I've got a roof terrace which was constructed about 20 years ago over the rear addition - it was previously a tiled sloping roof. All specified by a decent architect and carried out by (supposedly) the best roofing specialists in the area.
The construction is marine ply over joists with chicken wire surrounds covered in asphalt. The upturns have lead flashings, and the main surface GRP concrete tiles - IIRC called promenade tiles. Looks a pukka job.
Has been absolutely fine despite heavy use, but now a couple of tiles have lifted along one edge by quite a lot - an inch or so. But aren't loose. Like as though the asphalt has swelled in some way beneath them. Isn't leaking or anything - but I'm concerned as to the cause and a way of sorting it. I can imagine full replacement costing both arms and both legs.