Purlin vs Structural Steel: What Is The Best Steel Frame Construction?

Are you looking to build a new shed, but are confused by the different options out there?

A question we often get is: What’s the difference between structural steel sheds and purlin sheds?

Below, we compare purlins vs. structural steel sheds and help you choose the right one for your needs.

What Is A Purlin?

What Is A Purlin

Image Source: Shedblog

A purlin is a roll formed structural section. It is roll formed out of flat sheet steel. A bunch of rollers shapes the flat sheet into the specific purlin shape.

There are two types of purlins: 

  • C purlins, which are in the shape of a C
  • Z purlins, which are in the shape of a Z.

Purlins are also known as cold-form, or rolled-form steel.

A purlin shed is a structure where the main portal frames are made out of rolled-formed sections, usually C purlins. These purlins are made out of steel sheets that range between 1mm, 2mm or 3mm steel.

What Is Structural Steel?

What Is Structural Steel?

Structural steel is hot-formed steel. It is a thicker steel that is heated up and fed through rollers. The thickness of hot rolled structural steel members can be made from 5mm to 20mm.

In a structural steel shed, the main portal frames and roof rafters are made from strong steel sections which are welded together and later bolted together.

Purlins and girts are used longitudinally to support the roof sheeting (which they were originally designed for). This design is a lot stronger and more robust way of making a steel shed.

There are a couple of types of structural steel beams including:

  • UB: Universal Beam, also known as I-Beam or H-Beam. This beam is in the shape of an I. You can get it in various shapes, sizes, and thickness in the material.
  • Welded Web Truss: They provide the rigid skeleton to span your area without the wasted materials of heavy beams. Their superior strength eliminates the need for knee braces and ridge ties which infringe on your space. Trusses are welded open web design made from AS1163 Grade Galvanised RHS. They are engineered to provide the perfect balance between weight and strength

Purlin Shed Pros & Cons

Purlins frames are better suited to smaller sheds (less than 12 meters) because Rafter members have sufficient strength for these spans. The labor costs involved in manufacturing the smaller size can be streamlined, resulting in savings for the consumer. 

When a purlin shed gets to the building site, everything is bolted and screwed together (just like a Meccano set). This isn’t a problem for smaller sheds; however, for larger sheds, it can take a lot longer to construct, resulting in significantly higher erection costs. It’s a cheaper way of making smaller sheds, however, for larger sheds, the costs of erection can significantly increase.

Purlin sheds are not ideal if you have lots of machinery moving around that could potentially damage the structure. For example, if you use a forklift and it accidentally bumps into a column, because the metal is relatively thin, it is more likely to bend. When it bends it endangers the structural integrity of a shed.

Steel purlins have strength limitations. To strengthen larger spans they have to add extra bracing to stiffen the structure. You might notice on bigger span sheds they will need a knee brace and an apex tie. The knee brace usually cuts across at a 45-degree angle from the column and the rafter. That isn’t an efficient use of space.

Purlin Shed Space

Image Source: Shedblog

How Far Can C Purlins Span?

In general, C Purlins sheds are best for smaller spans. However, some manufacturers sell designs to do up to 24-metres in span. This is right at the limit of a purlin’s capacity and does no leave much margin for coping with nasty weather conditions

This design usually involves a lot of extra bracing. It also means they often can’t get the higher wind ratings due to limited structural strength at the larger sizes.

To strengthen the shed, manufacturers put two C sections back to back resulting in extra pieces. The extra parts needed results in significantly increased building costs due to 3 to 4 times the extra time required to put all the pieces together.

Structural Steel Pros & Cons

Structural steel is usually a lot thicker, making it ideal for warehouses and machinery sheds where there could be forklifts or heavy machinery moving about. If someone accidentally bumps into a column, the extra strength and durability of structure steel means it will be less likely to bend.

Due to everything being cut and welded to engineers specifications you also can build a structural steel shed to suit any shape and size. They are fully customisable to any need, including curved roofing. This differs from purlins which are limited to standard material sizes, resulting in less flexibility to customisation.

Unlike a purlin shed, in a structural shed, all the pieces are welded into large prefabricated assemblies For larger sheds, this results in a faster install, saving you thousands in steel construction costs.

Overall a large structural steel shed is more cost-effective than large shed made of purlins.

Structural Steel Span

A large industrial shed we built.

How Far Can Structural Steel Span?

In a structural shed, all pieces are manufactured to size and engineering specifics. As a result, the length of a structural steel span can be any size.

With the addition of lower erection costs, this makes structure steel the ideal choice for your large shed.

Our #1 Tip To Constructing A Large Shed

Structural Steel vs Purlin

A farm shed built to store large rural machinery.

Make sure you get site-specific engineering, whether you choose to go with purlin or structural steel. It is crucial your large shed is designed and engineered, not only your area, but also the block of land you are putting it on. 

If you are putting it on a hill, the top of the hill can be a significantly different wind rating than the bottom of the hill.

We make sure to engineer everything to suit the exact spot and what the shed will be used for.

Wrapping It Up

By reading this guide, you will have a better understanding of the different steel frame constructions available.

Purlins are more efficent for sheds smaller than 12 meters in span. While larger purlin shed kits might be appear cheaper, the added costs of supply and installation significantly increases the price for larger sheds. 

However, structural steel is great for larger sheds. It's strong, more customisable, and the quicker install means that you won't waste money on shed installation.

If you are interested in a large structural steel shed, contact us today.