How Can Z Section Steel Overlapping Improve Purlin System Continuity?

A steel roof system fails at the purlin connections more often than at the purlin midspan. A single purlin may be strong enough to carry the design snow load, but if the purlins are butt-jointed at the rafter, the roof behaves as a series of simply supported spans. The deflection accumulates, the roof sheeting separates, and the end connections are overloaded. This is why many steel roof systems use Z Section Steel with overlapping ends. The Z shape allows one purlin to nest over the next at the support, creating a continuous load path. This guide explains how the overlap works and what to verify when specifying it.

1. Why Does Purlin Continuity Matter More Than Individual Member Strength in a Roof System?

A roof system carries wind uplift, snow load, live load, and the weight of the sheeting. These loads travel from the sheeting to the purlins and from the purlins to the rafter frames. If the purlins are simply supported at each rafter, the maximum bending moment occurs at midspan, and the maximum deflection occurs at midspan as well. The deflection of each span adds to the deflection of the adjacent spans, and the roof surface becomes uneven. If the purlins are continuous over the supports, the bending moment is redistributed, the midspan moment is reduced, and the deflection is controlled. The continuity also reduces the number of connections that must transfer the full load. In our factory, we produce Z Section Steel for purlin systems and supply the overlap details. Tianjin Shunchen Hongye Enterprise Management Co., Ltd. has been manufacturing section steel for over 18 years.

The key point is that continuity is a system property, not a member property. A stronger purlin with a butt joint is still a discontinuous system. A lighter purlin with a proper overlap can perform better because the load path is continuous.

2. How Does the Z Shape Allow One Purlin to Nest Over the Next at the Support?

The Z Section Steel has a web in the center and two flanges that point in opposite directions. The top flange points one way, and the bottom flange points the other way. This geometry allows the top flange of one purlin to sit directly on the top flange of the adjacent purlin at the support. The two purlins nest together, and the overlapping flanges can be bolted through. In a C section, the two flanges point in the same direction, so the sections cannot nest. They can only be butt-jointed with a connecting plate. The table below compares the two sections for purlin continuity.

The nesting also provides a natural alignment. When the purlins are nested, the top surfaces are flush, so the roof sheeting sits flat. If the purlins are butt-jointed with a connecting plate, the plate may protrude and create a high point in the roof. This is why the Z section is preferred for systems that require both continuity and a flat roof surface.

3. What Happens to Bending Moment and Deflection When Purlins Overlap Instead of Butt-Joint?

When purlins overlap, the system behaves as a continuous beam over the supports. The bending moment diagram changes from a series of positive parabolas to a continuous curve with negative moments at the supports and reduced positive moments at midspan. The deflection also changes. The maximum deflection of a continuous beam is typically 30 to 50 percent lower than the deflection of a simply supported beam of the same span and load. The table below shows the comparison for a typical purlin span of 6 meters under a uniform load of 3 kN per meter.

4. Why Does Overlap Length and Bolt Arrangement Determine Whether Continuity Is Actually Achieved?

The overlap only works if the overlapping flanges can transfer the negative moment and the shear without slipping. The overlap length must be long enough to accommodate the required number of bolts at the specified spacing. A short overlap with a single row of bolts may slip under load, and the system will behave as a simply supported beam despite the overlap. The bolt arrangement must be symmetrical about the web to avoid eccentric loading. The edge distance must be sufficient to prevent tear-out of the flange. In our factory, we control the hole position with CNC punching and verify the hole diameter and spacing with a gauge. Tianjin Shunchen Hongye Enterprise Management Co., Ltd. recommends a minimum overlap length of 1.5 times the purlin depth for light loads and 2.0 times the depth for heavy loads or high wind uplift.

We also recommend the use of a double row of bolts in the overlap zone. A single row of bolts can allow rotation of the overlapping flanges, which reduces the continuity. A double row of bolts provides a more rigid connection and distributes the load across the flange width.

5. How Should Designers Specify and Verify Z Section Steel Overlapping in a Purlin System?

The specification should include the section size, the steel grade, the plate thickness, the overlap length, the bolt diameter and grade, the bolt spacing, the edge distance, and the galvanizing requirement. The verification should include three checks. The first is the slip test of the overlap connection, which measures the relative displacement between the two purlins under the design load. The second is the deflection measurement of the installed purlin system, which should be compared with the design prediction. The third is the bolt torque check, which verifies that the bolts are tightened to the specified torque. In our factory, we perform the slip test on a sample of each production lot and provide the test report with the shipment. Tianjin Shunchen Hongye Enterprise Management Co., Ltd. also provides an installation guide that shows the correct nesting sequence and the bolt tightening pattern.

We recommend that the designer check the overlap zone for local buckling of the web and the flange. The negative moment at the support creates compression in the bottom flange, and if the flange is slender, it may buckle locally. The section should be selected so that the flange width-to-thickness ratio is within the limits of the applicable code.

Summary for Structural Designers and Roofing Contractors

Z Section Steel overlapping improves purlin system continuity by allowing the flanges of adjacent purlins to nest at the support. The nesting creates a continuous load path that redistributes the bending moment, reduces the midspan deflection, and lowers the total steel consumption. The continuity is only realized if the overlap length and the bolt arrangement are designed correctly. The specification should include the section size, the overlap length, the bolt diameter and spacing, and the edge distance. The verification should include a slip test, a deflection measurement, and a bolt torque check. Tianjin Shunchen Hongye Enterprise Management Co., Ltd. has been manufacturing Z Section Steel for purlin systems for over 18 years and provides full overlap design and verification support for our customers.