Shed Rafter Spacing – 16″ vs. 24″ OC with PV and 7/16
Hey all,
I am designing a shed/pump-house to serve in the short-term as temporary storage for building supplies related to house build. The space will later be divided, by adding an interior wall, and will house the H2O gear (pressure tank/filters/pumps); with the other side acting as a conditioned hybrid work location for me. Eventually, solar panels will go on the roof to power various equipment/systems.
The structure itself doesn’t require a permit in my area (less than 200 sq ft). However, in my current design I’ve spec’d 7/16″ OSB roof sheathing sitting on top of 2×8 rafters spaced 24″ OC. I do plan on installing a standing seam metal roof on which the solar panels will rest.
Do I need to, or should I, put the rafters 16″ OC?
Please advise.
Thanks.
WD.
P.S. I will have permit for plumbing and electrical in the space; just not for structure.
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Replies
7/16 is rated for a 24” span, but it’ll sag after about 4 years if you live in any climate that has snow, or heat, or anything.
I’d suggest going with 19/32 instead of 16” oc rafters. It’ll probably cost less than $100 at current lumber prices, vs much more with the 2x8’s.
It’ll be stronger, less sag, and save time.
Thicker OSB, or better yet plywood, will also give you more meat to attach the roof panels to. I hate either shingles or metal on 7/16 OSB.
+1 for thicker sheathing. Plywood probably has an advantage over OSB here too for holding fasteners.
If you are careful with your rafter placement, the solar loading won't be an issue. What you want to do is plan out your solar installation prior to framing the roof, then make sure to put a rafter under every support point for the solar panels. This way when you put up the solar panels, the screws from each part of the solar panel rack will line up with a rafter so that the load of the solar panels transfers directly to the rafters and doesn't go through the sheathing. This will make a stronger assembly, and it just takes a little more planning up front.
I would consider putting some "hard points" in too, which are roof penetrations placed to support the solar racks. This can keep the solar rack from having to mount directly to the roof which can also be a plus.
Bill
Thanks all,
Sounds like I should, at a minimum, increase the thickness of the sheathing to 19/32 plywood. For an even better, more trouble free experience in the future, I will also consider adjusting the placement of the rafters to fall directly under the panel mount points.
Bill W. - Since I'm doing this roof install myself I'm trying to keep the roof simple (see atch'd). So, I may skip the 'hard points' as they sounds like penetrations that will complicate the roof install. I will, however, hold off on the insulation under the roof deck until after the panels are mounted to provide easy access in case additional support is needed.
Thanks All!