Chapter one
Slate, brick and mansards in a rebuilt city
The 18th-century rebuilding left Chichester with brick walls under slate, and slate rewards a survey that looks at its fixings.
Medieval Chichester was probably built mainly of timber-framed buildings, according to the Victoria County History, with brick employed from the 17th century. By the end of the 18th century the town had been almost entirely rebuilt, a rebuilding the Victoria County History remembers for its 18th-century doorways and fanlights, with roofs of the same age set behind them. The Royal West Sussex Hospital, begun in 1825, is a useful example of the next phase: stuccoed brick under a slate mansard. A mansard has a steep lower slope and a flatter upper one, which gives more headroom in the roof but puts more joints, valleys and lead dressings into the weather.
At a glance
- Brick came into use here from the 17th century
- The hospital has a slate mansard roof
- Slate slips when its nails fail
1.1 Why a slate slips
A natural slate is held by nails or hooks through its upper edge, and the stone itself usually outlasts the metal. MyBuilder says slate laid well can run to 80 or 100 years or more, a span the metal fixings rarely match. Once a nail rusts through, the slate drops a few centimetres and leaves a gap, and the next gale can take it. A single slipped slate can often be re-fixed with a small strip of lead called a tingle, which holds it without a nail, but a roof where slates are slipping in every row is telling you the fixings are finished. We lift a few at the survey to see what holds them, and our slate roofing page sets out the options.
1.2 Pitch and headlap
How well a slate roof sheds water depends on pitch and headlap, the overlap between a slate and the one two courses below it. Cupa Pizarras says BS 5534 indicates a minimum pitch of 20 degrees for natural slate, in its example of a 500 mm slate with a 115 mm headlap, and warns off hook fixings on pitches below 25 degrees. A slate laid with too little overlap for its pitch will let in driving rain however good the stone is.
1.3 Lead on mansards and parapets
A slate roof is only as dry as the lead around it. Where a mansard meets its dormers, chimney stacks and parapets, the weather seal is sheet lead, and the thickness is named by a code: code 4 is the usual choice for flashings, with code 3 kept for soakers, the small bent pieces tucked under each course. We check for splits, ridges lifted by heat and old sealant smeared over a failing joint. Replacing the lead properly takes more than a dab of mastic, and our lead roofing page explains what that involves.
Slate facts for a Chichester roof
- Natural slate minimum pitch
- 20 degrees in the BS 5534 example (500 mm slate, 115 mm headlap)
- Hook fixings
- Not on any pitch below 25 degrees
- Well-installed slate
- 80 to 100 years or more
- Slate re-roof, typical UK
- £100 to £180 per m²
Source: Cupa Pizarras; MyBuilder slate roof cost, updated 21/08/2026






