Thatched Roof Shear Capacity?

[ehl]

I have a project in Hawaii in which a African thatched roof will be used. It uses compressed Berg or Mountain grass that is about 10-12" thick. It is tied to 2" dia. purlins that are 10" on center and the purlins are supported every 3.5'. I need the roof to have an allowable shear capacity of 200plf. If I calculate the shear capacity ignoring the thatch I can only get about 40plf. The Thatching is pretty solid and stiff. I know it must contribute to the strength and stiffness of the roof assembly.

I have been unsuccessful in finding any shear testing information although I know thatch roof construction is very common in some parts of Europe.

Anybody have experience with thatched roofing?

Thanks.