lumber roof sheathing

[eng003]

Looking for general formation on 1x, 2x, & 3x lumber roof sheathing. Pro's con's, design considerations, etc.
-I note awc has a technical pub on 2x & 3x but what about 1x what design values do you use for that?
-the tech pub shows some installation patterns where they appear to stagger with ends not on a support presumably just locking into the the adjacent tongue & groove? is this right/typical?
-Is all of it typically tongue & groove?
-What are typical grades/species?
-Fastening considerations?
-I note awc has diaphragm capacities for 1x & 2x what about 3x though?
-What about roofing material consideration, I heard you can't do asphalt shingles? why not?

Thanks for any information!

 

[Ron]

For sloped roofs, 1x dimensional lumber was commonly used, usually 1x4 or 1x6. If the width gets much over 6", then "cupping" can occur,
For flat or low slope roofs, thicker dimensional lumber and tongue&groove lumber was commonly used. For steep slopes, tongue&groove or straight edge could be used, as long as the edges were butted with little or no space between.

Asphalt shingles can be used with dimensional lumber sheathing. If you are concerned about all the joints in the sheathing, then use No. 30 felt underlayment or two layers of No. 15 felt.

As for species, any structural grading will work. For fastening, you will need 2 fasteners (8d ring shank galvanized nails for 1x lumber) per board at every support (prevents cupping and provides adequate structural fastening) and terminate boards only at a support. Do not cantilever as you have suggested.

Tongue&groove is nice but not necessary.

There are several reasons that dimensional lumber fell out of favor for construction. The first and most significant is the labor cost. It is faster to install plywood or OSB sheathing than to install dimensional lumber. In general, it is a bit safer for the installer to used plywood or OSB sheathing as more area gets covered quicker and the surface of the plywood or OSB can be more slip-resistant than smooth surfaced dimensional lumber. Next is the dimensional movement of individual boards as compared to plywood or OSB sheathing. Each joint gets a small amount of movement, either by moisture gain/loss and/or thermal gain/loss. This movement can, with long periods of time, wear on the felt underlayment, particularly if cupping of the lumber occurs. That is why it would be good to use two layers of No. 15 felt or 1 layer of No. 30 felt for the underlayment.

There are some advantages to using dimensional lumber. First, you will get a thicker section and correspondingly more strength than using thinner plywood or OSB sheathing, provided fastening is done correctly. Next, because of all the joints, you will get better air flow, ventilation and drying of the underlayment, so it will perform better (at least in theory!).

In short....nothing wrong with using dimensional lumber for sheathing a roof and you CAN use it with asphalt shingles.

 

[hokie66]

A disadvantage is that individual boards don't provide much in the way of diaphragm capacity...if you are depending on a diaphragm.

 

[eng003]

Ron - Thanks for all the great information, interesting point about the ventilation. Wonder if I was reading that awc tech pub right about staggering the joints...I definately agree I would want the end joints on a support. Hokie - Regarding the diaphragm capacity good point I see that if the boards are laid of horizontal to the framing the capacity is very low. A diagonal lay up appears to be have a much better capacity.

If you wanted bulky rafters spaced at say 4' o.c. and wanted the underside of the planking exposed for ascetics, any ideas on how one could insulate such a section if the roof is sloped?

 

[a2mfk]

Like Hokie66 said, just be careful with the diaphragm, pretty weak in shear parallel to the decking. I had to check it before on a older roof, and I think I found tables from AWC.

If you want to expose the underside of the decking, you have to make sure the boards are thick enough for the roofing material fasteners to NOT penetrate, which can be difficult. Especially if you want to insulate, which likely means rigid foam insulation screwed into the decking.

Though it may seem a bit wasteful, when an exposed wood plank ceiling is desired, it is fairly common to go with a conventional roofing system and then install a separate wood plank ceiling below this to provide a plenum for insulation and to avoid the fastener penetration issue.