Flashing in Building Construction

Flashing is a flat and thin material used to seal joints to prevent water from entering or seeping into a building. When building materials intersect, joints are formed that have the potential for water infiltration. 

Flashing are provided at important junctions of a building like roof hips and valleys, joints between the walls and the roofs, roof intersections with chimneys, vent pipes, window openings and so on. 

The two general categorise of flashing are:

a. External or exposed flashing

Exposed flashing are partially exposed to the outside and are usually made of a sheet metal. For example, flashing for joints where masonry wall intersects with the roof.

b. Embedded or Internal or concealed flashing

An embedded or concealed flashing are are mostly hidden and are made of metal or non-metal. For examples, used in the heads of windows or doors and window sills. When a building leaks, an embedded flashing redirests the water outside through drainage pipes or weep holes.

In addition to the general categorise, flashing can vary based on the material and location they are used. This article broadly takes through the construction, features and types of flashing employed in building construction.

Functions of Flashing for Buildings

The main functions of flashing for buildings are:

1. To direct water that is penetrated behind the cladding on a wall out to the exterior ( by means of wall base flashing).
2. To protect an interface between two different enclosure elements like roof to wall.
3. To shed water over protruding building elements
4. To reduce moisture loading on any of the building enclosure

Construction of Flashing

The primary objective of any flashing is to lead water away from the building. A basic flashing construction follows an "down" and "out" arrangement as shown in figure 3 below. The down direction harnesses the force of gravity and the out direction gets water away from the building enclosure assembly or openings.

Fundamentals of Flashing

Flashing Materials

The common materials used for flashing in buildings can be metal or non-metal materials. 

Metal flashing materials include lead, aluminium, copper, soft zinc and arrangement of metals that are coated, for example; lead-coated copper, terne-coated copper, anodized aluminium, polyvinylidene fluoride.
Non-metal flashing materials also called as flexible flashing materials include PVC, acrylic, butyl rubber and rubberised asphalt.

 

Flashing Application

Based on the location flashing is applied, flashing can be:

1. Roof flashing
2. Wall Flashing
3. Sill Flashing
4. Channel Flashing
5. Through Wall Flashing
6. Pipe flashing
7. Valley flashing

Types of Flashing Construction 

Flashing is engineered and constructed such that water is deflected away from the building. Improper design and installation can result in water being accumulated within the building. Based on the way flashing are constructed, it can be:

1. Base flashing
2. Continuous flashing
3. Stepped flashing
4. Head flashing

1. Base Flashing

Before laying shingles over the roof, a waterproof membrane is laid first and it forms the base flashing. These membrane flashing is attached to the up side of the wall as well. 

2. Continuous Flashing

Continuous flashing is a flashing provided by a single piece of material without any joints. It is mainly provided where a sloping roof and vertical sidewall meet to form a horizontal line. 

Flashing Construction Types
Image Credits: ©Don Vandervort, HomeTips

3. Step Flashing

These are flashing where the pieces of flashing materials overlaps in the form of steps. It is used when a sloped roof meets a vertical wall as shown in the figure below.

Step Flashing for Chimney Wall and Roof Joints

As shown in the figure, these are made from individual metal pieces that are overlaid and woven such that it ensure to prevent water penetration into the roof-to-wall joint. Hence, step flashing come over the base flashing.

4. Counter Flashing

A counter flashing or cover or cap flashing is a final layer installed over the step flashing that is made of aluminium, galvanized steel, copper, tin or even plastic.

Counterflashing for Chimney Roof Joints

In most cases, step flooring is enough, but for extreme joint conditions a counter flashing can help to completely seal the joints.

5. Kickout Flashing

Kickout flashing or diverter flashing is a flashing provided to kick water away from the structure. These flashing helps to divert the rainwater  from the cladding into the gutter as shown below.

Kickout Flashing

6. Drip Edge Flashing

Drip edge flashing is a strip flashing that is applied to the edge of roofing prior to the installation of other items. These flashing helps to protect the roof's sheathing from water damage caused by rain, snow or heavy winds rolling back the edge of the shingles.

Conclusion

Flashing construction is one of the important step taken to increase the durability of the building, that is often disregarded or not given importance it deserve.

The type of flashing used varies based on the location and extend of water sealing required. The material used for flashing varies based on the exposure conditions and budget of the project. 

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