Polycarbonate Roofing: Where It Works and Where It Does Not

Polycarbonate sheet is a roofing material for the places where a solid roof is more than the job needs and no cover at all leaves the space unusable. It is commonly used over pergolas, verandahs, carports and walkways, and it is chosen for three practical reasons: it is light, it admits daylight, and it does not demand the structural support that tile or concrete requires. It also carries two conditions that decide whether it suits a particular job. The first is how much heat builds beneath it under an Australian summer sun. The second is how much noise it makes when rain falls on it. Both are manageable. Both are cheaper to understand before the sheets are ordered than after the frame is built.

What polycarbonate sheet is

Polycarbonate is a thermoplastic. For roofing it is supplied as rigid sheets in several constructions, and the differences between them matter more than the brand on the packaging.

Twin-wall sheet is two layers joined by internal ribs, with air channels running between them. Multi-wall sheet repeats that structure through several layers, which makes it stiffer and slower to pass heat through, at the cost of some clarity. Solid sheet is a single layer and behaves differently again: it is clearer, heavier and more expensive for the same area. Sheets intended for outdoor work carry a UV-resistant face, and that face is installed upwards. The supplier identifies which side the coating is on, and the manufacturer’s instructions should be followed on that point without exception.

Relative to glass of comparable thickness, polycarbonate withstands impact far better, which is why it is common where hail or falling branches are a realistic concern. It is also lighter, which is what allows a domestic timber frame to carry it.

Where it works

A covered outdoor area is the natural use. A pergola over a courtyard, a verandah along a wall, a carport, a walkway between the house and the shed, and a shelter over a barbecue or a pool pump all suit a material that lets light through and can be fixed to a simple frame.

Three conditions decide the details. The first is span. The wider the unsupported distance, the thicker and stiffer the sheet must be, and the closer the support timbers or purlins must sit. The second is pitch. Water must be able to run off, and the fixing method changes with the slope. The third is attachment to the house. A structure fixed to a dwelling, or built close to a boundary, can require approval depending on its size, its setback and the rules that apply where the property sits. Councils rely on plans prepared by a licensed surveyor for matters of boundary and siting, and what such a plan certifies, and what it does not, is set out in our guide to council and building compliance.

Where it does not

There are jobs for which polycarbonate is the wrong answer, and it is better to know them at the start.

As the primary roof over a habitable room, sheet plastic is rarely the first choice. A metal roof or tiles insulate and muffle sound more effectively, and they hold their appearance over a longer service life. A clear sheet also lets in light that a bedroom or living room may not want, and it will show dirt and pollen in a way that an opaque roof does not.

Long spans without intermediate support are a second poor fit. The material is light, not rigid, and it flexes. Where the span is long, the frame has to do more work, and at some point metal sheet becomes the simpler proposition. What the profiles are, and what to check before buying them, is covered in the guide to metal roofing options.

A third case is the position directly beneath a bedroom window, where the noise question below decides the sheet choice.

The heat question

Clear polycarbonate admits daylight, and daylight carries heat. In full summer sun, the space beneath a clear sheet can become uncomfortable in the middle of the day, and the effect is strongest where the covered area is enclosed on two or more sides and the air cannot move through it.

There are three practical responses, and they are usually combined. A tinted sheet, in bronze, grey or opal, cuts both glare and heat, at the cost of some light. A heat-reflective sheet is made to reject more of the sun’s heat while keeping more light than a dark tint. And ventilation, whether open sides, a ridge gap or a fan, does as much as either sheet choice, because trapped air is what turns a warm space into an unbearable one.

The choice between them is a matter of what the space is for. A carport can afford a darker tint. A sitting area needs the light tested in the afternoon, not chosen from a sample under a shop’s lighting.

The noise question

Rain on any roof makes noise. On polycarbonate it is louder than on tile, and the frame underneath changes how loud it seems. A metal frame transmits vibration more readily than a timber one, so the same sheet over the same span can sound noticeably different depending on what holds it up.

Two adjustments reduce the problem. A thicker multi-wall sheet is quieter than a thin twin-wall sheet, because the extra layers and air channels absorb more of the sound. A timber frame, or an insulating layer between sheet and frame, removes some of the drumming as well. Where the covered area sits beside a bedroom, the sheet choice and the frame choice deserve to be made together, and with the noise in mind rather than as an afterthought.

Choosing between the sheet types

The decision comes down to what the space needs from the roof.

Twin-wall sheet suits a carport, a walkway or any cover where light is welcome and the span is modest. It is the lightest and usually the least expensive of the roof-grade options, and it does the job where the space below is not somewhere people sit for long periods.

Multi-wall sheet suits a sitting area, or any position where the noise matters or the span is longer. It is stiffer for the same thickness, quieter in rain and better at slowing heat transfer, and it costs more and passes less light. Where a supplier stocks both, the two samples side by side, held up against the sun, answer more questions than a specification sheet does.

Solid sheet is the third option and the least common over a whole roof, because it is heavier and more expensive. It earns its place in smaller positions, such as a highlight strip within another roof, where clarity is the point.

Fixings and movement

Polycarbonate expands and contracts with temperature more than the frame it is fixed to, and the fixing method must allow for that movement. The practical points are standard across manufacturers: holes drilled slightly larger than the screw, washers that cushion the sheet, screws set so the sheet can move without being held rigid, and no over-tightening at any point. The manufacturer’s fixing instructions exist for this reason and should be followed as written.

The frame deserves the same attention. Purlins or battens at the spacing the sheet manufacturer specifies, timber that is dry and straight, and a fall that takes water to a gutter or clear of the structure. Most complaints about a plastic roof trace back to the frame or the fixings rather than the sheet.

What to check before you buy

• Confirm the sheet thickness and profile suit the span and the fixing centres, using the manufacturer’s span table rather than an estimate.
• Confirm which face carries the UV protection and that it will be installed upwards.
• Decide the tint with the afternoon sun in mind, and ask to see the sheet against the light.
• Ask what the warranty covers, particularly yellowing and hail, and what it requires of the installation.
• Check the frame material and the fixing method together, since the two decide how loud the roof will be in rain.
• Establish whether approval is needed for a structure attached to the house or close to a boundary before any work starts.
• Plan how the sheets will be cleaned, because dust, pollen and leaf litter all show on a translucent roof.

Polycarbonate sheet is usually bought through a roofing supplier such as Singh Roofing Supplies rather than a general hardware store, and it is worth taking the frame measurements with you when you go.

What it comes down to

Polycarbonate roofing is a good answer to a specific question: how does a light, open, covered outdoor space get built without the weight and cost of a tiled or metal roof? It is a poor answer to a different question, which is how a habitable room gets a permanent roof. Within the first question, the sheet type, the tint and the frame are the three decisions that decide the result, while heat and noise are the conditions that determine whether the space will be pleasant or merely covered. A buyer who settles those five matters before ordering has the job under control.

Sources: Wikipedia, “Twinwall plastic” (en.wikipedia.org/wiki/Twinwall_plastic); Wikipedia, “Polycarbonate” (en.wikipedia.org/wiki/Polycarbonate).