Even seamless gutters have seams. These seams are found at the end of each section of gutter where the end cap is attached, and in the corners where miters are used. These seams, no matter how expertly installed, are subject to leaks. Properly sealing these seams during installation can prevent or delay leaking. Once a leak is discovered, the seam can be easily resealed to prevent further damage.
Coat the inside of the piece of gutter that will be the bottom piece in the seam with gutter seal or silicone caulk. The bottom piece will be the miter, the end cap, or the run of gutter that is closest to the downspout.
Attach the two pieces of gutter together and press firmly on the seam to ensure that the sealant is distributed evenly. Secure the pieces together with pop rivets.
Apply additional sealant inside the gutter at the seam, pressing the sealant into the entire crevice. Pay special attention to the pop rivets or screws holding the pieces together.
Allow to dry completely. Drying time will vary depending on the type of sealant used. Then test the seam with a garden hose to be sure that it is properly sealed.
To correct a seam that has failed and is leaking, scrape away the old sealant completely and clean the area before attempting to seal the seam. New sealant applied to a leaking seam will not work, as most sealants used in gutter installations will not attach to themselves. First remove the old sealant. Then begin the process of properly sealing the seam.
When overlapping gutter, make sure that the gutter maintains a continuous flow toward the downspout. The bottom section of the seam should always be the piece closest to the downspout to avoid creating areas where debris will get hung on its way to the downspout.