Last edited 15 Sep 2020

Flashing in building construction



[edit] Introduction

Flashing is a sheet of thin, impervious material used to prevent water penetration or seepage into a building and to direct the flow of moisture in walls. Flashing is particularly important at junctions such as roof hips and valleys, joints between roofs and vertical walls, roof intersections or projections such as chimneys, dormers, vent pipes, window openings, and so on.

There are two categories of flashing, exposed and embedded. Exposed flashings are partially exposed to the outside and usually made of a sheet metal, while embedded (or concealed) flashing can be metal or non-metal and are typically used at the base of cavity walls, above openings, at sills and shelf angles (a structural steel member that supports and transfers the brick’s dead load back to the building frame), and under copings.

[edit] Types of flashing

There are different terms given to flashing, often determined by where they are used or their shape.


[edit] Construction of flashing

Flashing has to be engineered and installed with care so that water is deflected away from the building. Improper installation can result in water being directed into a building.

Expansion joints should be provided for long runs of metal flashing to allow movement without causing deformation.

Continuous flashing is one solid piece of material, and is sometimes seen where a sloping roof and a vertical sidewall meet to form a horizontal line.

Step flashing allows a roof to abut brickwork, instance when a lower height roof meets a wall, a roof meets a chimney, or a conservatory roof meets a house wall. Step flashing offers far better protection from leaks than continuous flashing, because even if a single piece of step flashing fails, the water hits the next lower piece, which directs the water onto the shingle and drains down the roof.

[edit] Materials

There are many materials that can be used for flashing; the important thing is that they do not degrade from coming into contact with incompatible materials and that they are impermeable to water. The most common flashing materials are metal, plastic, or rubber.

[edit] Metal flashing materials

Lead is traditionally used as a flashing material around upstands in roofs, such as chimneys because of its durability. The minimum specification that should be used is Code 4 lead (1.8 mm) which is both malleable and resistant to fatigue splits that could be caused by expansion and contraction. Code 3 (1.32 mm) is thinner, lighter and cheaper, and is commonly used for lead soakers which are the concealed waterproof sections positioned underneath slates and plain tiles.

Other materials that can be used as metal flashings include aluminium which is also durable and relatively inexpensive. However, aluminium will corrode if left in contact with pressure-treated wood, and also wet mortar and plaster.

Copper is malleable, strong and has a high resistance to the corrosive effects of mortars and wood-preservatives. Cold rolled copper is recommended rather than soft copper as it offers more resistance to expansion and contraction stresses, although soft copper is useful where there are complex roof shapes.

Soft zinc is a very malleable material which makes it suitable for complex roof shapes.

There are also arrangements of metals that can be coated for use as flashings, such as:

Neither copper nor lead can be used in contact with or above aluminium, zinc or coated steel without an increased risk of premature corrosion. Coastal areas face the risk of accelerated corrosion due to salt spray and so stainless steel, copper or coated aluminium are often recommended.

[edit] Non-metal flashing materials

Also known as flexible flashing materials, these include rubberised asphalt, butyl rubber, PVC and acrylic. They have different chemical compatibilities, resistance to ultraviolet light exposure, application temperature ranges and so on. Adhesive-backed materials are easily installed but consideration should be given to their long-term water resistance and most should not be left permanently exposed to the weather unless laminated with a top surface of aluminium foil.

[edit] Related articles on Designing Buildings Wiki

[edit] External references



Is it possible to add information on the pointing material for flashing. I was asked today if the zinc flashing should be pointed with mortar or mastic

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