Adding Solar Panels to Untied Roof 1

[Madavids2010]

I got hired to give an opinion on adding Solar Panels to the roof of an old barn. The barn roof is somewhere in between a stable and unstable roof, and 150 years old. it is basically 2"X6" @24" OC spanning 15 feet. Normally I would just say of course you need to reinforce the roof to support the new loads, but in this case the snow load plus dead load is 45 psf and the solar panel only weighs 2.5 pounds per square foot. By the book it seems like I would not only need to reinforce the members but also stabilize the roof with tension ties at the base. However, engineering judgement tells me that this is only a 6% increase in load to a roof that has been there for 150 years, and that is unfinished so no worrying about plaster cracking. Let me know your guys's thoughts!

 

[Nor Cal SE]

If your client, the barn owner, will need you to stand by your recommendation in a legal sense (i.e. provide stamped and signed letter or other means of justification to the local jurisdiction) stating that the roof would be adequate for the added solar panels, then I would have reservations about doing so. If anything goes wrong, the liability will be yours, and it sounds from your description like you are unable to show the typical roof members as structurally adequate for the added loads or even the current loads (which doesn't surprise me, based on 2x6's spanning 15 feet).

In reality I expect nothing would go wrong based on the described history, but I just don't think it's worth taking the chance. Your client might not be thrilled to hear the news, but I think he would respect that you are giving your honest engineer's evaluation.

 

[SlideRuleEra]

Agree with Nor Cal SE.

Forget the solar panels... take a look at the bending stress in the 2x6 from just the 45 psf snow load/dead load:

15' span, simple supports.

Tributary load on one 2x6 = 2' x 45 psf = 90 lb/ft

Bending Moment = 30,400 in-lb

Say, 2x6 are rough cut (exactly 2" x 6"). Section Modulus = 12.0 in³

Bending Stress = 2530 PSI

Even considering that lumber from 150 years ago was excellent quality, allowable bending stress is probably < 1750 PSI (Dense, Select Southern Pine - this value from the 1934 Carnegie Pocket Companion).

Snow load duration factor = 1.15, so allowable bending stress = 2010 PSI

Ok, quality lumber stressed to 2530 PSI (with 2010 PSI considered allowable) is not going to fail... but no way would I add any additional load to it.

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[Madavids2010]

Thank you for your input guys. I have considered just sistering the rafters which I think is reasonable. My concern though comes with it not being a tied roof. But I agree anything other than just some advice would be hard to put a stamp on.

 

[NorthCivil]

Old undersized roofs scare me. If I touch them, I always upgrade them to something I can stomach.

see link. a 1938 roof that was re-roofed with 4" of rigid insulation a few years ago. This past winter there was a big snowfall and...

http://www.news1130.com/2016/12/20/community-centre-roof-collapses-weight-snow-new-westminster/

 

[SlideRuleEra]

I have considered just sistering the rafters which I think is reasonable.

Instead, suggest performing some calcs based on the actual (field measured) size of the rafters, including proposed sistering members. I have a "hunch" there will still be "trouble" with the rafters.

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