New plywood roof deck installed over old rafters:

I'm having the old roofing material stripped off -- 3 layers, the oldest being cedar shakes installed in the 1930's. The original 2x4 rafters are in good condition; the original 1x8 boards on top of the rafters are mostly in OK condition, however the nails (80 years old) attaching these to the rafters are in poor condition (rusted, brittle).

My question is this: a 1/2" CDX plywood roof deck will be installed over the 1x8 boards. I want to be sure that this plywood is very securely attached to the rafters. This is important to me because some of the rafters have been doubled-up (where skylights were installed) and I want these new rafters attached to the plywood, for maximum structural integrity.

The roofing contractor (sales rep) wants to use staples. He says that nails would pull out. I want the contractor to attach the plywood to the rafters using either threaded nails, screws, or some product with equivalent strength. It may be overkill, however it's cheap insurance.

I would greatly appreciate hearing from anyone with roofing experience. What kind of fastener should I specify? What spacing?

 

Ring shank nails shouldn't pull out. On occasion, I've had to remove them and it wasn't easy to say the least.

 

Thank you for the recommendation.

 

The APA specifies that: sheathing "Should be fastened with a minimum of 8d common nails spaced at a maximum of 6 inches on center at supported panel ends and edges. at intermediate supports, fasten panels 12 inches on center. In high wind areas more fasteners may be required. Fasteners should be 3/8 from panel ends and 3/8" from panel edges"

Proper Installation of APA rated Sheathing
For Roof Applications

With regard to fasteners for "high wind areas" (including alternate fastener types), see for example:

Roof Sheathing
Installation - HOME BUILDER’S GUIDE TO COASTAL CONSTRUCTION - Technical Fact Sheet No. 18

However... when I encounter problems related to plywood roof sheathing detaching from the underlying structure at home inspections here in Chicago it's usually because 1) the fasteners were too short (most common on "roof-overs"), 2) the manufacture's nailing requirement were not followed (too few fasteners or fasteners to close to the panel's edge) 3) the fasteners missed the structure below, 4) the panels were not properly spaced (too close together) or 5) some condition produced a high moisture condition at the sheathing (failed vapor retarder, bath fan exhausting below sheathing, etc.), not because an otherwise incorrect fastener was used.

So IMO it's as least as important to pay attention to these sorts of details as it is specify more aggressive fasteners, for example in cases where excessive moisture is present fasteners can pull right through plywood sheathing as it warps.



Fig 1: Localized sheathing detachment from the rafters below.



Fig 2 Tear in vapor retarder allowed moisture from bath on opposite side of knee wall to collect in insulation between rafters, warping plywood roof sheathing above.