Pergolas and Cabanas: Components of built shade

Built shade consists of:

• the supporting structure, which is required to maintain the shading element in position

• the primary shading element, ie the material that comprises the canopy or roof.

The properties of the shading element, such as mass and span, will strongly influence the design of the required supporting structure. For example:

• relatively lightweight materials such as metal or translucent roof sheeting, which are capable of spanning up to 1.2 metres, will require less supporting structure than a roof of terracotta tiles, which are significantly heavier and require supporting battens about every 300mm

• a project that uses structural fabric or shade cloth as the primary shading element will require less supporting structure than metal roof sheeting, as fabrics are lighter and span greater distances when in tension

• solid fabrics such as canvas or reinforced PVC do not allow wind to pass through in the same manner as open-weave shade cloth, and therefore require supporting structures that can resist a much higher level of wind loading.

This relationship between the shading element and supporting structure is significant in determining the functional performance of built shade. Short-span structural systems necessarily result in a high number of supporting columns. While in some situations this may not present a problem, locations such as school playgrounds require shaded areas substantially free of columns. The design brief should define the requirements of the area to be shaded, and the designer should ensure that span characteristics of the supporting structure comply with the requirements of the brief.

The final cost of built shade is also significantly affected by the relationship between the shading element and supporting structure. Selecting a cheap material for the shading element will not be costeffective if an extensive and costly structure is required to support the selected material. In order to achieve the most cost-effective outcome, the designer must select materials and structural systems that together provide the optimum solution for the brief.

In planning built shade, the required life of the shade structure should be determined and designed for. The post-life use of the materials, ie how they may be re-used, recycled or disposed, should also be considered and designed for.

For all built structures, large or small, it is essential to seek professional structural advice and certification from a qualified structural engineer. This certification is required for building applications and will ensure structural integrity and safety.