I don't know the answer to your question - I'm hoping someone else will chime in here. I will say that for grading around houses and for grading streets, the number used is 2% grade. 4% seems like it would be good. Let's see what some of the green builders say.
Subtropical desert (Köppen: BWh)
Elevation: 1090 ft Annual rainfall: 7"
First, "Timber frame" in most of the world outside the U.K. means traditional wood building with timbers and all wood joints. What is called "timber framing" in the U.K. really isn't...it is "stick framing" or more technically called "light stud framing" and in its original mid nineteenth century "balloon stud framing."
What I see in your photos is a "hybrid" of "post and beam" (which uses metal fasteners) and some "light frame lintels." Regrettably you have relied on the shear strength of the metal fasteners to take the load of the roof, and this WILL NOT be sufficient to support the weight of even most simple flat roofs over time...let alone...the massive weight of a living roof.
I do not intend to be a "downer" or sound negative (sorry about that ) yet I am not really seeing a long life for the structure as it has been built. The approaches used are out of context for intended application, as the post bottoms are entirely too close to grade which will lead to "dry rot" fungus in short order in that biome type.
I would, while you still can, read and study a bit more about traditional timber framing methods (or mixed modalities) and perhaps embrace them as you could be using your building resources more efficiently and effectively. I would also recommend having some good design plans drawn up by yourself (or someone else) and submit them here for feedback from folks.
If the frame, as it is now, will be encapsulated in a thick Cob wall, with a larger roof overhang...it might be possible to salvage the current frame with some augmentation.
Again...apologies for coming across as being such a downer to your plans...but the structure does not seem built for your planned intent.
posted 6 years ago
Thanks Jay C. White Cloud for your valued input.
>First, "Timber frame" in most of the world outside the U.K. means traditional wood building with timbers and all wood joints.
>Regrettably you have relied on the shear strength of the metal fasteners to take the load of the roof, and this WILL NOT be sufficient to support the weight of even most simple flat roofs over time...let alone...the massive weight of a living roof.
You're right. I'm dropping the green roof idea. I will do a bit more measuring and geometry and, if necessary, will increase the gradient on the roof to ensure no water can collect. As you say, it won't be able to take any significant extra weight on top.
As for rot, my neighbour has been using similar chestnut poles for about ten years without a problem (and his are touching the soil, while mine are an inch above it on concrete footings with a copper gasket between the wood and the concrete).
It's only supposed to be a bit of shelter from the rain, so the standard of building required is not to the same level as for a home. Nevertheless, thanks for making me stop and think about the weight issue - a green roof was never going to work, even with very thin soil.