BRANZ sheds light on some key considerations when adding shade structures to your clients’ property
Tensile fabric roof structures and sun shades must be specifically designed by the manufacturer for the particular application, the type of fabric proposed, the loads acting on it and the means of supporting or securing it.
A true tensile fabric structure is defined as a structure where all of the fabric is in tension. To do this, the fabric must curve equally in opposite (vertical) directions to provide three-dimensional stability – the simplest form being a hyperbolic paraboloid. The most efficient proportion of horizontal span to vertical height is 4:1 – as the shape of a tensile structure becomes flatter or more irregular, the greater the loads that will need to be applied during erection to maintain the shape.
The supporting structures are usually constructed from aluminium or steel and can be finished in a range of options, such as galvanising, powder coating or painting. Tensile structures can also be designed to be supported in place by steel cables anchored to existing buildings or specifically erected poles or columns.
The final choice of fabric will depend on colour preference, environmental conditions, location, life expectancy, physical qualities, UV protection, shading, construction and translucency.
The main options are:
PVC-coated polyester – comparatively inexpensive, available in a range of colours and typically with a life of 15–20 years. Fabric is available in waterproof and perforated grades.
PVC-coated glass fibre – typically 50% more expensive than PVC-coated polyester but with higher tensile strengths and a longer life.
Silicon-coated glass fibre, which tends to be self-cleaning.
PTFE (polytetrafluoroethylene or Teflon) coated glass fibre – tends to be self-cleaning and is only available in white.
HDPE (high-density polyethylene) shade cloth – a fabric knitted filament that is permeable to light, wind and fluids and relatively inexpensive. Shade protection ranges from 50% to over 95% as a result of the direct heat from the sun being reflected or absorbed by the material and dissipated up through the fabric, keeping the area underneath cool. UV stabilisers are added to provide protection against UV degradation, with the typical serviceable life being from 5–10 years. When using HDPE:
Avoid exposure to chlorine and other corrosive chemicals or gases.
Ensure it is kept taut to prevent sagging when it rains – HDPE stretches more than other fabrics.
Keep a separation between the fabric and plants or structure, because the fabric will deteriorate where abrasion occurs
Have a maintenance programme in place to ensure seams remain in good condition. Seams typically require resewing once in the serviceable life of the structure.
Natural canvas – used for small spans where texture is important. However, it is less stable and durable than synthetics and is difficult to clean.
Modified acrylic canvas – similar in texture and maximum span to natural canvas, but more stable and can be treated with a fire retardant.
All these fabrics are inelastic, so that they do not distort under wind and snow loads. Complex shapes must be created by cutting and sewing, or specialised welding, as the fabrics are not able to be tensioned to the desired shape.
Shade cloth sails are likely to sag in the middle when they are installed with a low pitch and they will shed rainwater efficiently. All shade sails should have a cross fall of at least 1 in 6, so that water is shed when it rains. Large shade sails need catenary cables to stop the sail sagging excessively under its own weight and to provide for adjustment when the materials stretch with age.
The design of textile and shade structures requires particular attention to the detailing of:
Connections to posts or buildings to ensure they (and the restraining structure) will cope with the design wind and snow loads and meet durability requirements.
Seams in the fabric.
Resistance to uplift.
The three-dimensional curvature in the fabric.
The span of the structure – typically spans greater than 15m require specially reinforced fabrics
Also important are:
The durability of the fabric – the expected life of cloth is 8–12 years, but it will depend on local conditions.
Thermal expansion and contraction in the fabric.
Fabric strength and loss of strength with ageing.
The level of resistance to water penetration – not all fabrics are waterproof.
For shade structures, consider:
The level of UV protection provided by the fabric chosen.
The amount of shading provided.
The colour, as this can influence both the amount of shade and the UV protection provided – darker colours tend to give higher levels of protection.
This article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.
The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at www.branz.co.nz.