" wrote:

My house was built with a flat roof. The previous owners had pitched
roof constructed on top of it so that when you enter the roof space
you can walk anywhere on the original bitumen & gravel finish. We
have broken through the old roof to fit ventilation trunking for a
bathroom extractor and I see that the old roof is made up of a
boarding material about 50-75 mms thick which seems to have been made
with a straw-like material.


Probably "woodwool", made of long wood fibres bound together with
cement.