08/07/2020
Tensile Fabric Roofing
Pros and Cons of Fabric Structures
While tensile structures offer great opportunities for covered areas, there are of course other construction methods by which a covered outdoor area can be achieved.
Good light transmissionThis brief articles aims to highlight the benefits of tensile fabric structures over other types of cover in an unbiased but practical manner. There will also be situations where alternative means to cover the area may be better, and we aim to bring these to your attention too.
One of the greatest benefits of tensile structures is that they are translucent. The woven basecloth combined with the appropriate coating allows a light transmission value of around 10%. This provides a very comfortable level of illumination compared to the full brightness of outside.
Glass and polycarbonate are also translucent, but the light levels under the canopy are going to remain quite similar to those outside, so on very bright days, these materials offer little respite.
The other factor that clearly puts fabric ahead of other materials is its clear span capabilities. Whereas all other possible roofing materials require rigid intermediate support, fabric can span from one boundary right across to the opposite side in one unbroken sweep. The advantages of this are that there are no awkward or difficult sealing details that need to be addressed.
Since the fabric has this amazing tensile capacity, the effect is to reduce the supporting framework to a minimum number of components, all working efficiently as a whole system. This in turn leads to a much more lightweight structure than other types of construction. A downside of this is that large foundations are often required in order to prevent the wind lifting the canopy and taking it away. However, in terms of cost, foundations are a lot easier and cheaper to prepare than visible, above ground constructions that are exposed to weather and damage.
A tensile canopy is a great solution for an entrance canopy. It acts both as a shelter for those leaving and entering the building and makes the entrance area clearly visible from a distance.
Focus on entranceThe 3d nature of most canopy structures is quite unique as a building form, so it quickly forms a visual focus of attention. If you use a more traditional structure, such as a tiled roof, or a flat glass or polycarbonate canopy, these are unlikely to stand out so clearly. In all potential canopy locations, you have to take into account the type of structure you wish to use. If you are connecting to a building, you need to know that there is adequate supporting structure to accommodate the expected loadings.
If there is no suitable structure within the existing building, it would be difficult to make any meaningful connections to that building for a tensile canopy, but sufficient anchorage might be achieved for a rigid type of construction. That said, it is nearly always possible to design the canopy in such a way that it imposes identical loads onto a building as a rigid canopy structure would, but in these cases, the cost of the additional engineering, steel fabrication and installation difficulties is likely to make it an uneconomical solution.
Another situation where a tensile canopy may not offer the best solution in terms of cost is where the fabric structure, although bespoke, is very small. It would still require that all connections are engineered and checked, it would still require for patterning to be carried out, the steel fabricator would still need to fabricate awkward angles and special corner plates, so the overall cost per square metre could be much higher than if a fixed structure was used instead. Essentially, fabric structures offer great economies of scale – small structures can cost three or four times more per square metre than larger structures. While this is true to a degree with all construction types, we believe the effect is much more extreme with tensile structures.
Fabric structures, if properly engineered and installed are virtually immune to damage and weathering. Providing that they are not likely to be susceptible to acts of vandalism or accidental damage from passing vehicles or falling objects, they provide a reassuringly simple and durable solution.
It’s also reassuring for the client that for all the bespoke structures we install, we supply full calculations to justify every component of the system, which means we’re not relying on a generic specification to cover all eventualities.
An often overlooked advantage of fabric structures is that all of the manufacturing takes place off site, in specialist membrane fabrication units and steel workshops. When the membrane, the steelwork, cables and all other associated parts are complete and taken to site, the actual site work can be completed very swiftly and with little disruption to other trades or the public.
There can’t be any other ways to cover a large piece of land as quickly as with a tensile structure. All other construction methods involve a lot more costly work on site, and often cause considerable disruption. If you think about it, with a fabric structure, it’s quite normal that the area to be covered is only “out of bounds” for a short while just prior to lifting the fabric and the bulk of the installation work will take place at the perimeter, out of most people’s way. Compare this with a rigid structure that needs the construction work to be carried out exactly where you don’t want people working and obstructing the route.
All roofing materials will of course collect dirt and debris on the surface, whether glass, polycarbonate, fabric or an opaque material. With glass or polycarbonate, the dirt can be clearly seen, however the light diffusing properties of the fabric mean that the surface dirt is not easily identified. It’s quite surprising how good a membrane can look from below, and then when you actually look at the top surface you find there’s quite a build up of dirt.
Coated fabrics have very good UV absorbing and reflection properties. That means it offers safe shade to all users, whilst providing very comfortable light levels.
Bright lightsThe unique properties of light reflectance and transmission also offer exciting possibilities for lighting after dark. Directing lights under the canopy to reflect off the underside is a great way to use uplighters, but more even lighting can be achieved under the fabric by shining lights down on the fabric from above.
Finally, one biased opinion about fabric structures – they’re beautiful to look at, they’re great fun to design, exciting to install, and a delight to own. And they sound great in the rain!
