How a pitched roof works, from rafters to roof covering

The frame collects loads from the covering and transmits them to the building supports. Snow and wind act on the slopes. Forces can press the roof down or lift it. Timber sizes, connections and anchors must work together to resist these actions.

The design defines a load path through some combination of wall plates, walls, purlins, posts or trusses. Removing a post to fit furniture or cutting a beam for a service changes that path. Have a structural engineer assess such alterations even if the covering stays in place.

A rafter roof uses pairs of opposing rafters. A collar-beam roof connects the rafters with horizontal collar beams. A purlin roof supports them on additional longitudinal beams and their supports. Each arrangement carries forces differently and leaves a different usable space beneath the slopes.

Prefabricated trusses allow much of the frame to be made off site. Their drawings define the geometry and connections. Cutting a diagonal member or chord for loft access changes the structure. Storage or occupation is possible only where the intended use and capacity permit it.

Battens support and secure the covering. Counter-battens usually follow the slope, creating a ventilation space above the underlay. Provide both an inlet and an outlet for air to move through the gap.

For metal roof tiles, batten spacing must match the module. For trapezoidal sheet, the sheet capacity and support spacing are critical. When changing product, check the first batten position, eaves detail and flashing fixings as well as the spacing.

Ridges, hips and valleys need separate details. Valleys drain two adjoining slopes, while the ridge often provides a ventilation outlet. Filling every visible gap with foam can obstruct airflow or drainage. Use the geometry and layer arrangement specified for each junction.

The covering sheds rain. Beneath it, the underlay has to suit the roof and covering material. At valleys, roof windows and penetrations in particular, provide a continuous drainage route that takes water clear of the insulation and wall plate.

A heated loft needs insulation and a layer that limits uncontrolled movement of moist air. PIR boards can offer high thermal resistance at modest thickness. Make the board joints and transitions to other components carefully. Between-rafter and over-rafter insulation require different junction details.

Check timber dimensions and grade against the design. Inspect moisture, surface condition and damage. A twisted or excessively notched member needs assessment even if an extra angle bracket has been fitted. Verify fastener type, number, positions and edge distances.

Inspect bearings and anchorage closely. Separate timber from damp locations using appropriate details. During refurbishment, expose critical points near walls and chimneys. Preservative treatment and a sound-looking loft surface cannot establish the condition of concealed timber.

Inspect the structure before it is covered. Check the underlay and preparation for the covering at the next stage. Photograph connections, anchors and flashings while they are accessible. These records complement inspections and help trace later faults.

At handover compare the work with the drawings and check that changes were agreed and recorded. The timber frame, insulation and metal covering must function together. A concealed fixing defect may cost more to put right than an upgrade to the exposed coating.