Graphical Construction Glossary >> Roofs and roofing. >> Roof Framing >> purlin bridging

Purlin bridging

Lightweight roll formed metal channel sections with proprietary end fittings used to stiffen Cee and Zed purlins

Also known as "Hook lock" bridging and by other trade names.

Steel purlins are designed to carry load in one direction only, that is at right angles to the roof plane. They can not be allowed to deflect from that line. Bridging is a method of stopping deflection and thereby increasing the carrying capacity of any particular section.

For instance an engineer will look at his load tables and deem that a purlin can span 3600 with no bridging, with one central roe of bridging the same purlin will be able to span 3900 and with two evenly spaced rows it will span 4200.

In some cases the stiffening may be done in other ways. The top edge of the purlin is said to be held in place by the roof sheeting screwed to it. If a ceiling is fixed to the underside of the purlins then bridging is not needed as the ceiling battens hold the purlins from deflecting under load. Similarly flat straps (say 40x3.0) screwed to the underside of the purlins can also be used.

Bridging is by far the quickest and most common method though. In the hook lock system holes are pre punched in the purlins and the bridging pieces are made to size at the factory. The hooks on the ends of the bridging pass through the holes and engage each other.

Here is one of the various accessories available. This an extension piece that allows odd spaced bridging to be made up on site.

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