I could be wrong, but my own thoughts are that a concrete roof on a wooden structure would be too heavy. ( I'm thinking of the story of the original Dr who box collapsing on Liz Sladen's head...Concrete would be very messy)The possibility of the boxes simply not having had the rooves painted is far more likely to my mind at least. The bloke told to do the painting might not have had a step ladder! Or perhaps they were just undercoated or even a paler shade of blue? ( or green, or even white) I suppose we shall never know! ( Unless one of us builds a real Tardis and we go back to find out!)
( And yes, Alexcutler, it is great shame about those ugly watermarks! Quite spoils the picture!)