Attic flooring

[Yankee Chronicler]

Looking at a plan for an oversized garage/barn accessory to a single family residence. The plan calls for a walk-up attic, accessed by a permanent stair, for storage. Attic floor is called out as 1/2" nominal (15/32") plywood, 32/16 span rated. Joists are spaced 24" on center.

My immediate reaction was that the span rating for floors is 16" so you can't use 15/32" plywood for a floor with joists at 24" o.c. But then I looked at the table in the IRC:



Reading in the commentary, it repeated refers to the 32/16 span ratings as "recommended." Well, "recommended" is not the same as "required." The last column on the right seems to be telling me that the joist spacing can't exceed 16 inches, but then I look at the allowable live load column and it shows me 70 psf for 24" joist spacing, and the next column over tell me that the maximum joist spacing is 28 inches.

So which column governs? Even for the main floor in a house, the required design live load is 40 psf. If 15/32" 32/16-rated plywood can support a live load of 70 psf on joists spaced at 24", which does the last column limit the span to 16"?

This is one of those cases where the commentary comments are completely useless.

 

[wwhitney]

The last column on the right seems to be telling me that the joist spacing can't exceed 16 inches

That last column has the subheading "Subfloor", so it applies to your subfloor application. 1/2" sheathing is limited to 16" span between joists unless the provisions of footnote h apply.

but then I look at the allowable live load column and it shows me 70 psf for 24" joist spacing

Yeah, I don't know what to make of those columns, but they just seem to be for reference.

and the next column over tell me that the maximum joist spacing is 28 inches.

The subheading on that column is "Roof," so it does not apply to your application.

Cheers, Wayne

 

[Yankee Chronicler]

The subheading on that column is "Roof," so it does not apply to your application.

I realized that about 3.5 nanoseconds after posting ...