8.5. Flat Seam Roofing

Description: Flat seam roofing systems are typically used on roofs that are flat or have a low pitch or alternatively on very steep slopes. They are also used on curved surfaces such as domes and barrel vaults.

Flat seam roofing is constructed of 18" by 24", 20 oz. rectangular cold rolled copper sheets. Two adjacent sides of the sheets are folded over and two are folded under to form 3/4" locks. Copper cleats of 16 or 20 oz. cold rolled copper are installed in each of the longitudinal and transverse seams. The finished pans are interlocked longitudinally and transversely, with staggered transverse seams. For applications of this system to wall cladding, see Section 12.7. Horizontal Flat Lock Systems.

Where slopes are 3:12 or less, seams are soldered, see Roofing Systems - Special Roofing Design and Installation Considerations. The edges of the sheets should be cleaned, wire brushed, fluxed and pretinned to a width of 1-1/2" before folding. After locked seams are engaged they should be malleted or dressed down and thoroughly sweated full with solder. For furnace heated coppers, each pair of coppers should weigh at least 5 pounds. Heavier coppers retain heat better, typically resulting in more efficient soldering of flat seam joinery. The head of a torch-heated soldering copper should weigh at least 3 pounds.

To accommodate the accumulation of expansion, roofs over 30 feet in the direction of continuous, longitudinal seams should be divided by expansion battens. These tapered battens should be spaced no more than 30 feet apart. They are covered with 20 oz. copper sheets in 8 to 10 feet lengths, locked and soldered together. See Detail 8.5E for additional information.

Special Conditions: Where roof slopes are greater than 4" per foot, seams may be filled with a rubber or synthetic based sealant instead of solder. See Roofing Systems - Special Roofing Design and Installation Considerations.

Decking Requirements: Nailable deck or nailing strips.

8.5A. Standard Roofing Square

Shown are the typical roofing square and the formed pans. The corners of the sheet are clipped before the edges are folded.

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8.5B. Typical Flat Seam Roof

This drawing illustrates the overall concept of flat seam roofing. The longitudinal seams are typically continuous, whereas the transverse seams are staggered. Each pan is cleated to the roof deck. The pans should be installed so that water always sheds from one pan to the underlying pan.

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8.5C. Edge Detail

A typical edge detail is shown. This design allows water to shed from the roof at this edge. Other methods, preventing this flow, use battens similar to the gable rake detail for batten seam roofs (Detail 8.3A).

figure detail

8.5D. Eave and Gable End

This detail shows the 20 oz. copper rake or eave sheet. The lower edge of this sheet hooks over the edge strip. The upper edge is locked and soldered into the typical roof sheets.

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8.5E. Expansion Batten

The tapered expansion batten shown is typically 3" wide at the top and 2-1/4" at the bottom. The height should be at least 1-1/2". The 20 oz. copper cover sheets are continuous from 4" on one side of the batten to 4" on the other. They are bent at right angles where they meet the tapered batten, leaving some room for movement. These sheets are locked and soldered into the flat seam roofing.

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8.5F. Detail at Valley

The copper flashing is laid into the valley and its upper edges held by cleats spaced no more than 12" O.C. The flat seam roofing laps the flashing at least 6". Two methods are shown for engaging the lower edge of the roofing. The one on the right uses a continuous lock strip soldered to the valley flashing. The one on the left uses a double fold in the valley flashing.

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8.5G. Hip Detail

The upper ends of the copper roofing sheets are turned up to form a 1" high standing seam. Two cleats are used, at the top, to secure each pan. A 20 oz. copper cap covers the hip joint.

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