Roof construction: rafters, battens, undertake, and ventilation

A roof structure is not just a couple of rafters with roof tiles on. It is a cohesive system of layers each with a specific function: to bear, to keep water out, to insulate, to seal air and to direct moisture away. If you do not understand the sequence and interaction of the layers problems arise with moisture damage which only shows up years later — and which is expensive to fix. Therefore roof structure is one of the disciplines a carpenter must have completely under control.

The load-bearing part of the roof consists of rafters, which are usually made of structural timber in strength class C24. The rafters are erected and braced, after which underlayment is laid, which functions as the roof's rain jacket and catches the wind-driven rain and water that might escape past the outermost roof covering. On top of the underlayment, spacing strips and battens are screwed on, which the roof tiles or sheets rest on. At the bottom — towards the building — comes insulation, vapour barrier and an internal lining.

• 01Rafter – the load-bearing beam structure, typically C24, that absorbs self-weight, snow, and wind load
• 02Underlay—water-repellent layer that directs water and condensation out to the gutter.
• 03Distance strips and battens — create the air gap and support the outer roof covering
• 04Roofing — tiles, concrete roof tiles, roofing felt, or steel plates, the roof's outermost shield
• 05Insulation – limits heat loss and must lie tight without thermal bridges
• 06Vapour barrier — PE film 0.20 mm

There is a distinction between the cold and warm roof. In the cold roof (typically an unused attic space with insulation on the ceiling), the entire attic space is ventilated so moisture is removed by the air. In the warm pitched roof, insulation runs all the way to the underlay, and here a continuous ventilation gap between insulation and underlay is crucial so the structure can shed moisture. As a starting point, an air gap of at least 50 mm and ventilation at the roof edge of at least approximately 25 cm² per meter of roof edge is recommended.

The Building Regulations (BR18) set out in chapter 14 on moisture (§ 334) requirements that buildings — including roofs — must be designed and constructed so that water and moisture do not cause mould rot or reduced insulation capacity. In practice this means that the moisture balance must be coherent: water coming from outside must be kept out of the roof underlay and roof covering and water vapour from inside must be slowed by the vapour barrier and ventilated away. A roof that traps moisture without a way out is doomed to cause problems.

For the carpenter a good roof is about understanding that each layer is dependent on the next. A perfectly laid underpayment does not help if the air gap is squeezed and the world's best vapour barrier is worthless if perforated by nails and screws. That is why checking each layer — before it is closed off by the next — is a fixed part of good carpenter work.

“A roof is built from bottom up but thought from top down: where does the water end and where does moisture escape?”