
Chapter one
Rafters, battens and the layers under the tiles
The covering gets the attention, but the timber frame, underlay and battens beneath it decide whether a pitched roof stays dry.
A pitched roof is a sloping roof made of two or more faces that meet at a ridge, a hip or a gable end. The slope does the first job, because rain runs off it rather than pooling. Everything under the covering does the second, which is to catch whatever gets past. Working from the inside out, the layers are the ceiling, the roof timbers, the underlay, the battens and finally the tiles or slates. A fault in any one of them can show up as a stain a long way from the cause.
At a glance
- Rafters carry battens, and battens carry the covering
- Cut roofs and trussed roofs are framed differently
- Eaves and ridge openings let loft air move
Note Permitted development caps a dormer's added volume at 40 cubic metres on a terrace house and 50 on any other, under Class B.

The timbers come in two forms. A cut roof is built on site from individual rafters, a ridge board and purlins, and it leaves an open loft. A trussed roof arrives as factory-made triangular frames set at regular spacing, with diagonal timbers webbing the loft. Trussed frames carry load through every member, so cutting one for a loft room needs a structural engineer's design. Our guide to how a pitched roof is built follows both from the walls upward.
1.1 Roof shapes on Sussex houses
The shape sets how many hips, valleys and joints the covering has to cross, and each is a place to look when a leak appears. A gable roof has two slopes and a triangle of wall at each end. A hipped roof slopes on every side, the shape of many post-war bungalows near the coast. The hips are the angled ridges at the corners, with cut tiles beside them. Bays, dormers and rear extensions add valleys, the inside channels where two slopes meet. Each shape is laid out in the guide to gable, hipped and mansard roofs.

1.2 Underlay, battens and ventilation
Underlay is the membrane stretched over the rafters, and it catches wind-blown rain and snow that slips under the covering. Old bitumen felt perishes, tears and sags between rafters, and it is often the first thing to give up in a roof that is otherwise sound. Battens are the timber laths nailed across the rafters, spaced to the gauge of the tile or slate. Where they have gone soft, nothing new will hold in them.
A cold pitched roof also has to breathe. Marley's guidance for a high-resistance underlay asks for a continuous opening of 10 mm along the eaves where the pitch is over 15 degrees, and 25 mm at 15 degrees or less, with a further 5 mm at high level where the pitch exceeds 35 degrees.
The layers of a pitched roof, inside to out
- Timbers Rafters or trusses carry the whole load down into the walls.
- Underlay A membrane over the rafters catches anything that passes the covering.
- Battens Timber laths set to the gauge of the tile or slate hold each course.
- Covering Clay, concrete or slate sheds the rain down to the gutter.
Stains on a ceiling often start in the timbers and membrane, not the tiles.
A roof installation on an extension or a loft conversion follows the same order. The frame is fitted and braced first, then underlay is laid with its laps facing the fall, then battens go on, and only then the covering, ridge and verge. Rushing any step shows up later as a leak, which is why we photograph each stage before the next one hides it. On a new roof the underlay laps, the batten gauge and the fixings at the eaves are checked against the specification at that point, while they can still be seen.























