The Complete Guide to Roof Sheeting for Steel Structures

A steel frame gives a building its strength. The roof sheeting is what actually gets tested every day, by sun, wind, hail, and in coastal areas, salt air that will find any weak point in a fixing or a lap joint within a season or two.

At B&T Steel, roof sheeting isn’t a line item we add once the frame is up. It’s specified alongside the structural design, so pitch, purlin spacing, and sheeting profile are worked out together instead of one dictating awkward compromises to the other.

In this guide, we cover what roof sheeting actually needs to do, the profiles we work with most, and the details that determine whether a roof lasts fifteen years or forty.

What Roof Sheeting Actually Has to Do

Roof sheeting is the metal cladding fixed to the purlins across the top of a structure. On paper that sounds simple. In practice, a roof in Mpumalanga has to handle hail loading that a coastal roof in the Western Cape doesn’t, while a coastal roof needs corrosion resistance that an inland warehouse can often do without. The sheeting and the fixings both need to be specified for where the building actually stands, not a generic spec sheet.

Roof sheeting is doing several jobs at once:

• Weather protection. UV degradation, wind uplift, and hail impact are the three most common causes of premature sheeting failure.
• Structural bracing. Correctly fixed sheeting acts as a diaphragm, adding lateral stiffness to the roof structure, not just a cosmetic layer sitting on top of it.
• Thermal performance. Paired with insulation, sheeting choice affects heating and cooling costs for the building’s entire life.
• Water management. Pitch, overlap, and gutter design determine whether water clears the roof or pools and works its way into laps and fasteners.
• Corrosion resistance. Ammonia from livestock, chemical exposure in processing plants, and coastal salt air all shorten the life of the wrong coating choice.

A roof spec that ignores any one of these is a roof that will need attention sooner than the client expects.

Types of Roof Sheeting Used in Steel Structures

The right profile depends on the span, the budget, and what the building is used for. These are the three we specify most often.

IBR (Inverted Box Rib) Sheeting

• The standard for industrial and commercial buildings, typically laid at a minimum pitch of around 5 degrees
• A rib profile that gives strong load-bearing capacity for its weight, with good water drainage along the ribs

• Available in CHROMADEK®, ZINCALUME®, or plain galvanised finish, depending on exposure and budget

Concealed Fix Sheeting

• Fixed through clips rather than pierced through the pan, so there are no exposed screw heads for water to track through over time
• Better wind uplift resistance than exposed-fix systems, which matters more as spans get wider
• Usually specified where appearance matters as much as performance, such as street-facing commercial buildings

Corrugated Sheeting

• Lighter gauge and lower cost per square metre than IBR
• Common on agricultural buildings such as piggeries and general-purpose warehouses where budget matters more than finish
• The rounded wave profile handles thermal expansion well over long, unbroken runs

Please note on roof spans over 30 metres, concealed fix sheeting stops being a design preference and becomes a structural requirement. Exposed-fix sheeting relies on screws piercing the pan at regular intervals, and at that span the thermal expansion and contraction across the sheet works those fixings loose over time, along with the wind loading a span that size generates. Concealed fix systems allow the sheet to move independently of the fixing, which is the only way to keep a wide-span roof watertight over its design life.

Where Roof Sheeting Fits Into a B&T Steel Project

We spec sheeting at the same stage we spec purlins, not after. That matters because the roof pitch, the purlin spacing, and the sheeting profile all constrain each other. Choosing sheeting after the frame is finalised means either accepting a compromise or reworking structural elements that were already signed off.

What that looks like in practice:

• Sheeting, fixings, and flashing detail confirmed during Tekla 3D modelling, before fabrication starts
• Wide-span roofs specified with concealed fix systems from the outset, not retrofitted once a client asks why an exposed-fix roof is leaking at 35 metres
• Corrosion protection matched to the site
• One project manager carrying the roof spec from design through to the final fix

Whether it’s a piggery, an agri-processing plant, or a cross-border warehouse project, the roof gets the same level of engineering attention as the frame underneath it.

Where Roofs Actually Fail

Most roof sheeting problems trace back to a handful of installation errors, not the material itself:

• Insufficient side or end lap, letting wind-driven rain track back under the sheet
• Fixings installed off-centre on the rib, which crushes the sealing washer instead of compressing it evenly
• Flashing details rushed at penetrations, ridges, and valleys, which is where the majority of leaks start
• Insulation omitted or poorly fitted, which shows up later as condensation rather than an obvious fault

None of these are material defects. They’re installation errors, which is why our teams install to manufacturer fixing specifications and schedules rather than site judgement calls, on every roof, at every span.

If you’re specifying a steel structure and need the roof detailed properly from the start, not fixed after the fact, contact B&T Steel or see our full steel structure services.