Pergolas and Cabanas: Providing shade

There are numerous types of shade systems, incorporating natural or built components, or a combination of these. The following sections describe the major factors and considerations for each of these approaches. Built shade is designed and constructed of manufactured components, unlike natural shade solutions that use trees, large shrubs, vines and ground covers to block direct UVR and absorb indirect UVR. A comprehensive shade strategy will most likely incorporate both built and natural shade solutions.

Natural shade

The use of natural shade can be one of the most effective and aesthetically appealing ways of providing shade. Vegetation offers seasonal variations in perfume, colour and sounds. Many species produce colourful flowers or have attractive foliage or bark, some make good habitats for wildlife and many species can be used to screen unwanted views, give wind protection and provide privacy. Other materials cannot accomplish these things as well as vegetation can.

The use of vegetation for shade also has a number of environmental benefits including:

• less need to use non-renewable resources (used in many building materials)

• energy saved in comparison with built shade systems, which often have high embodied energy (this is the sum of all energy used to produce a material, product or structure including extraction and processing of raw materials, manufacturing, assembly and transportation)

• fewer disposal problems as plants generally act as nutrients during decomposition

• absorption of carbon dioxide in the atmosphere, thereby potentially counter-balancing the ‘greenhouse effect’.

The effectiveness of natural shade depends on the density of the foliage. Foliage and timber will block direct UVR, but gaps in the canopy will allow UVR to penetrate. The size of the canopy (of a tree or group of trees) is also an important consideration. The larger the canopy diameter, the greater the opportunity for protection from scattered or reflected UVR. The height of the branches from the ground can also influence the effectiveness of natural shade, with low branches providing better protection.

It should be noted that introducing complete, or even partial shading by vegetation may affect the viability of existing under-storey vegetation. The landscape of shaded areas, as a result, may need to be treated differently to that of non-shaded areas.

Built shade

Built shade comprises all shade systems that are constructed, as distinct from natural shade. One advantage of using built shade systems, especially permanent systems, is that they can often be used for a number of purposes besides providing shade. For example, a shade structure could be used to collect rainwater for irrigation; or a structure could support photovoltaic cells, (either as a mounted array or as a laminated roofing material) for the generation of electricity.

Built shade systems can be either stand-alone structures, or systems which are incorporated into existing buildings and other facilities. They can be categorised as follows:

• permanent systems

• demountable systems

• adjustable systems.

Permanent systems

Permanent systems are considered to be those which last for at least 10 years. Their structure commonly comprises a roof with associated supporting structure and sometimes side protection, to reduce the effects of indirect UVR.

It is very important that permanent roofing systems are durable as they need to withstand the harshest of conditions, eg exposure to sun, rain and wind uplift. From an environmental and economic point of view, regular maintenance of these systems is essential to ensure their long life span, thus reducing the need for replacement of materials.

The component parts of a permanent system should either be cheap and easy to replace, or they should have a life span equivalent to that of the main parts of the structure. There is no point having a metal roof with a useful life of 60 years that needs to be replaced after 10 years, because the supporting structure or fixings did not last.

Permanent systems are likely to gain more favourable consideration from funding bodies that provide capital works subsidies.

Demountable systems

Demountable systems are likely to be a more practical and cost-effective alternative to permanent structures where:

• shade needs are infrequent

• temporary shade is required at varying locations

• a permanent structure is incompatible with the range of activities that take place at a site.

Some demountable systems can be adapted for use in a variety of situations such as above tiered seating as well as over large flat surface areas. Some can be placed on a variety of ground surface conditions such as sand, grass and pavement.

Demountable systems may be designed in modular forms that can be extended or contracted depending on the circumstances. This may consist of a lightweight framework and fabric infills which provide overhead cover, as well as walls where they are required. Alternatively, they may comprise tent-like forms, such as large marquees or lightweight tension membrane structures.

The demountable system should be easy to erect and dismantle. Complex assemblies are time-consuming and increase the risk of incorrect installation. For example, tension membrane structures are quick to erect because the canopy is in one piece. Structures using conventional materials usually take longer to erect because there are more components and fixings.

Demountable systems need to be strong enough to withstand the wear resulting from frequent transportation, assembly and dismantling. The availability of suitable storage facilities is essential to maintain the product in good condition. Their temporary nature means that they are less likely to be subject to vandalism.

Adjustable systems

Adjustable systems range from very simple devices to those which use sophisticated technology. They offer a high degree of flexibility, allowing protection levels to be modified according to the time of day or season, and to satisfy a variety of user needs. They can be either permanent or demountable.

Adjustable shade systems should be easy and convenient to operate. For example, if operation is time-consuming, difficult or requires specialist attention, use of a device may be discouraged. Systems should also be easy to operate in storms when prompt dismantling of the structure may be necessary

Components such as pulleys and cables should be corrosion resistant; use of stainless steel is recommended where possible.

Adjustable shade systems are usually attached to a permanent structure and fall broadly into the following two types:

• retractable devices such as canvas awnings

• louvred devices

retractable devices

These can cover large areas and can, in some cases, offer rain as well as UVR protection. The most common of these is of the fabric ‘roll-out’ type. One simple form is the fabric awning, cantilevered or supported by a pergola-like frame. Fabric awnings can also be supported on folding or telescopic arms, which allow adjustments to achieve the required vertical shadow angle. These devices are available in manually operated or motorised form.

Where fabric canopies are tensioned on extension, it is important that they are as taut as possible. This is because movement and flapping of the canopy will reduce the life span of the device. The canopy should be able to be tightened and adjusted as the fabric stretches over time.

It is also important to ensure that the canopy can be adequately stored when retracted. Folding back into a well-ventilated box will extend the life span of the device and prevent accidental damage.

There are positive and negative environmental impacts associated with retractable devices. The negative impacts include those resulting from the manufacture and disposal of plastic and other synthetic materials commonly used in canopy devices. On the other hand, only a small volume of material, and therefore less embodied energy, is required to produce the canopies.

louvred devices

A range of manually adjusted louvre roof systems are commercially available. These systems offer a high degree of control and flexibility for shading of outdoor areas. They can be adjusted to totally exclude the sun’s rays or to create partial shade. Most louvre designs have the added advantage of providing protection against rain.

The louvre blades are usually an aerofoil design, fabricated in pre-painted steel or aluminium. Other materials can also be used for the louvre blades. For example, corrugated polycarbonate sheet could be used in situations where UVR protection and ventilation are required, without excluding warmth and light. Louvres of expanded metal mesh could be used where waterproofing is not a priority

If the louvres are placed vertically, the louvre wall can be an effective wind deflector, or wind gatherer, in addition to providing protection against direct and indirect UVR.

Also available in the marketplace are louvred roofs which open and close automatically in response to sensors triggered by wind, rain and solar radiation.