Chapter one
Gently pitched slate on converted barns
A low-pitched slate roof leaves little room for error, and a barn turned into a home adds insulation and condensation to the list.
The Hunston conservation area appraisal describes the former manorial barns as flint with red brick dressings, their roofs gently pitched and covered in natural slate, and it notes that some now serve as homes. The gentle pitch is the thing to understand. Slate sheds water by overlap, and the headlap is the distance each slate reaches down over the slate two courses below it. The lower the roof, the more headlap is needed so that wind-driven rain cannot be pushed up between the courses. For a 500 x 300 slate on a sheltered or moderate site, Cupa Pizarras lists 115 mm of headlap at 20 degrees, 95 mm at 25 degrees and 80 mm at 30 degrees.
At a glance
- The barns are flint and red brick under slate
- BS 5534 points to 20 degrees as the slate minimum
- Headlap has to grow as the pitch falls
1.1 What we look at on a barn roof
On a long barn slope we sight along the ridge for a dip, which tells us the timbers underneath have moved, and then walk the courses looking for slipped, split or missing slates. Daylight through the boarding or the underlay shows where a gap has opened. We also check the rafters and battens near the eaves, where damp tends to collect first, and the lead where a roof finishes against a flint gable, stepped to follow the courses of the wall. Rain usually enters well away from where a stain finally appears inside, so we trace the water back along the slope before deciding how much needs doing.
1.2 Barns that became homes
A conversion changes how the roof behaves. A barn was built to breathe, and a home holds warm, damp air that can condense on the cold underside of the covering. Marley's guidance for cold roofs asks for ventilation at the eaves, and with a high-resistance underlay it gives 10 mm along the eaves at pitches over 15 degrees. If a roof has been insulated or lined since conversion, we check whether that gap is still open. Barns suit roosting bats and nesting birds, both legally protected, so before any slate is lifted we look for droppings and nests. Our slate roofing page covers relaying, and roof inspections and surveys explains what a written survey records.
Slate headlap against pitch, 500 x 300 slate
| Roof pitch | Headlap, sheltered or moderate site |
|---|---|
| 20 degrees | 115 mm |
| 25 degrees | 95 mm |
| 30 degrees | 80 mm |
| 35 degrees | 70 mm |
