I’m still looking for reasonable numbers on roof loads for a living roof. I’ve run across people claiming as little as 7 lbs per square foot per inch of soil, and living roofs in as little as 4” of soil, and numbers as high as 140PSF.
The online calculators and charts for joist calculations (I’m treating this like a floor because a living roof wants to be a fairly flat roof) I’m finding go to a max of 100PSF live load and 20PSF dead load. I’m finding calculators into which I can type a snow load, but my impression is that snow loads are treated like live loads, and what I really want is to calculate a reasonable span (12-16 feet) for a 140PSF dead load and a 20-30PSF live load (ie: someone walking around on the roof).
Of course the other thing I need to do is double-check those living roof weight numbers, because that’s saying I’m going to be supporting (and carrying up a ladder to install) 22.4 tons of material (A human can generate 1/4HP, right? So if the roof is 12 feet high I should be able to do that in 1 hour, 5 minutes and 10 seconds, right?).
And, yes, I will eventually end up having to hire an architect or engineer to sign off on my calculations, but when it comes to my house I’ve run into too much incompetence from professionals to just lob the whole project over the fence to one.
-- Dan Lyke, Petaluma California, http://www.flutterby.net/