Exposed roof sheathing

[kzq]

I've been laboring under the assumption that roof sheathing exposed on the underside must be exterior plywood. OOPS! Section 803 of the IRC allows any exposure 1 panel to be exposed on the underside such as roof overhangs.

I know there used to be a requirement for exterior plywood. Does any one know the last code that mandated exterior plywood in such an application?

TIA

Bill

[Uncle Bob]

Bill,

" I've been laboring under the assumption that roof sheathing exposed on the underside must be exterior plywood. OOPS! Does any one know the last code that mandated exterior plywood in such an application? "

I have from 2000 thru 2009 IRC; and no requirement for exterior plywood.

But, they do require 803.2.1.1 Exposure Durability "All wood structural panels, when designed to be permenantly exposed in outdoor applications, shall be of an exterior exposure durability. Wood structural panel roof sheathing exposed to the underside may be of interior type bonded with exterior glue, identified as Exposure 1."

Now, I don't have the 1998 CABO; but, the 1995 CABO does require "exterior type plywood";

803.2.1.1 Type. An exterior type of plywood is required when the plywood is exposed to the environment. Plywood roof sheathing exposed to the underside, such as eave overhangs is permitted to be of the interior type with exterior glue because of the decrease of exposure hazard relative to moisture and because the lower grade of inner piles will not adversely affect the glue-line performance.

Note: this is from the 1995 CABO "Application and Commentary".

This does not mean that OSB could not be used for roof sheathing. Just that it could not be exposed to the elements; at that time.

Hope this helps,

Uncle Bob