Flat roofs: how to detail them so they do not leak

Not really flat

A "flat" roof is a roof with a low slope. It should always be designed with a fall so that water runs to an outlet. A roof that is level or sags will collect pools of water ("ponding"), which can shorten the life of the waterproofing and add load. The fall, the outlets and the structure must be designed together.

Three build-ups

• Warm roof. The insulation sits above the structure and below the waterproofing. The structure stays warm, which reduces the risk of condensation inside the roof. It is generally the preferred approach.
• Cold roof. The insulation sits between or below the structure, with a ventilated void above it. It needs good ventilation and a vapour control layer to avoid condensation, and in practice is harder to get right.
• Inverted roof. The insulation sits above the waterproofing, protecting it from sun and impact, held down by ballast, such as gravel, paving or planting. Suitable insulation and enough weight are needed.

It should always be designed with a fall so that water runs to an outlet.

The layers

From the top, a typical warm roof has a finish (such as ballast, paving or planting, or a surface treatment), the waterproofing membrane, insulation, a vapour control layer, and the structural deck. The vapour control layer stops moist indoor air from reaching cold parts of the roof.

Waterproofing choices

Common choices include single-ply membranes, built-up bitumen systems, liquid-applied coatings and mastic asphalt. Metal roofs, such as zinc and copper, are also used with a slight pitch. Each has its own rules about slopes, joints, detailing and maintenance. The manufacturer's guarantee often depends on the installer and the details being approved.

The details that matter

• Outlets and overflows. Enough outlets, placed at the low points, and an overflow in case the main one blocks.
• Upstands and parapets. The waterproofing must turn up at edges and walls to a height set by the manufacturer, and be protected at the top by a flashing.
• Doors and thresholds. Level access to a roof terrace is a common point of failure. It needs a considered detail.
• Penetrations. Pipes, rooflights, vents and supports are weak points. Fewer and better detailed is best.
• Edges and drips. Water must be thrown clear of the wall below.
• Movement joints. Large roofs need joints to cope with expansion.
• Access and protection. Walkways and protection where people or maintenance cross the roof.

Green roofs and terraces

Planted roofs, paving and plant need extra structure, drainage and protection, as well as a root-resistant membrane and a maintenance plan. See Biodiversity on a roof.

Maintenance

All roofs need inspection, at least once a year and after storms. Keep outlets clear of leaves, check the membrane and flashings, and repair small defects before they grow.

In short

Design a flat roof with a fall, choose a warm build-up where possible, detail edges, outlets and penetrations carefully, use experienced installers and maintain it. See also Pitched roofs.

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