Wood Purlins: Complete Guide to Design, Construction, and Installation

Wood purlins are a type of structural element used in traditional framing methods, particularly in timber framing. They are horizontal beams that provide support to the roof's rafters.

Purlins are typically spaced between 2 to 4 feet apart, depending on the design and load requirements of the structure. This spacing helps to distribute the weight of the roof evenly.

A common material used for purlins is wood, often in the form of 2x8 or 2x10 lumber. The choice of material depends on the desired strength and durability of the structure.

In traditional timber framing, purlins are often used in conjunction with a system of beams and joists to create a sturdy roof. This method has been used for centuries and is still effective today.

Wood purlins come in two types: C purlins and Z purlins. These types are similar to those offered by Steel and Stud, which also provides cut-to-length painting and punching services to their clients.

C purlins are one of the two types of wood purlins available.

Z purlins are the other type, offering a solution for construction projects.

C-Purlins are a type of wood purlin that features a curved or tapered design. They are often used in curved or irregularly shaped rooflines.

C-Purlins are typically made from solid wood or engineered wood products. They can be used to create a variety of rooflines, from simple curves to complex shapes.

The curved design of C-Purlins allows for greater flexibility in roof design. This can be especially useful when working with non-traditional roof shapes.

C-Purlins can be used in conjunction with other types of wood purlins, such as I-Purlins or T-Purlins, to create a strong and stable roof structure.

Z-purlins are distinguished by their "Z" shape and ridges, providing heightened structural strength.

Their unique shape allows for secure attachment to rafters or trusses, making them a reliable choice for load-bearing applications.

The decision to use Z-purlins often depends on the required load-bearing capacity of a project.

Their structural strength is a significant advantage, especially in projects where stability is a top priority.

Z-purlins are a great option for projects that require a high level of structural stability and secure attachment to rafters or trusses.

Purlins play a crucial role in the design and construction of a wood roof, and their spacing must be carefully calculated to ensure proper support.

The spacing of purlins should follow the nodal pattern for truss support, typically no more than 24" on center. This is especially true for post frame buildings, where purlins are spaced between trusses to provide framing for sheathing material attachment.

Purlins can be either Z shapes or C purlins, and their design can be reached depending on the supply length of the sections, loading, and roof span. The dimensions of purlins are often 2 feet by 4 feet, and their installation is similar to that of metal roofs.

The connection details between purlins and the main structural elements are critical for ensuring load transfer. Welded or bolted connections are standard, and their design must account for the specific loads and forces involved in the structure.

Purlins transfer loads from the roof covering to the main frame of the building, which typically consists of columns and beams. This load transfer is crucial for maintaining the structural integrity of the entire system and preventing the overloading of individual components.

Here are some key factors to consider when designing purlins for a wood roof:

Purlins should be placed standing up. This is a crucial step in the construction process.

When installing wood purlins, it's essential to follow the manufacturer's instructions and take necessary safety precautions to avoid accidents.

Purlins should be placed standing up, and not laid flat, as this can compromise their structural integrity.

In post frame construction, purlins play a crucial role in your pole barn's structural integrity. Purlins are horizontal beams that span between each of the trusses to provide framing for sheathing material attachment.

To ensure purlins carry the structural load adequately, they must be properly spaced. The maximum spacing of purlins should not go beyond 24" on center.

Purlins should be placed standing up, not laid flat. This is essential for their structural integrity.

Using screws to secure purlins to trusses is a better option than nails. Screws provide increased uplift resistance and ensure a structurally sound pole barn.

At FBi Buildings, we use 6" FastenMaster HeadLOK screws as our standard connection detail for post frame projects. These screws have proven to increase uplift resistance for the purlin-to-truss connection.

Proper spacing and orientation of purlins depend on how far apart the trusses are spaced. This is essential for carrying wind loads, dead loads, and gravity.

When you're working with longer spans, it's essential to overlap the ends for maximum support. Overlapping ends is stronger and preferred compared to butting.

A general rule of thumb is to overlap the ends at 12 inches. This provides added stability and security to the structure.

Butting longer spans using a thin gauge splice plate is not recommended, as it can compromise the integrity of the structure. Instead, overlap the ends for multiple supports.

Structural integrity is the backbone of any building, and wood purlins play a crucial role in ensuring it. Purlins are horizontal beams that span between trusses, providing framing for sheathing material attachment and resisting gravity and uplifting wind loads.

To carry structural loads adequately, purlins must be properly spaced, with a typical spacing of no more than 24 inches on center in post frame buildings. This spacing is dependent on the distance between trusses.

Purlins must be designed with load-bearing capacity in mind, with sectional properties such as cross-sectional shape and dimensions determining their strength. Cold-formed steel sections, like C or Z profiles, are often used due to their favorable strength-to-weight ratio.

Connection details between purlins and main structural elements are critical for ensuring load transfer. Welded or bolted connections are standard, and their design must account for specific loads and forces involved in the structure.

Proper engineering and design ensure loads are allocated efficiently to prevent overloading, and deflection limits are considered to control bending or sagging of purlins under load. Design calculations aim to maintain the roofing system's integrity by preventing excessive deflection within acceptable limits.

Here are some key factors to consider when designing wood purlins:

Screws are a more viable option than nails for securing pole barn roof purlins, as they provide better structural security and can help reduce damage from extreme weather.

Using screws instead of nails can make a big difference in the durability of your post frame building. In fact, testing has proven that screws offer increased uplift resistance for the purlin-to-truss connection.

At FBi Buildings, they use 6” FastenMaster HeadLOK screws as their standard connection detail for post frame projects. This fastener is designed to replace the use of standard 60d nails.

These screws have been shown to provide added structural security, which can give you peace of mind during intense summer storms.

Wood purlins are a crucial component in many construction projects, and understanding their materials and specifications is essential for a successful build. The rafter timber used for wood purlins has a bending strength of 30 N/mm², which is a critical factor in withstanding external loads.

The tension strength of the rafter timber is also noteworthy, with a parallel-to-grain strength of 24 N/mm² and a perpendicular-to-grain strength of 0.5 N/mm². This highlights the importance of proper orientation and installation of the purlins.

Here are the key material specifications for wood purlins:

Rafter timber material is a critical component in many construction projects. It's essential to understand its properties to ensure the stability and safety of the structure.

The bending strength of rafter timber is 30 N/mm^2, which is a significant factor in withstanding external loads.

Tension strength parallel to the grain is 24 N/mm^2, while tension strength perpendicular to the grain is a mere 0.5 N/mm^2, indicating a significant weakness in this direction.

Compression strength parallel to the grain is also 30 N/mm^2, which is a reassuring figure. In contrast, compression strength perpendicular to the grain is only 2.5 N/mm^2.

Shear strength of rafter timber is 3.5 N/mm^2, which is essential for resisting forces that cause deformation.

Here's a summary of the key properties:

Galvanized steel purlins are a popular choice for construction due to their accuracy, stability, and light weight.

They can contract and expand suitably in extreme temperatures, making them a better option for all climates.

Galvanized steel purlins are formed of cold-formed steel, which is thin enough for drilling screws through.

This type of material is made by rolling multiple sheets together into the desired shape.

A benefit of galvanized steel is its cost, which is far less than hot-rolled steel.

Designing a timber roof requires careful consideration of various factors, including wind, snow, dead, and live loads. You can find step-by-step guides that walk you through the process.

To ensure your wood purlins are safe and sturdy, you'll want to design and verify them for bending, shear, deflection, and size the connectors between the beams. Double timber beam design guides can help you with this process.

The partial factor γM is a crucial part of design calculations. According to EN 1995-1-1 Table 2.3, it's defined as 1.3.

It's worth noting that these factors can vary from country to country. You should always check the values with your National Annex.

Please keep in mind that this value is specific to EN 1995-1-1 Table 2.3 and may not apply to other design guides or calculations.

ULS Design is a critical aspect of designing timber structures, especially when dealing with complex systems like our purlins. This is because they have 3 supports, making the statical system statically indeterminate.

To calculate stresses in purlins, we need to calculate Bending Moments and shear forces due to different loads. This requires more work than designing a flat roof or beam, which is simply supported.

Our purlins are subject to various loads, including point loads, which adds to the complexity of the design. Calculating these loads is essential to ensure the structural integrity of the purlin.

In ULS Design, we verify the stresses in timber members due to bending and shear. This is a crucial step in ensuring the safety and durability of the structure.

Designing a purlin system requires careful consideration of the loads and stresses involved. By following the ULS Design methodology, we can create a robust and reliable structure that can withstand various environmental conditions.