Existing Roof Investigation

[cruzinbear]

I am mounting solar panels on an existing structure that has 3 roof areas with distinct member sizes and spans.

Since I do not have the existing plans, I conducted a site visit to size the roof purlins and check the overall structure condition. I am finding 2 areas of the roof where the roof members are adequate with wind and 20 psf minimum live load conditions. However, one area, I am finding the purlin stress ratio to exceed 1.0 for live load but not wind loads. Is it acceptable anywhere in the code for an existing structure to not meet minimum roof live load of 20 psf? The tributary area to the purlins are only 70 sq. ft. so I cannot take advantage of roof live load reduction. This is a one story structure and the roof dead load is approximately 2.5 psf. I know the code allows for 5 psf live load for awnings and canopies but this is an actual industrial structure but very light indeed.

Any suggestions or recommendations would be much appreciated. Thank you.

 

[Signious]

How does the weight of the new solar panels compare with the weight of the existing limit loads?

If you are within 10% I believe you are in the clear.

ie.

{1.25[15psf + (weight of solar panels)psf] + 1.5[21*psf]} >= 1.1[1.25(15psf) + 1.5(21psf)]

*code minimum of 10kPa = 20.9psf, not 20psf

 

[Signious]

switch that >= to a <=

Mondays are a charm.

 

[XR250]

The solar racks usually attach at 4' o.c. so isolated rafters get loaded up. I usually put a strongback under the rafters to distribute the load back to all rafters.

 

[chamokinawan]

@Signious: I believe it's 5% for gravity. 10% is for lateral. I'm in California so it might just be our building code.
@cruzinbear: Did you increase your C_D to 1.25 for the roof live load? That might help.