[Taf]

One of my late uncles was a roofer who worked on a row of terraced houses in the Valleys. All had flat roof extensions added to the back (kitchen and bathroom). The houses were of stone construction, so the extensions were brick-built, leaving a narrow cavity between the main house and extension.

The last few rows of slates were slightly lifted so that the rain ran down to new guttering that was just further out from the old position to ensure that it could not run into the cavity, which was lead-capped (basically wider flashing).

A few years later he was employed to fit proper cantered and slated roofs over most of the extensions. He wasn't shocked to find that a lot of the lead flashing had disappeared, allowing wind-blown rain into the top of the cavities.