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Purlin Roof Structure

In architecture or structural engineering or building, a purlin (or purline) is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls, steel beams etc. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and both the ancient post-and-beam and newer pole building timber frame construction methodologies.
A section though lightweight timber frame construction showing the position of under purlins. Credit: Bill Bradley of
Figure 1
In structural steel or aluminum construction, purlins, usually W shapes or channels, transfer roof loads to the major structural elements supporting the roof; the type and spacing of purlins is a design consideration which depends upon the incident roof loads as well as the limiting lengths of sheeting to be used.
Ultimately, local building codes should always be understood and applied in the design of a structural roof system. The following are general rule-of-thumb practices in the layout and design phase of a roof system.
In roof design, Roof purlins supporting metal sheeting are frequently proportioned for a ratio of depth-to-length of 1/32. Other materials, unusual loadings, or deflection requirements must be investigated by the designer.
Corrugated metal deck and siding is used for roofs and walls, respectively, to span between purlins for roof loads or between girts for wind loads. The decking is sized to resist the bending caused by these loads. Roof decking is often also used as a diaphragm to transfer wind or seismic loads to the lateral bracing system below. Load tables specifying safe loads for different spans are available from metal deck manufacturers.

 The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft. Numbers 20 and 22, U.S. Standard gage, are generally used for roofing; No. 24 for siding.

When channels are used, the ridge purlin is placed as close to the peak of the truss as possible in order to shorten the connection to the purlin on the opposite side of the centerline (see Figure 2). This also serves to decrease the overhang of the roof sheeting where it extends beyond the purlin to the ridge.


Typical connections between ridge purlins.
Figure 2

Sag rods are usually furnished to transmit the gravity load of girts to a supporting member. Additionally, sag rods are used to control the deflection of and stiffen girts and purlins. Typical sag rods are 5/8-in. or 3/4-in. in diameter with lines spaced approximately six to right feet apart.

To be effective, the force in the sag rods must be carried across the roof ridge and must be balanced by a corresponding force on the opposite side of the ridge. Several ridge purlin connections are illustrated in Figure 2. Ridge purlins also are fastened together at other points along their length to increase their transverse stiffness, and thus permit them to be more effective if also used as struts.
In lightweight timber roof construction under purlins were used to support rafters over longer spans than the rafters alone could span. Under purlins were typically propped off internal walls. For example, an 8 x 4 under purlin would support the center of a row of 6 x 2 rafters that in turn would support 3 x 2 roof purlins to which the roof cladding was fixed.

In traditional timber truss construction purlins rest on the principal rafters of the truss.

It is the practice in the steel industry that structural shapes are assigned representative designations for convenient short-hand description on drawings and documentation: Channel sections, with or without flange stiffeners, are usually referenced as C shapes; Channel sections without flange stiffeners are also referenced as U shapes; Point symmetric sections that are shaped similar to the letter Z are referenced as Z shapes. Section designations can be regional and even specific to a manufacturer. In steel building construction, secondary members such as purlins (roof) and girts (wall) are frequently cold-formed steel C, Z or U sections, (or mill rolled) C sections.

Cold formed members can be efficient on a weight basis relative to mill rolled sections for secondary member applications. Additionally, Z sections can be nested for transportation bundling and, on the building, lapped at the supports to develop a structurally efficient continuous beam across multiple supports.