Roof Sheathing Span Ratings for Large Snow Load

[skimboard20]

Hi all,

I am designing a building in a high snow load region, and am hung up on sheathing specifications. The roof assembly is typical, with roof sheathing over manufactured roof trusses. Given the snow load of ~80 psf, the truss manufacturer says that trusses @ 24”oc will work. The roof sheathing we typically specify is 1/2" thick plywood, CDX grade, with a span rating of 24/0. But when I check the bending strength of this sheathing (Table 9 in the APA Panel Design Specification), it doesn’t work out. So I have a couple questions:

In this situation, if the architect would like to have trusses at 24”oc, then should the sheathing be designed to that span? Or should the trusses be reduced to 16” spacing so that our typical 1/2" plywood callout works? In short, trusses spaced further apart and thicker sheathing or trusses spaced closer together and thinner sheathing?

How are span ratings derived if it doesn’t seem to work in this case? Are the span ratings based on more typical roof loading scenarios?

Thanks for the help.

 

[RWW0002]

In my area 1/2" sheathing (likely 7/16 or 15/32") is more likely to be span rated to 24/16 or 32/16 than 24/0. I generally only see 3/8" plywood with 24/0. You should check with local suppliers and see what is available. That being said, I do not know that even 32/16 is going to quite get you what you need.

I would specify whatever thickness you need to meet the most economical truss spacing. In my area the difference in 19/32 and 15/32 sheathing is only about 12% of the sheathing cost. Forcing trusses to 16" may come at a much higher cost increase (maybe as much as 40%-50% increase in the truss cost plus you are having to set that many more trusses)

19/32 plywood is likely span rated to 40/20 and may get you much closer to the range you are looking for. See table 1 in APA Technical Note Q225D (Load-Span Tables for APA Structural-Use Panels).

 

[Craig_H]

The below image is from the CWC's Wood Design Manual. Canuck codes and units, but it should give you an idea of the sheathing selection for roof loads 4 kPa or less. You're likely right around that number, depending on how much occupancy load you need to add in Washington?

Depending on the roof finish type, it may be advisable to consider long term deflection. For example, well heeled clientele building their ski hill mansion don't like their roof to look like a tarp draped over the trusses. This seems to be more of an issue with shingle finishes than with standing seam metal, in my experience.

 

[RWW0002]

Here is the table referenced

 

[skimboard20]

Okay thanks for the insight.

RWW0002, I checked the number of trusses needed to move down to 16" spacing, and they would need to add 14 more trusses... So increasing the panel thickness to meet 24" spacing definitely seems like the more economical approach.

Thanks, Craig H, for a Canadian's opinion. From what I can tell from your table, the minimum panel thickness would 1/2" for the 4 kPa load. We are over 4 kPa (4.75 actually) in this area, so to me that sounds like more justification for bumping up the panel thickness.