Two profiles dominate outdoor structures in Australia: corrugated and Trimdek. Both are available in COLORBOND steel, both are manufactured to AS 1562.1, and both will outlast timber framing by decades when installed correctly. The choice between them comes down to four practical factors: minimum pitch, span capacity, aesthetic intent, and whether the roof needs to curve.
The Aesthetic Difference
Corrugated sheet has a sinusoidal (rounded wave) profile with a 76 mm pitch and 19 mm depth. It reads as traditional, rural and residential. On a Queenslander verandah, a farm shed or a backyard patio, it fits the vernacular without effort.
Trimdek has a square-ribbed profile with a 200 mm pan width and 29 mm rib height. The geometry is sharper and more contemporary. On a modern skillion carport, a commercial shed or a flat-roofed extension, it looks deliberate rather than default.
Neither is superior. The question is which suits the building it sits on. If the main dwelling has corrugated roofing, matching the patio to it is straightforward. If the home is a contemporary build with clean lines, Trimdek reads as the more considered choice.
Minimum Pitch
This is where the two profiles diverge most sharply for outdoor structures.
Corrugated steel sheet has a manufacturer-recommended minimum pitch of 5 degrees (approximately 1:11 fall). Below that, water pools in the valleys, capillary action draws moisture back under laps, and leaks follow. On a patio or carport where the owner wants a nearly flat roof for visual reasons, 5 degrees can feel steep.
Trimdek has a minimum pitch of 2 degrees (approximately 1:29 fall). The deeper, wider pans shed water at a much shallower angle, and the square ribs create a physical barrier that resists capillary creep at end laps. For a low-pitch skillion carport or a patio attached to a single-storey home with limited head height, that 3-degree difference is often the deciding factor.
Both figures assume correct end-lap length (150 mm minimum for corrugated at 5 degrees, 150 mm for Trimdek at 2 degrees per Lysaght installation data) and properly installed flashings at the high end. Reducing pitch without increasing lap length is a common source of leaks on DIY builds.
Spanning Widely Spaced Supports
Sheds, carports and patios frequently have purlin or rafter spacings of 900 mm to 1,200 mm, sometimes wider on portal-frame sheds. Span capacity depends on both profile depth and base metal thickness (BMT).
Corrugated at 0.42 BMT is typically rated to around 900 mm purlin spacing under standard residential wind loads. Push to 1,200 mm and you need to move to 0.48 BMT to maintain deflection limits and wind uplift resistance.
Trimdek at 0.42 BMT spans further than corrugated at the same thickness, owing to its greater rib height (29 mm vs 19 mm). It is commonly used at 1,200 mm purlin spacing in standard residential applications. For wider spacings or high wind regions, 0.48 BMT is the appropriate specification.
For shed builders working with portal frames where purlins sit at 1,200 mm or beyond, Trimdek at 0.48 BMT is the more common and structurally sound choice. Corrugated at 0.48 BMT will also work, but the span tables favour Trimdek at those spacings.
Always verify against the manufacturer's span tables for your specific wind region and terrain category. AS 1562.1 requires that sheet selection account for both positive (downward) load and negative (uplift) load, and uplift governs on most low-pitch outdoor roofs.
Wind Uplift on Patios and Carports
Open structures are more exposed to wind uplift than enclosed buildings. A carport with no walls, or a patio open on three sides, has no internal pressure to counteract negative external pressure during a wind event. The roof is essentially being pulled upward.
For BAL-rated properties or cyclonic regions, this matters considerably. AS 3959 and the NCC 2025 both influence material and fixing specifications in bushfire-prone areas, and cyclonic wind regions (C and D) require specific fixing patterns and fastener ratings that exceed standard residential practice.
Trimdek's deeper rib and wider pan give it a higher pull-through resistance at the fixing point compared to corrugated, which is relevant when fastener spacing needs to tighten up at eaves and ridges to meet uplift requirements. That said, both profiles perform adequately when fixed to the manufacturer's pattern; the difference becomes meaningful at the margins of the span and wind tables.
Curving for Verandahs
Corrugated sheet can be cold-curved across its width to create a bullnose or barrel-vault verandah profile. This is one of its genuine structural advantages over Trimdek for traditional residential work. A curved corrugated verandah roof is a standard detail on Federation and Queenslander homes, and it requires no special tooling beyond a sheet roller.
Trimdek cannot be cold-curved in the field. Its square ribs will kink or distort under lateral bending. If a curved profile is required on a contemporary build, Spandek or a purpose-made curved product is the appropriate choice, not Trimdek.
For a straight skillion patio or a gable-roofed shed, this distinction is irrelevant. But for a verandah with a traditional curved front, corrugated is the only practical option of the two.
Flashings for Patios and Shed Roofs
A roof sheet alone does not make a weatherproof roof. Flashings at every penetration, edge and junction are what keep water out, and they are the most commonly underspecified part of a DIY outdoor structure.
Ridge and hip flashings are required at the apex of any gable roof. For corrugated, the flashing must conform to the profile at the sheet interface; pre-formed corrugated ridge caps are available and should be used rather than flat sheet bent on site.
Apron flashings are needed where the roof meets a wall, whether that is the house wall on an attached patio or a gable-end wall on a shed. The flashing must lap over the top of the roofing sheet and be sealed at the wall with a compatible sealant. A common failure point is where the apron flashing terminates at the corner without a properly formed return.
Barge flashings seal the raking edges on gable ends. On corrugated, the barge cap presses into the profile; on Trimdek, it sits over the outer rib.
Eave trim at the low edge controls drip direction and prevents wind-driven rain from entering under the sheet. On open carports, this detail is often skipped, which allows water to track back along the underside of the sheet.
All flashings should be in the same steel grade and coating as the roofing sheet. Mixing ZINCALUME flashings with COLORBOND sheets, or vice versa, is not a defect in itself, but using dissimilar metals (for example, copper or bare aluminium in contact with COLORBOND) will cause accelerated corrosion.
Fixings
Corrugated sheet is fixed through the crown of the corrugation using hex-head screws with neoprene washers. The fixing point sits at the high point of the profile, which keeps it largely clear of water flow. Standard fixing is one screw per corrugation at end laps and every second corrugation in the field.
Trimdek is fixed through the pan (the flat area between ribs) using the same hex-head screw format. Fixing through the pan means the fastener sits in the water flow path, which is why the neoprene washer seal is non-negotiable. A worn or overtightened washer is a leak waiting to happen.
For both profiles, use fasteners rated for the coating system. COLORBOND steel requires Class 3 or Class 4 corrosion-rated fasteners depending on the environment. Coastal locations within 1 km of the ocean warrant Class 4 as a minimum.
Gutters and Rainwater
A patio or carport roof concentrates rainfall onto a small catchment area and delivers it to one or two eaves. Undersizing the gutter is a frequent mistake.
For a standard 6 m wide patio in a 150 mm/hour rainfall intensity zone, a 115 mm quad gutter is typically adequate. In higher-intensity zones (tropical north Queensland, for example), 150 mm quad or a fascia gutter may be needed. Downpipe sizing should be calculated to match, not selected by eye.
Gutter brackets for corrugated and Trimdek differ in profile. Corrugated fascia brackets clip to the sheet valley; Trimdek brackets are designed for the rib spacing of that profile. Using the wrong bracket leaves gaps at the gutter-to-sheet interface where debris and water accumulate.
For patios attached to the house, the downpipe must connect to the stormwater system, not discharge onto the ground adjacent to the footing. This is a basic NCC requirement that is routinely ignored on DIY structures.
Choosing Between the Two
The decision tree is straightforward:
- Pitch below 5 degrees: Trimdek.
- Curved verandah profile: Corrugated.
- Purlin spacing over 1,200 mm: Trimdek at 0.48 BMT.
- Traditional or rural aesthetic: Corrugated.
- Contemporary skillion or flat-roof look: Trimdek.
- Matching existing corrugated roofing on the house: Corrugated.
For most sheds and carports, either profile works at 5 degrees or steeper with 900 mm purlin spacing. The aesthetic preference and the pitch constraint will usually make the decision for you.
Getting the Right Specification
ACS supplies both corrugated and Trimdek in COLORBOND and ZINCALUME, cut to length, with matching flashings, fasteners, gutters and downpipes. For sheds, patios and carports, getting the sheet length, flashing profiles and gutter sizing right before ordering saves time and avoids offcuts that cannot be returned.
Visit acsupplies.com.au to request a quote or check profile availability for your project.