Not all steel sheds are created equal. When determining the structural integrity of your new building, the steel used inside the walls matters infinitely more than the cladding on the outside. Understanding shed frame types is the difference between a structure that flexes in high winds and one that stands rock solid for generations.
The Australian shed market is dominated by two primary structural approaches: the lightweight C purlin shed design and the heavy-duty Rectangular Hollow Section (RHS) steel frame. While both can meet engineering standards, they serve very different purposes on a property.
Here is what you must understand about structural steel sheds before you build.
The Mechanics of C Purlin Shed Design
A C purlin shed relies on cold-formed steel. This means flat sheets of thin-gauge steel (typically between 1mm and 3mm) are folded into an open ‘C’ shape.
The biggest advantage of a purlin framework is cost and weight. Because the steel is thinner and open-backed, it requires less raw material, making it cheaper to manufacture and transport. The design is exceptionally common for standard residential garages and light storage solutions.
However, because the profile is open, it has very little torsional rigidity. In plain terms, an open C channel wants to twist when put under heavy stress. To counteract this, manufacturers must rely heavily on internal apex and knee braces. While these braces keep the shed standing, they aggressively eat into your usable internal headroom and side clearances.
The Superior Power of RHS Steel Frame Sheds
In contrast, Rectangular Hollow Section (RHS) entails hot-rolled structural steel formed into a completely closed, four-sided tube. The wall thicknesses are generally much heavier (often 2mm to 6mm+).
Because it is a closed loop, an RHS frame refuses to twist; it is inherently rigid. This immense strength-to-weight ratio allows for massive “clear-span” designs. With an engineered shed frame built from RHS, you can bridge much wider gaps without requiring intrusive internal knee bracing. For agricultural machinery, industrial workshops, and vast open spaces, RHS is the undisputed champion.
Furthermore, RHS is highly impact-resistant. If a tractor or heavy forklift accidentally bumps a C purlin column, the thin steel can crumple, compromising the entire roof load. If that same equipment bumps a heavy-gauge RHS column, it will likely just scratch the paint.
Head-to-Head Comparison: Shed Purlins vs. Structural RHS
| Structural Feature | C Purlin Frame | RHS Steel Frame |
| Shape Profile | Open “C” profile. | Closed, 4-sided tube. |
| Twist Resistance | Low; requires extensive internal bracing. | Exceptional; naturally resists torsional forces. |
| Impact Resistance | Susceptible to crumpling from bumps. | Highly durable; withstands heavy impacts. |
| Best Application | Light residential storage, budget carports. | Farm machinery, commercial workshops, premium garages. |
Why the Framework Dictates Your Shed’s Lifespan
There is a common misunderstanding that if a shed is “engineered,” it must be indestructible. While a C purlin shed is engineered to survive specific wind ratings, surviving is not the same as thriving. During heavy storms, thin purlin frames can flex. This micro-flexing pushes and pulls on the fasteners securing your roof and wall cladding. Over the years, that constant movement can slowly widen screw holes, leading to leaks and premature wear.
A fully welded structural steel shed built with RHS frames doesn’t flex. It stands rigidly against the weather, reducing strain on the cladding and vastly increasing the lifespan of the entire building.
Making the Right Choice for Your Property
If you are simply looking to keep the rain off a lawnmower, a lightweight C shaped design is an adequate, budget-friendly solution. But if you are storing hundreds of thousands of dollars worth of farm machinery, operating heavy vehicle hoists, or building a premium workspace, compromising on the skeleton of your building is a costly mistake.
At CV Sheds, we build for longevity which is why our motto is ‘Built for Generations’. By utilising premium, heavy-gauge RHS steel for our main structural portals, we deliver a rock-solid, clear-span environment that you can rely on for decades. Don’t settle for a shed that bends to the weather. Contact CV Sheds to build it right from the ground up.