A Roof over our heads

Lets talk tiles. Specifically the tuille Romane, the typically red low-pitched roofs of France below the Loire. Everybody  takes photos of them, but how many understand the nuts and bolts of the system? Not that there are any nuts or for that matter  fastenings of any kind. The original point or the system was to create a roof with little iron (expensive in Roman times ) which could be built and maintained by the unskilled.
  The basic unit is the hollow Roman tile, about 15 inches long, heavy fired earthenware or clay,slightly tapered over its length.  You fix chevrons  (light rafters) down the slope of your roof, nail planks across these horizontally, then start tiling at the bottom. You run a line of tiles up the roof, tapered end down, each tile resting on its curved belly. You thus create a sort of rain gutter stretching down the roof. It will wobble as the tiles rock on their curved undersides. No problem, chock it in place with some bits of broken tile.  Now lay another one alongside leaving a gap of about 3 inches between the gutters. Continue this right across your roof, but before you go too far note that the next step is to link the gutters together by a new line of tiles this time the other way up and with the wide end down. The photo above left shows a short section of linked tiles and the photo to the right shows the ends of the gutters and capping tiles.
  Note that the tiles are not attached which is why the roof slope is shallow, they would all slide off otherwise. If a tile is broken it can be replaced with no tools by anyone with a head for heights.
  The disadvantages of the system are that it`s heavy, prone to clogging with moss and debris unless cleared from time to time and a really heavy rainstorm can cause the gutters to overflow causing torrential leaks! You cant have everything and underfelt (apparently unknown to the Romans) will cure this...