Overlapping of each course or row of thatch sheds any rainfall in the same manner as any shingle, tile or clay roof. Pili grass with its pleasant odor was a preferred material for thatch. Palm, Pandanus, sugar cane and Ti leaf bundles were other common covering materials. Consider this, it takes about 40 Ti leaves to cover a square foot. Palm branches with the lower leaves removed have an added advantage when used in construction. The stiff stalks add rigidity to the framework and with the large mass of the palm the surface can be covered quickly and with depth. Grass thatch will last for many years but being an organic material it will eventually decompose and have to be replaced.  Grass and leaves are naturally regenerative and a hale like this is a sustainable design. Pili grass also has a sweet aromatic scent that makes for a pleasant home environment. Grass is a natural sustainable resource and with a little help from the gathered family a new roof or walls can easily be replaced.