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What city and state are you located in for snow load consideration?
You say you have 16' span timber floor joists, what his your roof framing made of?
Your Building Code Specified Snow Load is 45 psf.
Would you be able to access the underside of the roof and snap a few pictures to share with me?
I agree with you that I would want to limit the penetrations into the roof for fear of future leaks.
The ballast mounting system would be my preference, but you need to verify that the roof structure can take the extra load.
Since I do not know the specific member sizes, I can not make a judgement on roof capacity.
The roof should have been designed to support the Live Load of 45 psf, plus the Dead Load of the roof system materials. They do not usually over-design to allow you to add a great deal of weight on top of that at a later date.
I would need to run a few numbers, can I get back to you in an hour or so?
I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.
I ran some quick numbers for 16' span 2x10's spaced at 16" o.c. using the 45 psf snow load and 15 psf dead load and they are overstressed at 131% of the allowable.
Since you said you may have 2x12's spanning the 16' and spaced at 16" o.c., I ran numbers for those and they are right at the allowable limit (98% of allowable bending stress).
So if you have 2x10's on your roof, they are overstressed under your existing load.
If you have 2x12's on your roof, they are right at the Code limit, so they work for your existing condition, but you can not add any additional Solar Panel and ballast load on the roof.
The heavy HVAC equipment may be located right over the top of one of your large steel beams, so that is why the heavier load is allowed in that spot.
Based on my rough numbers, you can not add additional load to your roof unless you strengthen it.
You will need to have a local Structural Engineer come out and inspect your roof and give you recommendations for strengthening.
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