the sheathing).....I may be wrong, but I do not believe there is anything additional that I can legally require for sheathing attachment for open vs. closed structures......

GBrackins said:
you said you couldn't require anything on the bottom of the truss, so if the bottom is opened and exposed on a porch can the wind enter and lift/push the sheathing from the trusses from below through uplift and nails in withdrawl?
Click to expand...
 
GBrackins said:
you said you couldn't require anything on the bottom of the truss, so if the bottom is opened and exposed on a porch can the wind enter and lift/push the sheathing from the trusses from below through uplift and nails in withdrawl?
Click to expand...
That load is accounted for by the increased interior pressure coefficient for partially enclosed structures. Of course if the structure is open, then interior pressure will be opposite that of exterior pressure, not additive to it.
 
except on the leaward side .... where it could act on both sides (no ceiling) of the roof sheathing, or may be I'm wrong, have been before, sure I will be in the future ....
 
There are three possible configurations: open, partially enclosed, and enclosed.

An open structure (such as a roof on posts with no walls) has no internal pressure (+/- 0.00 GCpi) and the least uplift.

A partially enclosed structure (such as an attached carport or porch) has the highest uplift because the interior pressure is highest (+/- 0.55 GCpi, typical).

Enclosed structures fall in between (+/- 0.18 GCpi, typical) and are the normal case.

The structural provisions of a prescriptive code which allows partially enclosed structures must be deemed to account for the highest load case.

To put it another way, if you allow porches or carports under the IRC, you already have the same loads.
 
I thought that a high velocity wind perpendicular to the ridge would have lift (negative pressure) on the leaward side of the roof even if the wind were allowed to pass unobstructed below the roof in the case of a roof support on columns?
 
GBrackins said:
I thought that a high velocity wind perpendicular to the ridge would have lift (negative pressure) on the leaward side of the roof even if the wind were allowed to pass unobstructed below the roof in the case of a roof support on columns?
Click to expand...
Did you come to think that based on your reading of ASCE-7 or similar works describing the engineering analysis of structures?
 
I think going back to the definitions of "open", "enclosed", and "partially enclosed" and then looking at GCip for each would clarify the requirements. Thinking about things as an airplane wing don't really help.
 
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