I would not really expect timber to be supporting concrete, except as temporary shuttering.
Concrete is a rigid material, with minimal flex before it breaks. Wood is a flexible material with considerable ability to bend before it breaks - therefore it is useless as a permanent support for a concrete roof.
Makes sense. I think the 1960s practice was to use cementized wood wool as formwork for concrete cast in situ, with the woodwool then left in place to act as insulation. My roof, given the regular position of lines, looks as if it was either cast in 1m/1yard strips or, as one roofer believed, it may be composed of precast slabs - I doubt that, as they would have been 1x12.5m in length.
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