Key takeaways
- Polycarbonate expands and contracts far more than steel or aluminium framing, so a fixing that holds steel sheeting firmly will crack a polycarbonate sheet.
- Fixing holes must be oversize, with the far fixings slotted in the direction of expansion, and the manufacturer’s allowance is not a matter of judgement on the day.
- Each screw should be driven only until its sealing washer seats, because a screw driven down hard creates a stress point and bulges the sheet.
- The coated face must be the one exposed to the sun, and it should be checked before fixing and again before the roof is closed.
- Translucent sheeting in a bushfire-prone area must meet the requirements that apply to the site, and that position should be confirmed in writing before it is specified.
Translucent polycarbonate roof sheeting is one of the most useful materials a residential builder can specify. It lets light into a carport, a verandah, a pool enclosure or a covered walkway without the glare and heat of a clear sheet, and it is light enough that the existing frame often needs little strengthening. It is also one of the easiest materials to install badly, because the fault does not show on the day. It shows a year later, in a cracked sheet, a leaking fixing or a bulged panel that nobody can explain.
The reason is movement. Polycarbonate expands and contracts with temperature far more than the steel or aluminium framing it sits on, and it does so in a sheet that is large, thin and fixed at many points. A fixing that is correct for steel sheeting can crack a polycarbonate sheet within a season. The installation rules for this material are not general roof sheeting rules with a different colour. They are a set of decisions about allowing movement, sealing the holes and protecting the surface from the sun.
This article covers expansion and fixing for polycarbonate roof sheeting on residential work: the fixing-hole allowance, the choice of washers, ultraviolet protection, support and laps, and the fire question in bushfire-prone areas. It does not cover general sheet metal roofing, which is a different job with a different set of fixing decisions. Always follow the manufacturer’s installation instructions for the product supplied, because the figures they give are the ones the warranty depends on.
Why polycarbonate moves more than the frame beneath it
Every material expands when it warms and contracts when it cools, but the amount varies widely. Polycarbonate has a much higher rate of thermal expansion than the metals most roof frames are made from. On a clear day a sheet can be noticeably warmer than the purlins beneath it, and a long run that is fixed at every point has nowhere to go. The stress has to be taken somewhere, and if the fixings do not let the sheet move, the sheet takes it, usually at a hole.
The movement is also directional. A sheet fixed at a single point will move freely in all directions from that point. A sheet fixed at many points along a run will be held at each one, so it bows and buckles between them. The longer the run and the more points it is fixed to, the more the sheet is constrained, and the more important it is that the fixings at the far end can slide as the sheet grows.
The builder’s job is therefore not to hold the sheet firmly in place. It is to hold the sheet down against wind uplift and rain while allowing it to move in the direction the manufacturer specifies. That is a different discipline from fixing steel, and it is the reason experienced installers talk about fixing as a choice rather than an action.
Fixing-hole allowance: the hole is bigger than the fastener
The most common expansion failure is the simplest. The fixing hole is drilled to the same diameter as the screw shank, so the sheet is held at the hole with no room to move. Polycarbonate fixing holes are drilled oversize, and the allowance is the difference between the hole and the fastener. The manufacturer specifies that allowance for the product, and it is not a matter of judgement on the day. A hole drilled to the wrong size, or drilled tight to save a fixing, is the origin of the cracked sheet that appears in the second summer.
Oversize holes are commonly slotted in the direction of expected movement at the far fixings, and the manufacturer’s guidance will say which fixings are fixed and which are free to slide. Central fixings usually hold the sheet in place. Fixings away from the centre allow it to grow. Mixing these up, so that the wrong holes are fixed hard, undoes the whole allowance, and it is the sort of mistake that is invisible until the sheet cracks.
Drill the holes with a sharp bit at the speed the manufacturer recommends. A blunt bit or a fast drill can melt the edge of the hole, and a melted edge gives the washer nothing to seat against and can start a crack. Check the first few holes for size and shape before drilling the rest of the sheet, and keep the drilled sheets clear of swarf that could scratch the surface.
Choosing the washer and deciding how tight is tight enough
Polycarbonate roof fixings use a sealing washer under the head of the screw, usually a bonded washer with a flexible seal that spreads the load and keeps water out of the hole. The washer is part of the expansion system, not just a seal. It has to be the type the manufacturer specifies for the sheet, and the seal has to be sound and undamaged. A washer that has perished, or one that has been replaced with a generic hardware item, can leak at the hole and fail to protect the sheet.
The tightening is where many installers go wrong. A screw driven hard enough to crush the washer and bulge the sheet around the head has created a stress point. The seal should be compressed enough to seal, and no more. The usual approach is to drive the screw until the washer just seats and the seal makes contact, then stop. The feel is learned rather than measured, so the first few fixings on each job should be checked by the lead installer before the crew works through the rest.
Use the fastener material the manufacturer specifies. Stainless or correctly coated fixings are the usual choice, and mixing metals on the same roof invites corrosion at the fixing. The screw length should allow the full thread to engage the purlin without projecting through the bottom of the purlin, and it should be fitted square to the sheet so the washer seats evenly. Screw selection for metal sheeting is a different job, covered in our article on roof sheeting fixing screw selection, and it should not be carried across to translucent sheets without checking.
Do not drive every screw hard. The mistake that costs the most is driving every fixing down until the sheet is held rigidly at every point, because it looks secure. The sheet has nowhere to expand, so it cracks or bulges around the head of the fixings, and the cost of the replacement and the access is paid by the builder, usually after the warranty period has been argued over.
Fixing polycarbonate against fixing steel sheeting
| Steel or aluminium sheeting | Polycarbonate sheeting |
|---|---|
| Expansion is small and the fixings hold the sheet firmly | Expansion is large, so the fixings must allow the sheet to move |
| Fixing holes are drilled to the size of the fastener | Fixing holes are drilled oversize, and the far fixings are slotted |
| Screws are driven down until the sheet is held flat | Screws are driven only until the sealing washer seats |
| Exposure to the sun does not change the face that goes up | The coated face must be the one exposed to the sun |
Ultraviolet protection and getting the exposed face right
Untreated polycarbonate yellows and becomes brittle when exposed to ultraviolet light. Most roofing-grade sheets are supplied with a protective coating on one face, which is the face that must be exposed to the sun. The coating is usually identified by a marking on the sheet, and the marking is the instruction. Installing the sheet the wrong way up puts the unprotected face to the weather, and the sheet begins to degrade long before its expected life.
Check the marking before the sheet leaves the pallet, and again before it is fixed. A sheet can be lifted, turned and laid back in the wrong orientation without anyone noticing, particularly on a long run where the sheets are handled in sequence. Mark the exposed face with a light tape on the first sheet so the crew can see it, and remove the tape only after the roof is complete and checked.
Cut edges are a further risk. A cut edge is not protected by the coating, and water and light can get into it. Seal the cut ends with the closure or the edge strip the manufacturer specifies, and do not leave bare ends exposed to the weather where the roof is open.
Purlin support, spans and the structure below
Polycarbonate is light, but the structure under it still has to carry the sheet, the wind and any load the owner puts on the roof. The purlin spacing, the sizes and the fixing of the purlins should follow the manufacturer’s span table for the sheet and the design wind load for the site, and the certifier will expect to see that the frame has been designed for the loads the roof will carry. A frame that was adequate for a steel roof may not be adequate for a polycarbonate one if the sheet is supported at spans it was never designed for.
Check the whole load path, not only the sheet. Roof trusses that are not braced or tied down correctly will move under wind, and a sheet fixed to a moving frame will crack no matter how well it was installed. Our article on roof truss bracing and tie-down checks covers that structure in more detail, and it is worth reading before the sheeting goes on.
The purlins must also be set out so that each sheet is supported at the points the manufacturer requires, with the laps falling on a purlin. A lap that falls between purlins is unsupported, and it is a place where the sheet can sag and collect water. Set out the purlins before the first sheet is ordered, and check the set-out against the sheet length.
Laps, edges, closures and drainage
Sheets are laid with side and end laps, and the manufacturer specifies both. The side lap is the overlap between adjacent sheets, and the end lap is the overlap where one sheet meets the next along the run. Both must be the amount specified for the product, and both must be sealed as the manufacturer directs. A lap that is too short lets water in. A lap that is over-tightened can crush the profile and trap moisture.
Edge and eave details decide whether the roof drains properly. Closures at the ridge, eave and gable ends keep insects, dust and wind-driven water out of the profile, and they must be fitted to the profile of the sheet rather than cut to fit. Gutters must be sized and fall to the outlets so that water leaves the roof rather than pooling on it. A sheet that holds water over a long period is under additional load, and it is more likely to discolour and to show fixing failures.
Allow for the movement at the edges too. A flashing or a trim that is fixed rigidly to the sheet at each end will restrict the expansion just as a tight fixing would. Use the sliding or free-fixing detail the manufacturer shows for the edge.
Bushfire-prone areas and the fire question
Where the site is in a bushfire-prone area, the roof material and any translucent sheeting must meet the requirements that apply to the site, and polycarbonate is not a material that can be assumed to meet them. The NSW Rural Fire Service guidance on planning for bush fire protection sets out the principles for building in these areas, and the certifier will check the materials against the requirements that apply to the site’s classification. Confirm the current requirement before specifying the sheeting, and get the product’s fire performance documentation in writing.
Translucent sheeting is also a vulnerable point on any roof, because it can ignite or deform in a fire more readily than the surrounding material. Even where the sheet is permitted, the details around it, including the gutters, the eaves and the gaps at the ridge, should be checked for ember entry. The principle is simple. The roof has to protect the building from embers, and a material that does not do so cannot be used where that risk exists.
Worked example: a covered walkway for a family home in western Sydney
A builder in western Sydney was asked to cover a long side walkway between a house and a garage with translucent sheeting, so that the family could move between the two in the rain without losing the light. The run was long, the frame was an existing one that had been built for a lighter covering, and the client wanted the sheets fixed tight so there would be no gaps for leaves.
The builder checked the frame against the manufacturer’s span table and found that the purlins needed to be closer together for the new sheet. The additional purlins were added and the set-out was checked before any sheet was ordered. The lead installer drilled the holes to the oversize the manufacturer specifies, slotted the far fixings in the direction of expansion, and fixed the central points hard. The crew were shown the seal on the washer and told to stop as soon as it made contact.
The client was unhappy at first, because the sheets looked loose when they were first fixed. The builder explained the expansion, and the client agreed to a short inspection after the first warm spell. The sheets had moved as expected, with no cracks and no bulges around the heads of the fixings. The builder recorded the fixing pattern and the set-out on the job file, and gave the client a note explaining the expansion allowance and the cleaning and inspection the roof would need.
Installation sequence for a run of polycarbonate sheeting
The installation follows a sequence that is the same on most jobs. Check the frame and the purlin set-out first, because any error in the frame is expensive to fix once the sheets are on. Mark the exposed face of each sheet, drill the holes to the oversize specified, and fix the sheets in the order the manufacturer shows, working from the centre of the run to the ends. Seal the edges and closures, and check the whole roof against the sequence before you leave the site.
Installing a run of polycarbonate sheeting
- 1Check the frame and the set-outConfirm the purlin spacing and support against the manufacturer’s span guidance before any sheet is fixed.
- 2Mark the exposed faceIdentify the coated face on each sheet so it goes up, and keep the marking until the roof is checked.
- 3Drill and slot the holesUse the oversize the manufacturer specifies and slot the far fixings in the direction of expansion.
- 4Fix from the centre outwardsFix the sheets in the order the manufacturer shows, driving each screw only until the washer seats.
- 5Seal the edges and closuresFit the closures and edge details to the profile and seal the cut ends.
Inspecting the roof after the first warm season
Inspect the roof after the first warm season, and again at intervals the manufacturer recommends. Look for cracks at the fixings, bulges between purlins, discoloured or chalky surfaces, and any water at the laps or closures. Check that the fixings are still seated and that the seals are sound. A small problem found early is a repair, and a large one found late is a replacement, so the inspection is worth scheduling into the handover.
The regulatory and licensing side of residential building work, including the obligations that come with the licence when a structure is altered, is set out in the NSW Government’s guidance on general building work licensing. PointCPD’s guide to how NSW builder CPD works explains how those obligations are kept current, and it is a sound reference for a builder who takes on this kind of work regularly.
Questions builders ask
Why does polycarbonate roof sheeting crack at the fixings?
The sheet expands and contracts with temperature far more than the frame beneath it. If the fixing holes are drilled to the size of the screw, or the screws are driven down hard, the sheet has nowhere to move and it cracks or bulges around the fixing. The manufacturer’s hole allowance and fixing method exist to prevent that.
Which way up should polycarbonate roof sheeting be installed?
With the coated face exposed to the sun, as marked by the manufacturer on the sheet. Installing the sheet the wrong way up exposes the unprotected face to ultraviolet light, and the sheet degrades early. Check the marking before fixing and again before the roof is closed.
How tight should I drive the screws into polycarbonate sheeting?
Drive each screw only until the sealing washer seats and the seal makes contact, and stop there. Crushing the washer or bulging the sheet around the head creates a stress point. The feel is learned, so have the lead installer check the first fixings on the job.
Can I use polycarbonate sheeting in a bushfire-prone area?
Only if the material meets the requirements that apply to the site, which must be confirmed against the current bushfire guidance and the certifier’s requirements. Do not assume the sheeting is acceptable. Get the product’s fire performance documentation in writing and check the details around the sheet for ember entry.
Does the frame need to change when I fit polycarbonate sheeting?
It may. Polycarbonate has its own span limits, so the purlin spacing and support must be checked against the manufacturer’s guidance for the sheet and the design loads for the site. A frame built for a lighter covering may need additional purlins before the new sheet goes on.
What should I check after the first warm season?
Look for cracks at the fixings, bulges between purlins, discoloured or chalky surfaces and any water at the laps or closures. Check the fixings are still seated and the seals are sound. Schedule the inspection into the handover so a small problem is repaired rather than replaced.
Sources
- General building work licensing · NSW Government · 2026-09-03
- Continuing professional development for builders and swimming pool builders · NSW Government
- Home Building Act 1989 (NSW) · NSW Legislation
Published 21 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
