From For a long-lasting repair of those annoying gutter leaks, turn to silicone sealants. Attach the end caps using the caulking and riveting techniques shown in Photos 1-3. Since I adopted this method, all my end caps have remained firmly attached to their gutters and watertight.
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1. Pry off the end cap and chisel off all old gutter sealant from both the end cap and the lip of the gutter. Use a straight-slot screwdriver that’s narrow enough to work down into the cap’s groove. Wipe them clean. Restore any bent adjoining metal edges with a screwdriver and pliers. Dry-fit the parts to make sure they will join properly.
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2. Fill the groove in the end cap with clear silicone caulk. Attach the end cap to the gutter by pressing the cap solidly against the lip, making sure the two are firmly seated together.
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3. Maintain steady pressure on the end cap and drill three 1/8-in. holes as shown through the cap’s flange. Then install the screw and two 1/8-in. dia. rivets. Wipe away excess silicone on the outside before it dries.