Uplift Wind Loading Values [RE-wrenches]

[Jeff Oldham]

PD - 

Yikes, what was the question again?! Remember the K.I.S.S. principle, don't further complicate something that is already complicated and site specific. Keep in mind, that live loads are temporary people and equipment on the roof. Dead loads are permanent "stuff", can be nearly anything, including our arrays.

Another Hot Tip: UBC allows for a second layer of roofing applied to an existing residence roof W/O structural engineering/re-clacs!! The catch (you know there has to be one) is like all of the UBC (Codes in general for that matter)it is up to local jurisdictions to adopt or not. Here in Mendocino Co., it is not adopted (of course) but in other areas it is working as it has been adopted. We are working with the local Building dept. and a local Structural Eng. to create some language that can work for everyone. As with most areas, especially rural, there are many sub-standard structures that can hardly handle a cat on the roof. This makes a "blanket" waivier difficult. We are talking about a $50 fee for the inspector to make a site visit and make a judjement call before issuing the permit. Standard 20 yr. 3-tab asphalt shingles weigh 22.5#/sf, this is about 4-5 times the weight of a typical PV array! I would call the Building Dept. in the area of your install and ask if addin!
g a second layer of shingles requires engineering. If they say no ask if you can put up a PV array with 1/4 of the weight that will prevent a second layer from going up there. Watch the fun from here!

Re-read Bills great KISS hit on it:

Jeff,

Back to the same page. I am absolutely talking about UPLIFT. Live loads are
irrelevant with PV systems and many inspectors will allow you to use the
elimination of live loads as a reason to add weight to a roof. My
calculations are specifically related to withdraw strength of fasteners in
wood. The uplift on a 1kW PV system can be much more than 1200# depending on
the conditions. Dead load is only relevant for the added weight to the
structure. The dead load increase of the wind is the same with or without
the PV system.

Two main issues:

1. Can the structure handle the additional dead load of the array?

2. Can the fasteners handle the expected withdraw loads imposed by the
uplift of the wind? (total uplift divided by number of fasteners)

Point loading is another issue, but a good design has enough attachment
points to keep this from becoming a problem.

Bill.




Jeff Oldham


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