Mechanically Seamed Standing Seam (Single and Double Lock)

In a mechanically seamed standing seam roof, neighbouring panels are first engaged loosely over concealed clips, and then a hand tool or electric seaming machine is run along each seam to fold the metal edges around each other. The fold locks the panels and the clip tabs together.

Single lock and double lock

• Single lock (90 degree): the seam is folded once. It is quicker to form and easier to undo but less resistant to water standing against the seam.
• Double lock (180 degree): the seam is folded a second time, producing a compact, multi-layer joint. A typical copper specification calls for a "5-ply" double lock finished about 1 in (25 mm) high. Double-lock seams are the usual choice for low slopes and for areas with heavy snow or wind-driven rain.

Traditional European zinc and copper roofs use the same principle, with the double-lock standing seam (sometimes called a double welt) formed by hand tools; an angled variant, the angle seam, is folded only part-way and is mainly used on steeper roofs and facades.

Advantages

• High weathertightness because the folded, locked seams resist water entry without relying on sealants alone.
• Strong resistance to wind uplift: the folded seam grips the clips firmly, and many engineered systems with mechanical seams achieve high uplift ratings in tests such as ASTM E1592 or UL 580.
• Suitable for long panels with sliding clips.

Trade-offs

• Mechanically seamed systems are generally the most expensive standing seam type because of the extra labour and equipment.
• Seaming tools must match the panel profile.
• Repairs require unseaming and reseaming.