Help with Green Roof on Pole Barn Style Shed

I am in the process of building an outdoor area to be used as a multi-use area - outdoor kitchen, chicken processing, whatever it may be. I am trying hard to make it a green roof with a native grasses and wildflowers mix. My main concern is that I have not engineered enough of the support structure to make sure this roof is safe. I based it off of our deck attached to the house and just upped the ante in an effort to over engineer it but am just not very sure of my efforts. Please excuse any misuse of construction terms.

Below is the building view from the top. The posts are 6x6 notched at the top for the front and rear header and for the side headers. All of these are 2x12's. The front 6x6 posts are anchored in concrete. The rear however sit on a concrete pad already at the location. I used metal anchors so the posts don't sit on the concrete pad directly and the anchors are set into the existing concrete with hydraulic concrete epoxy and galvanized 1/2" by 4 inch bolts. (I went through 2 bits getting these in!). The joists are 2x6s and are hung with joist hangars. I plan on doing a row of blocking straight down the middle with more 2x6s.

Once completed the roof will be separated in the middle (I hope) by a simple decking catwalk for the view and for maintenance.

Last attachment is a picture from today of how it is coming along.

So, am I on the right track for this structure?

Any thoughts on the plywood that will go on top - 1/2"?

Thank you folks alot.

A couple of side pictures as well.

And one very angry piece of lumber.

I would be mad too if I were expected to carry the loads of a green roof over those kinds of spans Its safe to say you will at the very least need to add some posts for the long span, keep your soil mix on the light weight side of the range, and get some lateral bracing into the mix. Try to find some expected weights of saturated soil mixes for green roofs, add in the snow loads for your area and compare that with the span tables for the lumber you picked out. It looks like your off to a good start!

Bearing on the concrete pad makes me nervous. I've seen that plan go badly. (Second story room gradually sagging off of the side of the house. In, fact, it was a very similar situation, along the length of a hill. The pad had been poured just for walking, and so there was no effort made to keep it from sliding down the hill little by little. It wasn't supposed to be structural. As it crept down, the posts resting on it did too, and the room sitting on the posts.)

Do you know anything about it? Thickness? Type and quantity of reinforcement? Presence or absence of a footer underneath?

Nothing to add to the topic, but +1 for the angry lumber.

Brian Knight wrote:I would be mad too if I were expected to carry the loads of a green roof over those kinds of spans Its safe to say you will at the very least need to add some posts for the long span, keep your soil mix on the light weight side of the range, and get some lateral bracing into the mix. Try to find some expected weights of saturated soil mixes for green roofs, add in the snow loads for your area and compare that with the span tables for the lumber you picked out. It looks like your off to a good start!

Thanks for your reply Brian! I was looking at a document from Penn State Center for Green research (http://aesop.rutgers.edu/~farmmgmt/virtual_conference/RBerghage_Greenroof_Brochure.pdf) and it listed the weight as 7.5 pounds per square foot per inch of media depth for an extensive roof using lightweight media.

Then I checked snow loads, it indicated 15 psf for Hendersonville, NC (hey neighbor).

I called an Extension agent here in NC and talked with him about the plans and he indicated a good average for a wet dead load of a landscaped roof would be 30 to 35 psf and to add the snow load on top of that so 45 to 50 psf is the dead load. He said to add 10 psf for live load but assuming that no one would go up there while it was snowing this could be added to the wet roof number so 40 to 45 live load. He did say to know the exact lumber size needed I should consult a structural engineer and he said it would be a quick job and relatively cheap.

The problem I have now is deciphering the span tables. I looked at this, http://www.awc.org/pdf/STJR_2012.pdf, and cannot figure any of that out. I think I will have some homework this evening and see what I can figure out. The NC Extension had a good document that I will look at and see what I can come up with, https://www.bae.ncsu.edu/topic/lidconference07/Post%20Conference%20Green%20Roof/3.Jones.Green%20Roof%20Structural%20Design.pdf .

By the way, no need to respond to these things, I just thought it would be good to share the info I have gathered as it aggregates.

Mike Cantrell wrote:Bearing on the concrete pad makes me nervous. I've seen that plan go badly. (Second story room gradually sagging off of the side of the house. In, fact, it was a very similar situation, along the length of a hill. The pad had been poured just for walking, and so there was no effort made to keep it from sliding down the hill little by little. It wasn't supposed to be structural. As it crept down, the posts resting on it did too, and the room sitting on the posts.)

Do you know anything about it? Thickness? Type and quantity of reinforcement? Presence or absence of a footer underneath?

Hi Mike, thanks for your reply. I was a little intimidated by putting the back two beams on the concrete. The concrete is close to 8" thick, I used up two masonry bits getting the holes bored for the bolts that hold the metal supports for the 6x6's. The original intent was to use anchors and screw the bolt down but the bolts sheared and I had to drill them out to get the holes back. So instead of the anchors I used hydraulic concrete epoxy. A house used to reside next to this but it burned down before I got the property. I think this is where a car used to be parked. The rock shape you see in the above picture used to be the entrance to the house till I dug it out for a fire pit. Would there by anyway to tell from that description any more information?

When I read through old threads like this, I often wish there was an update. How did this project go?