×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Adding Solar Panels to Untied Roof

Adding Solar Panels to Untied Roof

Adding Solar Panels to Untied Roof

(OP)
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!

RE: Adding Solar Panels to Untied Roof

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.

RE: Adding Solar Panels to Untied Roof

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Adding Solar Panels to Untied Roof

(OP)
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.

RE: Adding Solar Panels to Untied Roof

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-centr...

RE: Adding Solar Panels to Untied Roof

Quote (Madavids2010)

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources