Purlins are critical when installing a metal roof. They add valuable support and improve building stability by distributing the weight of the roof decking. Purlins can also protect the roof decking from being blown away by strong winds.
However, to get maximum value from your purlins, you must select the right purlin material and style and install them correctly. So the big question is, how far apart should purlins be for a metal roof?
When positioned, the distance between one roof purlin to the next should be no more than 1.2 meters when using 0.7 mm-thick sheeting and 1.0 meters when using 0.5mm-thick sheeting. You should consider closer purlin spacing for greater roof strength if you’re building in a location prone to heavy snow loads or high winds.
Read on to learn the different types of roofing purlins, what they do, and how to properly install and space your purlins for the highest impact.
What are Roofing Purlins
The term purlin in steel constructions refers to roofing framing members that span parallel to the building’s eave, supporting the roof decking or sheeting. Walls and rafters, in turn, support the purlins.
So, never confuse purlins for rafters. Rafters are members of the roofing structure that run perpendicularly from the top of the walls to the roof ridge. Meanwhile, purlins run horizontal or parallel to the ridge.
Two rafters, each running from the opposite side of the house or room, typically meet at the apex of the room and are held together by ceiling joists or rafter ties, forming a basic truss.
You need at least two, one at each end, for a rectangular or square room, though many projects require several trusses to form the shape of the roof.
Then several purlins run across the rafters, holding them together and providing the surface for roof sheeting.
What’s the Role of Roofing Purlins?
Roofing purlins perform several roles on the roof, which explains why they are an ever-present feature in most roofs.
- Support roof sheeting or decking: The main purpose of purlins in standard roofs is to support the sheeting or decking material.
- Support the weight of the roof deck: Purlins help spread the weight of the roof decking to enhance balance through the structure.
- They increase roof spans: Roof span is the distance from one end of the roof to the other. Purlins allow you to extend this distance beyond the walls.
Roofing Purlin Types
There are many different types of roof purlins based on material and styles. The following are the most popular options;
Types by Material
The two main materials used to make roofing purlins are wood and metal (steel). Here’s what you need to know about each option.
Wood Purlins
Wood purlins are very common. They are typically made from hardwoods for strength and durability. Wooden purlins combine well with fiber cement sheeting, creating a breathable room that you can use for any purpose. Unfortunately, timber is heavy and prone to rotting. Most woods are also not resistant to pests.
Metal (Steel) Purlins
Metal purlins are lighter and more durable than wooden purlins. The best one can last a lifetime with good maintenance. Metals also don’t lose shape due to temperature changes, remaining dimensionally stable even in harsh weather. However, they are more expensive, thus requiring a bigger budget.
Type by Styles
The two main purlin styles are C purlins and Z purlins. The names are based on the purlin shapes. Both are good styles if you use them in the right project.
C Purlins
C purlins, also known as C-section purlins, have a C-shape. They are primarily used to support walls and floors and help shape the section’s structure. They are also good for supporting beams used in flooring and can be used as joists. C-section purlins can span up to 20 feet and don’t require drilling.
Z Purlins
You should go for Z-section purlins if you’re shopping strictly for roofing purlins. They come in the letter Z shape, with an inclination of less than 90 degrees, making them a wonderful choice for giving a roof shape and structure. You can even overlap them during construction to build a more durable roofing frame.
How to Arrange Purlins When Roofing
Generally, most roofers arrange purlins depending on the roof slope. This means they are installed horizontally if the roof slopes at a 90-degree angle.
You also want to ensure the purlins are spaced evenly to provide adequate support for the trusses. The purlins shouldn’t be placed close to each other to prevent the roof from bending or the shape from being distorted.
You can achieve this by placing the purlins at the trusses’ nodes rather than on the members for even distribution and a straight roof.
How to Support Roofing Purlins
Although purlins are support structures, they too must be supported to remain in place and perform their duties accordingly. So, you should support your purlins with rafters to prevent curving. Install the purlins on the rafters with a strong underlayment material for maximum support.
Additionally, use struts to complement the purlins. For example, purlins should be placed eight feet from 2×4 struts and six feet away from 2×6 struts.
How Do you Attach Purlins to Metal Trusses?
You can use three different methods to attach purlins to the top chord of trusses, depending on the project;
- Ring shank nails: Use two nails per connection for trusses. The nails should be 20D ring shank nails, and there should be minimal spacing between the nails.
- Hang the purlins: When working on wide-spaced trusses, hang the purlins using purling hangers; LU26 hangers for 2×6 purlins or smaller and LU28 for 2×8 or larger purlins.
- Purlins on edge: Install the purlins on edge using one 60D galvanized ring shank nail for medium-spaced trusses.
Purlin spacing for Different Metal Roofing
Unfortunately, there’s no straightforward answer. Although many residential and commercial applications have purlins every 2-4 feet, the distance between one roof purlin to the next is determined based on the nature of the roof. The spacing (on sloped roofs) also depends on the truss spacing, truss type, truss slope, and roof covering.
For instance, you need purlins every four inches for a very light roof covering but can space your purlins up to four feet apart if you use a very heavy roof covering.
Nevertheless, the following are a few industry guidelines to help you choose a good purlin spacing pattern for your metal roofing project.
What’s the standard purlin spacing for metal roofs?
The standard purlin spacing for metal roofs is four to six feet. However, most residential buildings have purlins spaced three to four feet apart. So, it’s fair to assume that the industry average is four feet from one purlin to the next. The rule of thumb is to space your purlins closer (as close as under a foot apart) if you live in an area with heavy snow loads or high winds.
What is the purlin spacing for 29 gauge metal roofing?
When using 29 gauge metal roofing, your purlins should be 1.2 meters apart when using 0.7mm roofing sheets and no more than one meter apart when using 0.5mm roofing sheets. Note that you can have more or fewer purlins, depending on other factors. However, it’s a good place to start when working on purlin spacing.
What is the purlin spacing for 26 gauge metal roofing?
The purlin spacing for 26-gauge metal roofing is 4.5 feet. You’ll note that this is slightly larger than the recommended spacing for 29 gauge metal roofs. The reason is that 26-gauge metal roofs are a lot heavier than their 29-gauge counterparts. Heavier metal roofs don’t need as much support as lighter alternatives.
What’s the best spacing for 2×4 purlins on a metal roof?
Unfortunately, 2×4 purlins are some of the smallest in the industry. Therefore, you need quite a few for your roofing project, spaced closer than standard purlins for the highest roof quality. As a result, most roofing experts recommend spacing them about two feet apart.
What’s the ideal purlin spacing for corrugated metal roofing?
The best purlin spacing for corrugated metal roofs is 18 to 24 inches. This may seem too close. However, remember that corrugated roofs need plenty of support, especially where there’s a substantial slope. Don’t hesitate to place the purlins closer if you’re worried about the risk of winter ice or strong winds.
What’s the ideal metal building purlin spacing?
Metal building purlins are much stronger than their wooden counterparts, affording builders more room for maneuver. Therefore, metal roof purlin spacing length can go up to four inches (4′). However, you’re at liberty to find a spacing pattern that works best for your project. For instance, you can space the metal purlins further apart if you use stronger purlins.
How far apart are purlins put for a tin roof?
There’s common confusion about tin as a roofing material. While actual tin roofs existed a while back, tin (the metal) is rarely used in roofing anymore. The hype died in the 19th century. So, “tin roofs” typically refer to aluminum and galvanized steel roofs rather than roofs made from tin. The standard spacing for aluminum and galvanized roof purlins is 1.0 to 1.2 meters.
What Else Do You Need to Know?
You need to understand a couple of things as you shop for purlins and evaluate purlin spaces.
Purlin Span
Purlin span is like the length of the purlin. It’s the distance between the centers of the cleat bolts (type of bolt used to hold purlins) at each end of the purlin. Thus a standard/single-span purlin is supported at each end by cleats.
However, there are two more types of purlins spans – double-span purlins and continuous span purlins. Double span purlins are supported at each end and the center, while continuous span purlins are supported at each end, and several equally spaced intermediate supports.
Purlin Laps
In the case of double-span purlins, you may have a single purlin running the entire span (double unlapped) or two purlins overlapping over central support to provide continuity (double lapped). Where double lapped purlins are used, the lap length should take up 15% of the span.
Purlin Cleats
Cleats are metal plates used to secure purlins to the roofing structure. The cleat bolts go through the cleats and the wood/metal purlins. Single cleats (accepting two bolts) are standard. However, you can use double cleats (accepting four bolts) where successive unlapped purlins are butted together or to reduce bolt stress.
Purlin Spacing For Metal Roof FAQs
How far apart do purlins need to be?
Generally, roof purlins should be spaced one meter to 1½ meters, depending on the project. However, spacings smaller than a foot and as wide as five feet are common for weak purlin/roof materials and strong roof/purlin materials, respectively.
What is stronger? C or Z purlins?
Z purlins are the stronger of the two purlin types. Although C purlins are strong too, Z-section purlins are extremely strong and are used to support heavier structures. This explains why Z purlins are preferred for large industrial and agricultural projects.
Where should purlins be placed?
Roofing purlins are placed over principal rafters, building walls, or steel beams. It all comes down to the type of project and the architectural preferences. The most important thing is to ensure a strong roof structure.
Can you install standing seams over purlins?
Yes, you can install standing seams over purlins, though you can also install them over boards. You should also consider installing standing seams over underlayment felt.
Summary
In general, the distance between one roof purlin to the next should be no more than 1.2 meters when using 0.7 mm-thick sheeting and 1.0 meters when using 0.5mm-thick sheeting. You should consider closer purlin spacing for greater roof strength if you’re building in a location prone to heavy snow loads or high winds.
There you have it. The general standard for roof purlin spacing is one meter to 1.2 meters, with an industry average of 4 feet. However, it all comes down to the project requirements.
For instance, you can space your purlins less than a foot apart if using lighter roofing sheets and wider for thicker sheets. Give us a call to learn more.