In architecture, structural engineering or building, a purlin (or historically purline, purloyne, purling, perling) is a horizontal beam or bar used for structural support in buildings, most commonly in a roof. Purlins are supported either by rafters or the walls of the building. They are most commonly used in metal buildings, though they sometimes replace closely spaced rafters in wood frame structures.
The purlins of a roof support the weight of the roof deck. The roof deck is the wood panel, ply board, or metal sheeting that creates the surface of the roof. When made of wood, it is usually covered with some sort of weatherproofing and sometimes an insulation material.
Several kinds of purlins exist. They are divided into categories based on the material from which they are made and their shape. Different purlins are used for different purposes, including structural support of walls or floors. Purlin is important because without it, there’s no frame for the sheeting on the roof to rest on, making purlins critical to the structure of the roof.
1. Purlin Material
a. Wood Purlin
Wood Purlin is good for use with the fibre cement sheeting. The wood purlin and sheeting combine well to ensure that the room below is breathable and can safely store whatever you need to be kept safe in the room, from livestock to grain or other organic materials.
However, being made from wood, purlins can rot. Besides, the main problem with wood is it being dry when it goes up. Therefore, it is best dried before installation. Moreover, the moisture can add significantly to the weight leading to sag.
b. Steel Purlin
Steel Purlin is a direct replacement for wood purlin. They are light weight, dimensionally stable, accurate and straight. They expand and contract reasonably in extreme temperature changes.
|Characteristics||Steel purlin||Wood purlin|
|Cost||Generally cheaper||More expensive|
|Recyclability||Recyclable, thus “greener”||Less recyclable|
|Assembling||Quicker to be assembled||Slower to be assembled|
|Weight||Lighter than wood structures of the same size||Heavier than steel structures of the same size|
|Insect proof||Not affected by termites or other insects that feed on wood||Affected by termites|
|Life time||Last much longer than wood in most applications||Shorter life time than steel|
|Fire resistance||Greater fire resistance||Flammable|
Steel purlin is usually made of cold-formed steel that is thin enough to put screws through. Cold-formed steel is made by rolling or pressing thin sheets of steel into the desired shape. It is less expensive for the manufacturer than hot-rolled steel and is also easier to work with. Though cold-formed steel is stronger than hot-formed steel, it is more likely to break when under pressure rather than bend.
Purlins are manufactured from hot dipped galvanized steel with a coating, in line with other common lightweight steel structural building products. This gives good protection in most exposed internal environments. Run off from, or contact with, materials which are incompatible with zinc should be avoided.
To protect the purlin, they also apply a layer of paint outside them. Zinc and paint in combination (synergistic effect) produce a corrosion protection approximately 2X the sum of the corrosion protection that each alone would provide.
2. Purlin Types
There are three types of steel purlins: Rectangular hollow section (RHS), Cee or C purlin and the Zed or Z purlin which is shaped like a letter Z.
a. C Purlin (Cee Purlin)
Cee purlin or C purlin is shaped like a squared-off letter C. C Purlins are horizontal structures that are used to support the load from the roof deck or the sheathing. The plane surface of this purling on one side has made it a preferred material for cladding due to its easy installation on concrete structures or steel. Cee purlin should be light in weight and perfect for simple span construction.
C Purlins are often used for structural support in walls and as floor joists in addition to roofs. Cee purlins may also be used to form braces, ties, or columns in sheds.
- Optimum quality
- Easy installation
- High tensile strength
- Abrasion & corrosion resistance
- Ability to span length
- Purlin erection is easier than others
- Fast to erect and easy handling
- No side drilling/cutting required
- Assured dimensions and straightness
- High durability, versatility and uniform quality
- Low transportation cost due to decreased weight
- Close tolerances on sectional sizes due to process of cold roll formingC purlin (left) and Z purlin (right)
b. Z Purlin (Zed Purlin)
Zed purlin or Z purlin is shaped like a letter Z. Z purlin shape allows the purlin to overlap with others at the joints. This gives Zed purlins the potential to be much stronger than C purlins. They are mainly used in walls or for large roofing projects.
Z Purlins are made using cold-formed or rolled sheets for supporting roof. The flexible shape of these beams facilitates various designs solutions. These purlins are extensively used in huge roofing solutions such as godowns, workshops, industrials sheds and many more. The range is known for saving up to 50% on structural sheet in comparison with hot rolled angles. Z purlin should be crisp and clean in design and do not allow the scope of inaccurate lengths.
- Ability to span length
- Saving in Steel up to 40%
- Fast to erect and easy handling
- No side drilling/cutting required
- Assured dimensions and straightness
- Purlin erection is easier than others
- Saving in construction cost up to 30%
- High durability, versatility and uniform quality
- Low transportation cost due to reduced weight
- Close tolerances on sectional dimensions owing to process of cold roll forming
- Saving up to 35-40% in weight and 20% in cost when compared to hot rolled purlins
c. Rectangular hollow section (RHS)
Rectangular hollow section (RHS) is a type of purlin often used in roofs where the support structure will be visible when construction is complete. For example, decks and covered patios often use this type of purlin. As the bar is hollow, caps are welded to the ends of the purlin to keep moisture from getting inside and corroding the metal. The rectangular shape gives the roof the same aesthetic quality of a roof supported by wooden beams.
Rectangular hollow section (RHS)
3. Purlin vs Rafter
Rafter are also the basic member of any roof structure. Together with purlin, they are the load transmitting members of the steel roof. They transmit the live load, dead load, wind load (which is considerable in large steel structures with low permeability) and other loads acting on them to the roof truss situated below them which eventually transfers the load to the columns and then eventually to the foundation.
Purlin and Rafter
So basically, purlin and rafter are like two ways reinforcements of the roof. Purlins are one which are parallel to the ridge line or we can say they run along the span of the roof while rafters are perpendicular (plan) to the ridge line of the roof truss.
The roof truss is supported on the columns and the rafter is supported on the roof truss upon which the purlins are bolted/welded and upon the purlins lies the final covering material which may be asbestos sheet or cement asbestos sheet. Also note here that the rafters are present only where the truss are not present to support the purlins.
Principle rafter and common purlin frame configuration
Common rafter frame configuration
For easier distinction, the rafters are orientated vertically along the slope of the roof. Purlins are orientated horizontally, providing support to the rafters, usually at mid-height.
4. Purlin and Girt
Secondary framing is an important component of many pre-engineered metal buildings. Also referred to as “secondary structurals”, this type of framing runs in between primary framing elements, creating a structure-within-a-structure, much like cross-beams in a wooden building.
The purpose of secondary framing is to distribute loads from the building’s surfaces to the main framing and the foundation. Secondary framing can add longitudinal support that helps resist wind and earthquakes. And it can provide lateral bracing for compression flanges that are part of the primary framing, increasing overall frame capacity.
Secondary framing components are known as girts and purlins, and they work like this:
- Girts provide additional support for walls: They work in conjunction with columns and wall panels to support vertical load, improving both strength and stability. They also help attach and support wall cladding.
- Purlins provide additional support for the roof: They create a horizontal “diaphragm” that supports the weight of your building’s roof deck – whatever material you use for the roof itself. They also help make your entire roof structure more rigid. Because they add mid-span support, purlins allow longer spans, enabling you to create a wider building.
- Eave struts are another kind of secondary framing: Also called eave girts or eave purlins, these are essentially a combination of the two. They’re used where sidewalls intersect with the roof, using a top flange that helps support the roof and a “web” that helps support the walls.
Secondary framing comes in two configurations, CEE and ZED. They’re shaped on a bending press, to create a web with two flanges. They come in a variety of sizes; for instance, purlins can run over 30 feet in length.
Girts, purlins and eave struts are almost always made of cold-formed steel. It’s more affordable and easier to work with, but it also presents some structural stability issues that must be considered as part of your metal building framing options and overall design. In particular, local or distortional buckling or lateral displacement can occur, in which portions of the compression flange, web or connectors can buckle or shift from their initial position.
Problems can occur under extreme stress or even under relatively low stress if conditions are just right. However, you shouldn’t consider these engineering issues to be detractors if you’re considering a metal building. Additional stability or support can come not only from girts, purlins and eave struts but also from additional stiffeners.
Exactly how many and what size secondary framing elements your building may require will depend on your building’s dimensions, primary framing system and how you plan to use the building as well as other factors. Your metal building company can explain the nuances in detail and guide you toward the right decision.
Purlin and Girt
5. Purlin Roof
Roof purlin needs no introduction to anyone in the construction industry. In their design life, purlins are subjected to dead load (e.g self weight of sheeting materials and accessories), live load (e.g. during maintenance services and repairs), and environmental loads (e.g. wind and snow load). Therefore, a purlin should be adequately strong to withstand the loads it will encounter during its design life and should not sag in an obvious manner thereby giving the roof sheeting an undulating and/or unpleasant appearance. This post will be focusing on design of steel purlin using cold formed sections.
6. Purlin Span
The span referred to is the distance between centers of the cleat bolts at each end of the purlin. Each span type represents a complete purlin run system and recognizes that using separate component parts (e.g. internal span, end span) is not a valid procedure.
a. Single Purlin Span
Single purlin span is the span that is simply supported by means of bolting the web of the purlin to a cleat or other rigid structure. Under these conditions bridging does not influence inward capacities, but outward capacities vary dependent on the number of rows of bridging.
b. Double Purlin Span
Double purlin span are simply supported at each end and in the center. They may comprise only one purlin over the full length or two purlins lapped together over the central support to provide continuity. Both inward and outward capacities are influenced by bridging in double spans.
c. Continuous Purlin Span
Continuous purlin span are simply supported at each end and at a series of equally spaced intermediate supports. These tables are for spans in which purlins are lapped over each support, where the lap length is 15% of the span. Tables are given for 5 or more spans. For outwards loads on equal continuous spans the bridging shown is required in the end spans. One less row of bridging can be used for internal spans if inwards capacity, the recommended minimum bridging and practical spacing requirements allow. For inwards loads the required bridging in the table applies to all spans.
7. Purlin Accessories
a. Purlin Laps
It is generally recommended a lap length of 15% of the span. Where span lengths are unequal (e.g. reduced end spans) each purlin should have 7.5% of each adjacent span added rather than 7.5% of that purlin’s span.
Lap lengths of less than 10% (or 5% on any side of a support) may not provide full structural continuity and may also suffer from local failures not considered by this method. They must therefore be considered beyond the scope of this manual.
b. Purlin Cleats
Single cleats are used in most situations, including lapped Z purlins. Double cleats are generally only used where successive purlins (usually unlapped) are butted together. Double cleats could also be used in applications with a high reaction load, to reduce bolt stresses. In this situation, additional care would be needed in hole detailing.
8. Purlin Design
a. Purlin Spacing
By default, purlin sections assume the slope of the roof they are supporting. The spacing of purlins usually call for careful arrangement, in the sense that it should follow the nodal pattern of the supporting trusses. What I mean in this regard is that purlins should be placed at the nodes of trusses and not on the members themselves so as not to induce secondary bending and shear forces in the members of the truss. Furthermore, if manual analysis is employed to analyze a truss loaded in such manner, such secondary stresses cannot be captured since we normally assume pinned connections.
Cold formed Z (Zed) and C (Cee) sections are normally specified for purlins in steel structures (see their form in image below).
Z-Section and C-Section
As compared with thicker hot rolled shapes, they normally offer the advantages of lightness, high strength and stiffness, easy fabrication and installations, easy packaging and transportation etc. The connection of purlins can be sleeved or butted depending on the construction method adopted.
Sleeved connection and Butted connection
In terms of arrangement, we can have single spans with staggered sleeved/butt arrangement, single/double span with staggered sleeve arrangement, double span butt joint system, and single span butt joint system. The choice of the arrangement to be adopted can depend on the supply length of the sections as readily available in the market, the need to avoid wasteful offcuts, the loading and span of the roof, the arrangement of the rafters etc. Therefore, the roof designer must plan from start to finish. However, single and double span butt joint system are the most popular in Nigeria, due to their simplicity, and the culture of adopting shorter roof spans in the country. However, they are less structurally efficient than sleeved connections.
b. Purlin Bridging
Bridging provides resistance to purlin rotation during the installation of roof and wall sheeting. For this reason, a maximum bridging (or bridging to cleat) spacing of 20 x purlin depth, but no greater than 4000, is recommended. Failure to do so can lead to misaligned fastenings, causing additional stresses on the fasteners and roof sheeting. Excessive purlin rotation can be a safety hazard during construction. It is therefore recommended that at least one row of bridging be used in each purlin span. Span/bridging configurations which exceed these recommendations are shown on the left of or above the red line. However, in instances where sheeting is successfully installed outside of this recommendation the published values are valid for structural purposes.
9. Purlin Installation
Purlins are installed horizontally under metal roofs. They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top. Purlins are 2 by 4 feet and are installed much like metal roofing. They give added support to the roof and also provide a nailing surface for the end panels and drip edge.
|Step 1||Run a tape measure from end to end along the rafters of the roof to determine how many purlins will be needed for installation. Measure the width and height of the roof and record the measurements.|
|Step 2||Snap a chalk line horizontally across the roof two feet down from the top. Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner. Fasten the purlin with 16d common nails into each vertical rafter. Insert two equally spaced nails into the rafter. The rafters are generally spaced every 16 inches on center.|
|Step 3||Set the second purlin horizontally right next to the first and install it the same way. Continue down the row until the first row is complete. Cut the last purlin to size with tin snips, if necessary.|
|Step 4||Move two feet down the roof and snap a horizontal chalk line. Install the second row of purlins just like the first, cutting the end piece to fit. Snap the third chalk line two more feet down and continue down the roof until it is covered with purlins. Inspect the area to verify that no nails are sticking up and that all the purlins are secure to the rafters.|
a. Purlin Laps
Purlin laps must be bolted in the top web hole and the lower flange holes at both ends of the lap as shown below. Bolting only in the web of lapped purlins does not provide full structural continuity and excessive loads could be placed on to roofing screws that penetrate both purlins within a lapped region.
b. Purlin Flying Bracing
If the lower web hole in a lap is used for attaching fly bracing ensure that an additional bolt is used.
c. Purlin Bridging
Design Station Bridging can be installed either up or down the roof slope but cannot be mixed within a bridging run. However, as the starting and finishing components will be different, the direction of fixing must be established at the design/procurement phase. Girt bridging must not exceed its compressive capacity. Where more than one row is to be installed always complete the bridging for each girt before commencing on the next (i.e. do not complete one row of bridging before starting the next).
The welding or hot cutting of purlins, girts or bridging is not recommended. The heat produced in welding will affect the material properties of the high yield strength cold-formed steel used in purlins. In many instances considerable stress concentrations are likely to arise, even with good quality welding. In addition, welding will locally remove the protective coating, leading to a potential reduction in durability.
10. Purlin Handling and Storage
Roof Purlins must be kept dry during storage as water present between close stacked sections will cause premature corrosion. If they become wet, they should be separated and stacked openly to allow for ventilation to dry the surface.
The installation of Purlins can be hazardous and will require an adequate safety plan be in place prior to handling or installing of these products. All rigging, scaffolding and safety equipment must comply with the relevant codes, Australian standards and statutory requirements. It is recommended that good trade practice be followed such as that outlined in Australian Standards AS3828-1998 (Guidelines for the erection of building steelwork) and HB39 (Installation code for metal roof and wall cladding). Normally, purlins are not designed to be walked on unless fully covered by correctly installed roofing materials or the correct grade of safety mesh. The manufacturing or delivery process may result in oil or grease adhering to these purlins which could increase the potential hazard. Handling of this product must be carried out using a correctly supervised crane or appropriate lifting device. Safety harnesses must always be used during installation of purlins when working off the ground, and under no circumstances must any body weight be placed on bridging, or on purlins or girts that have not been fully bolted into position and with the correct bridging installed. Bolts must be the correct size and grade, all progressively fully tightened during installation. Laps must be bolted in the outer web hole (closest to the sheeting), and the inner flange holes provided.
11. Purlin Supplier
Last but not least, you have to choose the best supplier for you to order purlin. A reputable supplier with strong customer base can ensure the purlin’s quality to a certain extent. Other than reputation, there are some elements that make the best purlin supplier.
a. Production Capacity
Production capacity of the manufacturer decides how long it takes you to receive the purlin. The higher the production capacity is, the shorter time you have to wait. It depends on their factory size and technology they applied. In general, big manufacturer have higher production capacity.
In case of steel purlin, it’s essential to find a manufacturer which has production capacity of around 500MTS per day.
b. Production Process
The production is recommended to be closed from raw material to final product. If so, the manufacturer can completely manage the quality of the purlin. If they are not steelmakers and need other steel supplier, they cannot be sure about the exact material and production process. What will happen if your purlin are rusty after 3 months being used and when you contact your supplier, they blame it for the steelmaker. Moreover, it results in higher price because the products’ prices include more transportation costs and benefits.
Each production stage should use separate and modern production techniques. Modern machinery and advanced technology applied will save energy and reduce production costs. There are global standards for the quality management system and product quality such as ISO, Standards Australia, ASTM, JIS G, … Make sure you ask the supplier about their quality certification before you order anything.
Professional consultancy is very important when you merely know about purlin and steel. They will help you decide the purlin sizes and span according to your place’s weather condition. They had better provide free consultancy and it is convenient with 24/7 service.
Professional consultancy is very important when you merely know about purlin and steel.
e. Professional Staffs
If you are looking for a foreign supplier, they are needed to be familiar with exporting activities. You had better work with knowledgeable and dynamic staffs, which will push the process going much faster.
f. After-sales service
After buying purlin, you still need assistance in setting and maintenance. You will be in trouble if your supplier disappears right after sales because there will be no one to guarantee the quality of the products and take responsibility when they are faulty.
12. Chinh Dai Steel’s Purlin
Chinh Dai Steel is the professional supplier which specializes in producing purlin. Our products are popular in many international markets such as Australia, India, Myanmar, Indonesia, Laos … We have been collaborating with many reputable partners: Samsung, Toyota, Thyssenkrupp, Mitsubishi…
Read more about Chinh Dai’s Special Products.
Quality management system: ISO 9001:2008 International Standard.
Product quality: JIS G standard, ASTM International Standard, AS/NZS International Standard.
Technology: seven technology steps, modern machinery and advanced technology are applied to save energy and reduce production costs.
Production process: closed from raw material to final product. Each production stage uses a separate and modern production line.
Production capacity: 500MTS per day.
Read more about Chinh Dai’s Quality Certification.
Chinh Dai’s production process is closed from raw material to final product.
Product lifetime: more than 20 years.
Surface Treatment: chromated layer. The products are ensured to be shiny and beautifully patterned. The zinc and chromate layer is thick and resistant to corrosion and able to protect the inner layer for a longer period.
Payment: Letter of Credit, T/T.
Packing: Steel pallet for Purlin.
Delivery time: 30 – 60 days after receiving the deposit (depends on distance).
Communication: 24/7 hotline, 24/7 email, fax, website. Your inquiry will be replied in 24 hours.
Consultancy: free 24/7 consultancy. Your private information will be protected.
Read more about Chinh Dai’s Exporting Channel.