rafter size for grass roof

plschwartz

I am building a small retreat 10x10 with a curved gable roof. I couldn't find a flexible insulation for this curved surface so I decided to go all the way and put on a earthen roof and plant grass or flowers. Research gave an estimate of a dead load of about 100psi. My snow load is 30 psi. I plan
Regular 2x4 @16" studs.Doubled 2x6 top beam.
I need help with the rafters as I cannot find a calculator that will accept the 100 dead load.will 2x6 @16 in be sufficient?
Thank you

cleveman

Are we sure about the p square inch part?

paintdrying

Curved gable roof? Not sure on how you are building the structure.

plschwartz

Well there is this interesting professional discussion
http://www.eng-tips.com/viewthread.cfm?qid=206117
I took tyhe conservative estimate
From another direction
http://www.motherearthnews.com/moder...z72ndztak.aspx
also suggests 100psu but as I reread it it is an estimate for a LL
Finally http://www.environmentalleader.com/2...n-roof-design/
gives a much lower figure.
I will look some more but for now I will use the conservative 100 engineering figure. I mean the place is so small more wood is no a big deal. For looks I would like to keep it at or below 2x6. Doubled 2x4 or even landscape timber (but what is the wood?).

plschwartz

From plans of woodsshop.com
Cut out of 2x12

jagans

Roof loads are calculated in PSF not PSI. A Gable is not curved. You can get roll formed curved roof supports, and thin plywood could be laminated in the field in a curved configuration, as can metal panels. Segmented wood trusses can also be fabricated with a little ingenuity.

__________________
" A lot of men build things, and a lot of things fall down "

joecaption

http://02da44b.netsolhost.com/glulam/curvedbeams.htm

cleveman

Yeah, I think you should be talking ps foot, not inch.

So you're talking 130 psf. I think a residential floor is rated at 250, so if you build it as a floor, you will be plenty good to go up there and walk on it and run your push mower, etc.

Is your span 5' or 10'? Is there a structural ridge beam did you say?

Daniel Holzman

Typical loads on a residential floor are either 30 or 40 pounds per square foot (psf) live load, plus about 5-10 psf dead load, certainly not pounds per square inch (psi). So either the OPS made a typo, or the design is for load which is 144 times too high. As for soil, a commonly used unit weight would be between 100 and 130 pounds per cubic foot, so each foot of soil will add between 100 psf and 130 psf to the load. The OPS has not indicated how they derived the psi figure, but if the roof is to be 2 feet thick, it would carry between 200 psf and 260 psf dead load.

This is a very large load compared to normal residential floor loading. It is also much larger than conventional snow loading, so it is not a surprise that standard tables do not cover this type of load. It is similar to the load that might be imposed by a concrete roof deck on a commercial structure, so that would certainly be one way to design the framing, i.e. look at commercial design rather than residential design table.

I do not recommend that design of a heavily loaded roofing structure be performed by an internet chat room. My recommendation would be that the OPS either hire a professional, or if that is not in the budget, perhaps discuss the project with a lumberyard that will include the engineering design fee in their cost to supply appropriate sized framing lumber.

plschwartz

Thank you all.The psi was my late at night error. It is of course psf. The weight of moist soil indeed seems to be 120psF per cubic foot. Anyway hot a suggested link to US Green Building Council which has a phone in.