What is flashing on a roof?

Flashing is a thin strip of impermeable material fixed at the junction between a roof surface and a wall, chimney, dormer, or other upstand. Its job is to seal this vulnerable junction and prevent rainwater from penetrating the roof structure. Lead is the traditional and most durable material, though aluminium and GRP alternatives are also used.

What code of lead flashing is used on roofs?

Lead flashing is specified by code number which relates to its thickness. Code 4 (1.8mm thick) is standard for most roof flashings. Code 5 (2.24mm) is used for larger areas such as valleys and parapet gutters. Code 3 is sometimes used for soakers. The Lead Sheet Association publishes guidance on the correct code for each application.

What is the difference between flashing and a soaker?

Soakers are small individual pieces of lead tucked under each course of slates or tiles at an abutment, interlocking with them. Flashing is the continuous strip of lead dressed over the top of the soakers and into the wall above, forming the visible weather seal. The two work together - soakers handle the water at tile level, flashing handles the junction with the wall.

How far should flashing go into a wall?

Lead flashing should be chased at least 25mm into the mortar joint of the masonry wall and turned up so the top edge is secured. The Lead Sheet Association recommends a minimum 75mm upstand and the flashing should overlap the roof surface by at least 150mm.

Flashing - Construction Glossary

Definition

Flashing is a thin strip of impermeable material - typically lead, aluminium, or GRP - fixed at the junction between a roof surface and a wall, chimney, dormer, or other upstand. Its purpose is to seal this vulnerable joint and prevent rainwater from penetrating the roof structure below. It is one of the most critical weatherproofing details on any pitched roof.

In practice

Lead is the traditional material and remains the preferred choice for durability - it is soft enough to be worked into complex shapes and lasts 50-100 years when correctly installed. Lead flashing is specified by code number based on thickness: Code 4 (1.8mm) is standard for most roof flashings; Code 5 (2.24mm) is used for valleys and parapet gutters.

The top edge of the flashing is chased at least 25mm into a raked mortar joint in the masonry, turned back, and pinned with lead wedges before being pointed up. The bottom edge is dressed down over the tiles or slates with a minimum 150mm overlap. At abutments alongside a pitched roof, step flashings are cut to follow the tile courses, working with soakers tucked beneath each course to create a watertight stepped detail.

Building Regulations

Relevant PartPart C - Resistance to moisture

Standard code for most flashingsCode 4 lead (1.8 mm)

Minimum upstand75 mm above roof surface

Minimum overlap onto roof150 mm

Approved Document C requires roofs to resist the passage of moisture to the inside of the building. Flashing installation must follow the Lead Sheet Association guidelines or manufacturer's instructions for alternative materials. Re-roofing work that exposes flashings is an opportunity to inspect and replace failed leadwork before new tiles are fixed.

Full Building Regulations guidance

Related Calculators

Flashing Calculator

Flashing

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