Ood's wise man suggested bypassing the problem of the temperamental wood by 'seeding' the concrete with styrofoam cubes instead, which won't have the strength to crack the cement by either expanding when wet or disintegrating. Still, the principle is the same: the point is to reduce the mass of cement and thus the weight.
When Ood came back with the styrofoam, he proudly reported that the styrofoam man told him everyone buys it for that. There's at any rate an extra i-beam under that part of the floor, so perhaps all this expense of brain power was unnecessary.
For the roof we've used something called Flex Shield, which is apparently used as a cheap swimming-pool sealant, because Ood felt epoxy was too expensive. It sets a nice light grey so won't retain too much heat from ze burning sun of ze tropics.