Keith Roberts' article about durability issues in roofing (AJ Specification 12.05) has added much useful knowledge to the discussion.
There is, however, one small error that I would like to draw to your readers' attention. Where Roberts refers to the low water absorption of tiles made from Etruria marl he makes a connection between such low absorption and the tiles' ability to comply with BS EN 1304, the British Standard for clay roof tiles.
A test for water absorption, however, is not one of the test requirements of BS EN 1304. In earlier standards, water absorption was used as a test to establish the durability of clay tiles. But in recent times it has been found that, because clays from different parts of the country behave in different ways, comparing the water absorption of tiles made from one type of clay with those made from another type is not rational.
Water absorption is a useful indicator for one manufacturer to judge the quality of his product in-house, but it should not be used to compare different manufacturers' tiles. It is taken into account in the standard when testing for frost resistance, but its level is only relevant when dealing with a single clay type.
All clay tiles being made today in the United Kingdom should be frost resistant, whatever their water absorption, and it is not accurate to say that tiles with a lower water absorption are more frost resistant.
Martin Oldridge, Sandtoft Roof Tiles