I presume most of the red tiles you see on the Thames foreshore are medieval and they certainly are if they have one or two holes in them. But perhaps I'm wrong and many of the tile pieces are Roman for me it's just impossible to differentiate.
|The Thames foreshore is littered with tiles.|
|Tom's mudlarking find Roman Tegula.|
|Roman Londinium superimposed over modern London (Romanobritain.org)|
The imbrex were semi circular and were place over the joins between tegulae. If well laid the result was a fully waterproof roof.
|tegula (a) imbrex (b) (wiki)|
|Roman Roof with Antefixa (Grahams Potted History)|