Any structure can be built badly. What is useful about a tensile structure is that its failures are specific and repetitive - the same handful of things go wrong, for the same reasons - which means a buyer who knows the list can ask about them before signing rather than discovering them in the fourth monsoon.
None of these is a reason not to build one. They are the reasons a good one costs what it costs.
Ponding: the one that compounds
Water that cannot run off collects. On a membrane that is more serious than on a rigid roof, because the fabric deflects under the weight of the water it is holding - which creates a slightly deeper pool, which is slightly heavier, which deflects it further.
Left alone through a heavy monsoon, that loop is how a structure ends up carrying a load nobody designed it for. The consequences range from a permanently stretched panel to a torn one.
It is prevented entirely at design stage, by giving the surface enough curvature and fall that there is nowhere flat for water to sit, and by putting the outlets where the water actually goes rather than where they are convenient. It is the first thing to look for on a drawing: if you cannot see where the water leaves, neither can the water.
A pool that forms makes room for a bigger pool. That is the whole problem in one sentence.
Slack fabric, and why it wears itself out
A membrane that is not properly tensioned moves in the wind. Not dramatically - a ripple, a flutter at an edge - but constantly, and that constant working is abrasive. It wears the coating where the fabric passes over a fixing, fatigues the welded seams, and concentrates stress at corners.
Two things cause it. The first is a surface without enough double curvature, which was covered in why a tensile roof is held down: a flat-ish membrane has no way to stay taut. The second is relaxation - coated fabrics stretch slightly under sustained tension over the first year or two.
Relaxation is normal and expected. It only becomes a fault if there is no way to correct it. A structure detailed with turnbuckles and adjustable fittings can be brought back up to its design pre-stress in an afternoon. One where the membrane was fixed solid cannot be adjusted at all.
Worth asking directly: how is this structure re-tensioned, and by whom? An answer that involves specific fittings is a good sign. An answer of 'it will not need it' is not.
Corrosion, and where it actually starts
Hot-dip galvanising is a good protection system and it lasts for decades. What it protects is the surface it covers.
The weak points are everywhere that surface was broken after the steel left the galvanising bath: a hole drilled on site because a bolt did not line up, a bracket welded in place during erection, a cut end, a bolt hole reamed out to make something fit. Bare steel at any of those points will start to rust, and because they are usually at connections, it is rusting at exactly the place carrying the load.
Good practice is to do as little cutting and drilling on site as possible - which is why the steel is fabricated, drilled and galvanised in the workshop - and to treat any damage that does occur. On an existing structure, the connections are the first place to look and the base plates are the second, because that is where water collects.
Foundations that were never sized for uplift
This is the serious one, and the least visible.
A tensile roof is held down rather than held up, so the foundations are resisting a pull upward, not a push downward. Sizing them requires the design wind pressure for the site and an assumption about the soil. Skip that and you get a footing that looks adequate - concrete in the ground, column standing straight - and is not.
Nothing about it will be apparent for years. It will be apparent once, during a storm.
There is no way to inspect this after the fact without breaking out the concrete, which is precisely why it has to be a document rather than a reassurance. The foundation design, the wind speed it was based on and the soil assumption should all be in the handover pack.
Drainage that stops at the roof edge
A surprising number of problems are not about the structure at all. The membrane sheds the water correctly, into a gutter, and then the design stops thinking.
The water has to go somewhere. Discharged at the low edge of a car park canopy it lands in the drive aisle, exactly where people walk between the cars. Discharged off a home canopy it may land on a neighbour's side. Fed into a gutter with no fall it sits there and breeds.
- Where does the water leave the membrane?
- Where does it go after that - free edge, valley gutter, or downpipe through a column?
- Where does the downpipe discharge, and is that a place you are happy standing?
- Who clears the outlets, and how do they reach them?
Four questions, and they resolve most of what people complain about two years later.
The wrong material for the site
Plain PVC-coated polyester is the workhorse and it is the right answer for most commercial canopies. It is the wrong answer on a heavily polluted arterial road or a coastal site, where it will look grey and tired long before it is structurally finished.
That is not a defect. It is a specification choice, and the alternatives - a PVDF lacquer, or PTFE-coated glass - exist precisely for those conditions. The failure is choosing on rate alone and being surprised by the appearance five years later. The material guide sets out what each one is for.
And the one that is genuinely just neglect
Tensile structures need very little maintenance, which is not the same as none. Wash them periodically. Look at the fixings once a year. Clear the gutters and outlets before the monsoon, not during it. Treat any place the galvanising has been damaged.
A structure that has had none of that for ten years will look far worse than its age, and some of that will be permanent. It is the cheapest item on this entire page and the most commonly skipped.
What to do with this list
Use it as questions, not as a reason for doubt. Ask where the water goes. Ask how it is re-tensioned. Ask for the foundation design and the wind speed behind it. Ask what happens to steel that gets drilled on site.
A manufacturer who has thought about these will answer quickly and specifically, because the answers are just how they build. One who has not will answer in adjectives.