Pitched roofs: structure, insulation and ventilation

The parts of a pitched roof

• Structure. Rafters and trusses, or purlins and ridge beams, carry the roof's weight and loads from snow and wind, down to the walls. A structural engineer sizes them.
• Covering. Tiles, slates, metal sheets, zinc or other materials shed rain. The type decides the minimum pitch.
• Underlay or membrane. A secondary layer below the covering catches wind-blown rain and snow.
• Battens. They support the covering and allow air and water to move.
• Insulation. Keeps heat in, placed either at ceiling level or in the pitch.
• Ventilation. Lets moisture escape from the roof space.
• Details. Eaves, verges, ridges, hips, valleys, gutters and flashings, where most leaks start.

Cold roof or warm roof

• Cold roof (insulation at ceiling level). The loft is unheated and ventilated. Insulation lies on the ceiling joists, and air flows through the loft to keep it dry. It is simple and common, but it needs a clear airflow path and an airtight ceiling.
• Warm roof (insulation in or above the rafters). The insulation follows the slope, and the loft space becomes part of the heated building. Some designs ventilate a gap above the insulation, and some rely on vapour-open membranes. The correct approach depends on the build-up and the manufacturer's guidance.

Warm, moist air from the house rises into the roof.

Why ventilation and airtightness matter

Warm, moist air from the house rises into the roof. If it can reach cold timbers or underlay, it can condense and cause damp, mould and decay. Two things prevent this: an airtight ceiling that stops the moist air from getting in, and appropriate ventilation to remove any that does. See Airtightness and ventilation and Damp, condensation and mould.

Pitch matters

Each roof covering has a minimum pitch to shed water reliably, depending on the product and the exposure. Steeper pitches shed water and snow faster. Very low pitches need special coverings and details. Always follow the manufacturer's guidance.

The details

• Eaves. Allow ventilation, support the gutter, and protect the wall below.
• Verges and ridges. Fixed so that wind cannot lift them.
• Valleys and hips. Need careful lining and flashing.
• Flashings. Where the roof meets a wall or chimney, they must be properly formed and fixed.
• Fixings. In windy locations, tiles and slates need the right fixings for the exposure.
• Rooflights and dormers. Add points of weakness that need good detailing.
• Solar panels. Fixing them adds loads and penetrations. See Solar panels on buildings.

Looking after a pitched roof

Check the roof visually from the ground after storms, and have it inspected by a competent person at intervals. Keep gutters clear. Replace slipped tiles and failed flashings early. Do not walk on roofs without proper access and safety.

In short

A good pitched roof sheds water well, keeps heat in and lets moisture out. It needs the right pitch, structure, insulation, airtight ceiling, ventilation and careful details. See also Flat roofs.

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