[Cob] Can I lay tile over a cob subfloor?

[avjyoung at shaw.ca]

All of our subfloor and most of our finish floor is cob, but we are wanting to lay some tiles in the bathroom to stop any water getting in. I am OK with bedding them in a layer of the finish cob with a spacing between them to allow for cob drying (with cob well oiled and waxed), but my husband is not sure this will keep it dry enough. He is keen on a layer of thin waterboard over the cob subfloor, with the tiles laid on that, to prevent any water getting in. I don’t think we need to go this far, but would love some insight.

Ta muchly,

Anna
Cob/strawbale hybrid in Victoria BC. Ten years and counting....