Roof Diaphragm for Solar Array
Roof Diaphragm for Solar Array
(OP)
In order to determine the load a roof can handle, I typically analyze one of the rafters, and convert the uniform roof load to a distributed load on each rafter. Long story short, I check up to deflection criteria. My question is, how do I account for roof sheathing as a diaphragm which in turn reduces deflection to one rafter and transfers load to adjacent rafters. Is there a code reference?






RE: Roof Diaphragm for Solar Array
Maybe it is me that is confused, but I think you are referring to load sharing between adjacent rafters. Diaphragm traditionally refers to the horizontal structure of the roof decking that carries lateral loads, nothing to do with gravity loads.
I would think metal and wood decking is too flexible to redistribute the load once it deflects, unlike perhaps a concrete or steel frame and floor system.
Others may have another take on it... I personally would only do tributary area in a wood decking system and not push the envelope, the load goes into the nearest member.
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
Under the correct conditions - you can increase wood load capacity up to 15% for this...
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
In respect to load distribution, it can be assumed that any gravity/lateral loads (as applicable) are distributed to the roof sheathing/plywood as a uniform load, then transferred as a linear load distributed along each rafter/truss top chord which in turn bear on load-bearing walls or beams, etc. Typically plywood has a span-rating, often stamped on one face of the sheet. If someone has suggested using a higher span-rating, it would suggest to me that they are trying to eliminate some of the roof/floor rafters by spacing them further apart. Keep in mind, depending on the new spacing and new load distribution, you may find a bigger rafter is needed for the additional span loads now applied. Higher span-rated plywood AND bigger rafter sizes could cost more than your original layout. If the project budget is tight, the recently suggested route may not be the correct route in all cases.
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
You may be able to do this in a concrete floor with "T" beams or closely spaced composite steel beams with a very rigid floor, but not in wood sheathing IMO (not to an appreciable amount that I would consider). I have done this with X bracing between steel joists to redistribute point loads to adjacent joists...
An easier way to redistribute loads from a solar array to wood roof rafters is how you design your rack structure above. Place posts on as many rafters as possible in a manner that will spread the load out...
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
Have the panels already been procured? There are PV systems that are direct replacements for roofing tiles:
htt
http://inhabitat.com/solar-panel-roof-tiles/
h
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RE: Roof Diaphragm for Solar Array
jt- I agree with a2. Some load-distribution may be present but due to the lack of rigidity of the plywood, I would think it would be very minimal, not to mention quite time consuming to calculate even if you have all the variables.
Sure if the contractor (I'm assuming your client) wants to pay additional for a more rigourous analysis...knock yourself out and bill away. Otherwise, I would let him know it's understressed AND for additional services you can provide a reinforcing detail.
Lesson to contractor... "You're not an engineer. Just b/c it worked on the last project across town, doesn't mean it will work on the next one. Consult with the engineer FIRST!"
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
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RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
If you post a sketch of a typical cross section including your rafters maybe we can come up with some type of easy solution...
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
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RE: Roof Diaphragm for Solar Array
The load of the sheathing, tile, snow load, etc is shared by each rafter, but the sheathing can't be assumed to assist in redistributing point loads on an individual rafter.
RE: Roof Diaphragm for Solar Array
RE: Roof Diaphragm for Solar Array
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RE: Roof Diaphragm for Solar Array
Stressed skin analysis is just not used in design of residential structures.
RE: Roof Diaphragm for Solar Array
Or, can additional supports be inserted on the intervening rafters?
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