If you’re planning to build or renovate a roof, you might have heard of the term “purlin”. But what exactly is a purlin and why is it important for your roof? In this article, we’ll explain everything you need to know about roof purlins, from their definition and types to their benefits and installation. By the end of this article, you’ll be able to confidently choose the right purlins for your roof project and enjoy a strong and stable roof for years to come.
A couple of stats that make this a pertinent topic in the current day and age is that roof- related issues account for a substantial 39% of homeowner insurance claims in the United States (Insurance Information Institute) and, according to a report by Global Market Insights, Inc. the global metal roofing market size was valued at USD 16.5 billion in 2020 and is
expected to grow at a compound annual growth rate (CAGR) of 4.2% from 2021 to 2027. These stats both underline not only the importance of a strong roof, but also that building a roof using steel purlins is a much more accessible and progressively cheaper solution now than it ever was.
So, what is a purlin in a roof?
A purlin is essentially the horizontal structural member in a roof that supports the roof covering and transfers the load to the primary roof framing. It provides additional support and stability to the roof structure.
Purlins are essential in roof construction for several reasons:
● Structural Support: Purlins provide horizontal support to the roof rafters, helping distribute the weight of the roof covering and any additional loads, such as snow, wind, or equipment on the roof.
● Preventing Sagging: Without purlins, the roof rafters may sag under the weight of the roof covering, leading to structural issues and a compromised roof system.
● Preventing Spreading: Purlins help prevent the roof rafters from spreading apart, maintaining the correct spacing and stability of the roof structure.
● Longer Spans: Purlins enable longer rafter spans, allowing for wider buildings or more open interior spaces without the need for additional support.
● Attachment Surface: Purlins serve as a stable base for attaching the roof covering materials, such as metal sheets, tiles, or roofing shingles.
● Even Load Distribution: By distributing the load evenly across the rafters, purlins reduce the risk of localized stress points and potential structural failure.
Which are the best materials for roof purlin?
The most common materials used for roof purlins are steel and timber. Each material has its own advantages and disadvantages depending on the type of roof, budget, and preference.
Steel purlins (such as those produced by the UK giant voestalpine Metsec) are strong, durable, lightweight, and resistant to fire, corrosion, and pests. They can be easily fabricated into various shapes and sizes to suit different roof designs. They also offer high thermal performance and energy efficiency when combined with proper insulation. However, steel purlins are more expensive than timber purlins and require special tools and skills for installation. They also create potential thermal bridging issues if not properly insulated or ventilated.
Timber purlins are cheap, readily available, easy to install, and (arguably) environmentally friendly. They can also create a natural and aesthetic look for the roof. However, timber purlins are less strong than steel purlins and may warp, crack, rot, or decay over time. They also require regular maintenance and treatment to prevent damage from moisture, insects, or fungi.
Which is better? Well, timber is a sustainable solution, but it doesn’t last as well and you do have to kill a tree to manufacture it! It also takes a lot more maintenance than steel. Steel is more reliable, stronger, longer lasting, and recyclable. It may be more expensive, but you should factor in the cost of replacing a timber roof in 30-40 years, whereas a steel roof could last double that.
What are the main types of steel purlins?
There are two main types of purlins used in steel frame construction: Z purlins and C purlins.
Steel Z purlins have a Z-shaped cross-section and are usually used for roofs with slopes greater than 15 degrees. They have a high strength-to-weight ratio and can span longer distances than C purlins. They also allow for easy overlapping and nesting of sections, reducing installation time and cost.
C purlins have a C-shaped cross-section and are usually used for roofs with slopes less than 15 degrees. They have lower strength-to-weight ratio than Z purlins but offer more flexibility in design and layout. They also allow for easy connection of braces and accessories, enhancing the stability of the roof structure.
How to choose the right purlins for your roof?
Choosing the right purlins for your roof depends on several factors, such as:
● The type and slope of your roof: Different types and slopes of roofs require different types and sizes of purlins. For example, a flat roof may need C purlins, while a pitched roof may need Z purlins. You also need to consider the angle and direction of your roof when choosing the purlins.
● The load and span of your roof: The load and span of your roof determine the strength and spacing of your purlins. You need to calculate the weight of your roof covering, any additional loads, and the distance between your primary roof framing.
You can then use a purlin span table or a purlin design software to select the appropriate purlins for your roof.
● The material and budget of your roof: The material and budget of your roof affect the cost and availability of your purlins. You need to compare the prices and qualities of different materials and choose the one that suits your needs and preferences. You also need to factor in the installation and maintenance costs of your purlins.
We’ve explained what purlins are, what the most common materials purlins are manufactured from, as well as their advantages and disadvantages. We’ve discussed the considerations you need to make when choosing materials, as well as the potential factors that affect roof costs. Ultimately, if you have the budget, or your roof design is architecturally creative, shall we say, then steel is the answer. However, on the other hand, if you are budget-limited, and want to use a simple design with a material most builders know how to use, then go for timber. Just make sure you maintain it well and it’ll likely last a long time.
