Water can penetrate as rain from the roof, as driving rain through the side walls because of damaged gutters or damaged bricks, or as rising damp from the soil below.
Keeping out the water from above.
Roofs must always be good hats. They can resist water by;-
Lead, slate and zinc are all impervious materials, so the slope can be much less than with thatch, or wooden shingles. However, the water must still be thrown well clear of the walls by spouts, or gargoyles, or else be carried in sound gutters to down pipes, English cathedrals began to have flatter roofs when the Derbyshire lead mines were developed. Lead roofs were impervious to water so they could have a very low pitch, As a result, the shape of buildings changed and the Perpendicular Style was born. The lead roof, which is impervious to water, 'invented' the style, not vice-versa.
Many people think that the slope of a lead roof is low to save lead, It is true the area of a low-pitched roof is smaller than the area of a steep one but is another reason. Lead on a steep roof would creep away as these lead-lined gutters show. When this house in Park Village, Camden Town, was built about1838, the wooden gutters were covered with lead sheeting. When the house was restored in1990, thlead had torn away from its nails and the house was wet from leaking rainwater.
Lead is a very heavy material which expands in heat and contracts in cold. In Summer it expands downwards: in Winter it contracts downwards. This pattern of downward movement, called creep, is repeated year after year. Lead roofs have to have a low pitch and to be specially designed to reduce effect of the perpetual movement. Here is an example of a house ruined by moving lead. A house in Park Village East built by Pennethorne just east of Regents Park was being restored. The walls had been penertarted by water becuase the lead covered gutters had moved and the water poured into the walls. Lead creeps downwards. It shrinks downwards in cold weather and expands downwards in the summer. Therefore it gradually moves downwards and the illustration show how the lead pulled itself off the nails and the gutter become useless.
A similar roof shape to the Perpendicular sloope had arisen much earlier in the Greece and Italy as a result of using clay pantiles instead of flat tiles. Pantiles are curved to form a series of gutters which throw off water very efficiently, so the roof slope can be very low.
A Story about Thatch
Thatch will actually absorb water, but a thick thatch with a slope can throw it off. The thatch, of course, must be carefully applied. Samuel Butler, the Victorian writer of "Erewhon", emigrated to Rengitata in New Zealand as a young man to get away from his father. He bought land and developed it as a sheep station. There he had a simple house built with walls of earth sods and thatched with straw, the typical settler's building, but the builders were such amateurs that they put the thatch on upside down. Instead of starting at the eaves, they laid their bundles of thatch from the ridge of the roof downwards so that the ends of the straws pointed upwards. When it rained they acted as drinking straws. The rain ran straight through into the rooms below. Butler set "Erewhon", his famous utopia, in Rengitata and the nearby mountain pass, but there is no mention of the thatch.
The mythical flat roof which does not leak.
This would be a story for the Arabian Nights.
A flat rook in England which does not leak is almost a contradiction in terms. In summer the asphalt melts and runs down. In winter these thin pieces freeze and split. In summer the split pieces melt and run down. The roof gets weaker year by year. The life of a modern flat asphalt roof has proved to be about fifteen years, which is completely uneconomic. Flat roofs are for deserts. A headmaster once said, "Show me a flat roof in Britain and I will show you a bucket". Thousands of flat roofs in Britain have been, or need to be, replaced with pitched roofs.
A flat, bituninous roof
The Upside-Down Roof: An Unusual Solution.
Some refurbished flats in Kentish Town have interesting roofs. The architects could have put on new pitched roofs while leaving the existing roofs in place. This has been done successfully elsewhere. Alternatively they could have tried to improve the existing flat roof, In fact they decided to make a revolutionary Upside-Down Roof. This is a flat roof but of a new kind shown in detail below.
The Explanation to the Upside Down Roof System