The Sussex catslide

A catslide roof is one where a single plane sweeps much further down on one side of the building than on the other — typically at the rear, often to within a metre of the ground. You find them across Sussex, Kent, and parts of Surrey, more rarely in Hampshire. They are a South East answer to a universal problem: what do you do when the farmer or miller or blacksmith needs a lean-to scullery or pantry bolted onto the side of the house, and doesn't want the roofline to look like an afterthought?

The answer the local carpenters found was to extend the main rafters straight down over the lean-to as a continuous slope. Structurally simple, visually coherent, and it sheds water off the back of the building without requiring a second ridge or a valley junction. Done in long-straw thatch, with hand-hewn rafters and pegged joinery, the result is a form that reads as deliberate even though it grew out of a practical compromise.

The failure mode is almost always at the very bottom of the slope, where the roof covering meets the low masonry or cob plinth. Wind-driven rain finds the junction, sits, and works its way inward. If the thatch or slate is old and the plinth is cementitious, water goes sideways into the wall instead of shedding down the face. We've seen three catslides in the last five years where an otherwise sound building had rotted its ground-floor joists because of a sixty-centimetre failure at the junction of roof and plinth.

The repair is about respect for the geometry. You cannot straighten a catslide roof to match a modern expectation of right angles; the sag and the asymmetry are load-bearing facts. What you can do is replace the covering in period-correct material, repair the rafter tails if they're rotten (oak scarf, pegged, reversible), and rebuild the plinth in lime mortar so it can dry outward. The Keeper's Cottage in Ashdown Forest — project 658 — is a good example; long-straw thatch at the rear, a hot-mixed lime harl on the gable, and the chimney helical-tied where it was cracking.

Sussex has fewer catslides than it did fifty years ago. When owners decide to extend their properties, catslides are easy to sacrifice because planning officers sometimes don't recognise them as a protected form. If you own one, and you are thinking of altering the back of your house, please write to us first. We will come and look for nothing; we would rather save one than restore one.