Tension in a shading system is a complex balancing act. It’s both critical to the stability of the structure and also a significant source of risk when mishandled. Too little tension, and the structure risks damage from wind or pooling water. Too much, and the attachment points may break, supporting structures may bend, and the fibers of the rope or shade cloth can degrade, creep, or create injury risks in the case of a break.
Here are a few things you’ve got to know in order to walk the fine line between helpful and harmful tension in shading systems:
1. Every Attachment Point Needs Tension
Any loose attachments will lead to billowing and sagging in the fabric of the shading system in at least one dimension, which causes all sorts of trouble.
It’s easy to presume that loose attachments would put less stress on the ropes and fabric of the structure (ensuring its longevity). However, the loose fabric can bubble and catch wind like a parachute. This creates a risk of sudden shock loads on your sail shade hardware when strong gusts slam into the cloth. Such forces may exceed the intended load of the system’s hardware, ropes, or fabric, and cause it to snap, pull loose, or rip.
Shade sail sagging from low tension is also undesirable in calm conditions because rainwater on an outdoor shading system can collect and put far more weight on the fabric structure than it can safely support.
2. Some Stretch is Desirable—But Creep Is Not
The inherent stretch of knitted fabric helps shading systems create smooth, consistent surfaces without baggy or wrinkled areas. A small amount of stretch in ropes fastened to the fabric’s corners can serve a similar purpose. Without any stretch in the soft goods, you’d have a stable system, but it would be difficult to perfectly balance the tension on all attachment points or create helpful curvatures or 3D shapes in the shade (discussed in #3, below).
But with too much stretch, the structure would face the same issues as a low-tension system. Wind and water could pick up the slack in the stretchy fabric or rope and cause it to sag or billow in strong winds.
Another factor to consider is rope creep, defined as an unrecoverable stretch of the rope. High tension in shading systems exerts a constant pulling force on the soft goods. Over time, this force can cause permanent stretch that will gradually loosen the ropes and fabrics, requiring re-tensioning or replacement. The most successful designs will use forgiving but low-stretch ropes that have a minimal tendency to creep under tension, such as Dyneema® Ocean 5000.
3. Tension Should Create Hypar
“Hypar”—a term derived from “Hyperbolic Paraboloid”—refers to the saddle-shaped surface desirable in shading systems. Some level of twist or 3D shape is good for the shade (created by alternating high and low corners), because it increases tension, reduces flapping, and prevents water from pooling on the shade.
(Source: Formfinder - Typology)
Triangle shades always remain flat—they can’t use hypar. This means they’re more prone to flapping and holding water. At least four tensioned attachment points are best for the longevity of shading systems.
4. Select the Appropriate Attachment Hardware
Shading systems are typically tensioned one of three ways: via stainless steel turnbuckles, a shade sail pulley system, or “quick release” snap hooks affixed to the corners of the fabric.
- Turnbuckles are ideal for tensioning permanent shading systems because they generally allow the application of greater and more precise tension.
- Pulley systems are a more practical choice for temporary systems since they’ll allow for the shade to be erected or dismantled more safely and easily.
- Snap hooks on the corners may be an advantageous choice for domestic or consumer-grade shade sails because it can be taken down quickly in high winds or winter conditions.
Whichever hardware you’re using, be sure to use mounting points that are rated to absorb both the tension required to make the fabric taut and also the shock loads caused by strong gusts of wind.