Roman Tegula

Tegula is the twin of Imbrex the two types of tile which made up the Roman roof, a design nicked from the Greeks and still in use in Rome today.

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. 

If the tile has a return like the one below it and it is found where Roman London lay, it may be a Roman tegula. 

Tom's mudlarking find Roman Tegula. 

This find was one of Florida Tom's gifts to me. Later that day I found another. Funny how you spot things once their image and identity is imprinted in your brain. 

Roman Londinium superimposed over modern London (Romanobritain.org)

The tegulae were laid in overlapping fashion. Some were just plain flat tiles, others had raised edges on each side (like the ones I've found) preventing the water from seeping between the join and channeling the water to the bottom of the tile. Later tegulae were a v shape whereby the edges converged slightly each becoming a funnel which slotted into the one below forming a continuous channel. 

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)

The roofing area was usually surrounded by antefixa and these were frequently decorated, now that would be a nice find. 

Roman Roof with Antefixa (Grahams Potted History)