Standing Seam Roof Systems

September 14, 2014

Our clients often ask us about the difference between a standing seam roof system and a “through-fastened” also known as a “screw-down” roof system.

Through-fastened roofs are attached directly to purlins, usually by self-tapping screws or self-drilling or lock rivets. A standing seam roofing on the other hand, is connected indirectly by concealed clips formed into the seams. Though the through-fastened lapped-seam roofing is inexpensive, straightforward and easy to erect, it is penetrated by fasteners and hence susceptible to leaking.  The fasteners also prevent the sheets from thermal expansion and contraction. Repeated expansion and contraction may tear the metal around the connecting screws and eventually lead to leaking. Hence the width of building with through-fastened roofs should not exceed about 18m.

Standing-seam metal roof consists of metal panels running vertically on the roof deck. Each panel has two seams that stand up vertically and are snapped or crimped together to seal the joint, thus avoiding penetration of the roofing. A standing seam also avoids water from collecting on the surface. A factory applied sealant is normally placed in the female corrugation of the seam. To accommodate expansion and contraction, the panels are attached to purlins by concealed clips that permit the roof to move

The biggest disadvantage of the standing-seam roof is that it provides no lateral bracing to purlins and offers little diaphragm action. Hence a separate system of purlin bracings and a separate horizontal diaphragm structure are required. Moreover, standing-seam roofing is best suited for rectangular building layouts only. Though they may be expensive initially, life-cycle cost calculations may prove them to be economical in the long run.

Article Source: “Pre-Engineered Buildings” Selection of Framing System, Roofing and Wall Materials.” Masterbuilder, July 2008 Issue.

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