I have done this before a couple of times and while one was ok, the other one got a bit of damp through the concrete roof and the boards failed in a few places, I would reccomend putting a frame up, it wouldnt have to be a thick 3x2 timber, half inch slate lat would do screwed to the concrete or if its only a small out house then put a frame up screwed to the walls and knock the timber in tight that way.
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