hatched roof shear capacity
 

thatched roof shear capacity?
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.