Solder and Sealants: Copper Gutter construction methods have traditionally relied on solder to ensure water tightness and to strengthen joints and seams.The solder used is common 50 50 tin-lead bar solder for uncoated copper, and 60-40 tin-lead for lead-coated copper. It is typically applied to mechanically fastened or formed, rigid joints. Soldered seams and joints are permanent; they should last the life of the copper. Continuous, long runs of soldered seams should be avoided to limit stress fractures.
In the weathering process, the lead contained in solder turns gray. Exposed solder in the finished joints can be minimized with the use of blind soldering. In this technique, solder is applied to the back or concealed edge of copper surfaces.
An alternative to solder, where its additional strength is not required, is the use of sealants. Sealant filled joints have been used successfully for standing seam and batten seam roofing applications where roof slopes are less than three inches per foot. Sealants can also be used in joints that are primarily designed to accommodate thermal movement of the copper.
The sealants used should be tested by the manufacturer and designated as compatible for use with copper. Many elastomeric polyurethane, silicone, butyl, polysulfide or other inorganic or rubber based sealants have shown acceptable performance. Acrylic, neoprene, and nitrile based sealants have been observed to actively corrode copper. The use of such sealants is, therefore, not recommended.