Understanding Structural Terms for Your Glulam Project
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When you’re planning a glulam roof structure, you’ll notice a web of beams and supports working together, each with its own name and purpose. Terms like rafters, purlins, trusses, and beams can feel overwhelming if you’re new to glulam construction (or even if you’re not).
Understanding these terms isn’t just for engineers and architects; it’s essential for everyone. Whether you’re a builder, designer, or client, understanding the names and functions of these structural members helps everyone communicate clearly about your project.
We’ve therefore put together this guide to walk you through the most common structural timber terms you’ll encounter. Think of it as your friendly structural glulam glossary. We’ve even included some hand-drawn sketches and 3D visuals to show you exactly what we’re talking about.
What’s the difference between a rafter and a purlin?
Let’s start with the basics – rafters.
Rafters
Rafters are the sloping beams that form the skeleton of your roof. They run from the ridge (the very top peak of the roof) down to the wall plate at the eaves, carrying the weight of your roof covering and transferring it to the walls below.
What are the main types of rafters?
You’ll come across two main types:
1. Common rafters are your standard, regularly spaced rafters that create the main roof plane. They’re the workhorses of traditional roof construction.
2. Truss rafters are part of a prefabricated triangular frame called a truss. These incorporate not just the rafters themselves, but also a horizontal tie beam at the base and internal members like struts or posts that add strength and stability to the structure.
What is a Purlin?
Purlins are horizontal members that support your common rafters between the ridge and the wall plate. Running parallel to the ridge line, they reduce how far your rafters need to span, which stiffens the roof and helps distribute loads more evenly. In traditional timber roofs, purlins typically sit on internal posts or frames for added stability.
Think of purlins as the supporting cast – they might not be the stars of the show, but your rafters would struggle without them.
So in short – rafters slope, purlins run horizontally. Purlins usually support common rafters or other types of secondary roofing structure like insulated panels. Purlins can be supported by truss rafters.
Ridge Beam
A ridge beam is similar to a purlin, but positioned right at the apex of your roof. It runs horizontally along the very top, supporting the upper ends of the rafters and keeping your ridge line straight and true. It’s essentially the backbone running along your roof’s peak. This is probably the most common use for a glulam beam.
Hip Rafters
If your project involves a hipped roof – where all sides slope downwards rather than having gable ends – you’ll need hip rafters. These run diagonally from the corners of the building up to the ridge, forming the junction between two sloping roof planes. You can get valley rafters too.
Because hip rafters carry loads from intersecting rafters, they’re usually beefier than common rafters. They’re doing more work, so they need to be stronger.
What is a Truss?
A truss is a triangular framework that’s incredibly efficient at carrying loads. The beauty of a truss is that it distributes weight through its structure, allowing you to span greater distances with less material.
Let’s look at the different types:
1. King Post Truss
This is the classic truss design – simple, strong, and efficient. A king post truss has a single vertical post (the king post) that connects the ridge right down to the tie beam at the base. The tie beam is the horizontal member that stops the rafters from spreading outward under load.
You’ll also find struts – diagonal members that connect the king post to the rafters, adding extra support and stiffness to the structure. King post trusses are ideal for shorter spans and remain one of the most popular traditional timber frame designs.
2. Queen Post Truss
A queen post truss uses two vertical posts instead of one, standing between the tie beam and the rafters. This design allows you to span greater distances while maintaining strength and stability. It’s a step up from the king post when you need more room or a larger span.
3. Collar Tie Truss
In a collar tie truss, the horizontal member (called the collar – see image) sits higher up the rafters rather than at the base. This collar resists the outward spreading forces that try to push the rafters apart. This design can give you more usable space in the lower part of the roof, making it popular for buildings where headroom matters.
4. Steel Tie Truss
Looking for a more contemporary option? A steel tie truss replaces the traditional timber tie beam with a steel tension rod. This creates a lighter, more modern aesthetic and works brilliantly for open spaces where you want minimal visual obstruction. The steel efficiently handles the tension forces, while the timber rafters handle compression.
1. Pitched Curved Beam (Boomerang Beam)
Here’s where glulam really shines. A pitched curved or boomerang beam is thicker at the apex where stresses are highest, then tapers toward the ends. This elegant design is perfect for shallow-pitched roofs – typically around 15-20% pitch.
The curve isn’t just for aesthetics (though it does look fantastic). It’s engineered to match where the forces are greatest, putting material exactly where it’s needed most. This is the kind of sophisticated design that glulam manufacturing makes possible.
2. Cranked Beam
A cranked beam is formed from two angled timber sections joined at the apex with a rigid plate connection. The key thing here is that if there’s no tie beam connecting the two sides, it’s not technically a truss – it’s a cranked beam.
This creates a distinctive angular aesthetic at the peak and can be a striking architectural feature. The large connection plate at the apex creates a strong, rigid joint that holds the angled sections together.
3. Eaves Beam
Finally, let’s talk about the eaves beam. This is similar in principle to a purlin or ridge beam, but it’s positioned at the bottom edge of the roof where the rafters meet the walls. The eaves beam helps support the eaves and ties the frame together at this critical junction.
You can see the eaves beam clearly marked in our 3D visual – it runs horizontally at the base of the roof structure.
Bringing It All Together
By combining these different elements – rafters, purlins, beams, and trusses – glulam timber framers create structures that balance engineering precision with traditional craftsmanship. Each component plays its role in carrying loads, spanning spaces, and creating the finished roof structure.
At Buckland Timber, we work with all of these structural members in our glulam projects. Whether you need a traditional king post truss, an elegant curved beam, or a modern steel-tied design, our team of engineers and carpenters can guide you from initial concept through to manufacture and installation.
The hand-drawn sketches and 3D visuals we’ve included here should help you visualise how these components fit together. When you’re discussing your project with architects, engineers, or our team, you’ll now have the vocabulary to communicate clearly about what you need.
Got questions about which structural approach suits your project best? Our experts are always happy to talk through your options – simply get in touch!