How do I fix a leaking gutter seam?

A leaking gutter seam is almost always failed sealant at a joint: an end cap, a corner mitre, an outlet, or the joint where two sections meet. The fix is to strip out the old sealant, dry the joint, and lay a new bead of gutter sealant over the seam from inside the trough, then let it cure before water runs through it. It is a one-ladder job on a single-storey eave. If the joint has pulled apart, the gutter is sagging at the seam, or the eave is higher than the first storey, it is a repair for a pro.

Where gutter seams are, and why they leak

Sectional gutters are the ones bought off the rack. Amerimax's installation sheet lists gutter in 10-foot lengths, joined with seamers or slip-joint connectors,^[3] and it calls for sealant on the end caps, the inside mitre joints and all connecting points.^[3] For its 6-inch gutter, the 6-inch sections connect by cutting 3 inches off the top lip and sliding them together,^[3] which leaves an overlap that depends on sealant to stay dry. Seamless gutters are formed in one piece per run, so their only seams are at corners, end caps and outlets.

Sealant fails for a handful of reasons. It ages and shrinks in sun and frost. The metal expands and contracts every day, which works a joint loose over the years. Leaves and grit hold water against the seam. And where a gutter has lost its slope, water pools at the low joint instead of draining away. Gutters should slope at least 1/16 inch per foot of run toward the downspouts,^[4] so a seam that sits in a puddle after rain is a sign to check the slope as well as the sealant.

Choosing a sealant

Use a sealant made for gutters and matched to your gutter's material. OSI's gutter sealant, for example, is designed for sealing aluminum, galvanized steel and vinyl gutters and downspouts, including seam joints and cracks in gutter troughs,^[1] and its data sheet says it is not suitable for use on copper, rubber, silicone or butyl rubber.^[1] Bostik describes its Gutter & Seam Sealer Pro as particularly effective for sealing gutter lap joints, end caps and slip joints.^[2] Copper gutters need a product whose label names copper.

Read the temperature range on the tube before you pick a day. OSI says to apply it between 40°F and 120°F,^[1] while Bostik's hybrid polymer sealant can go on between 0°F and 140°F.^[2] Either way the joint has to be dry.

One tube goes a long way. Bostik estimates that a 9.5-ounce cartridge covers about 24 linear feet with a 1/4-inch bead and 11 linear feet with a 3/8-inch bead,^[2] far more than one leaking corner needs.

Getting the sealant to stick

Most repeat leaks come from sealant laid over dirt or old caulk. OSI's directions say all surfaces must be clean, dry and free of old caulk, grease, dust and other contaminants.^[1] In practice that means scraping the old bead out with a putty knife, brushing the joint to bare metal, and giving it time to dry fully after you rinse it.

The same sheet says to cut the nozzle at a 45-degree angle to the bead size you want, with 3/8 inch recommended,^[1] and that joints deeper than 3/8 inch need a backer rod.^[1] Lay the bead inside the trough, over the seam, in one continuous pass, and work it into the joint and up the sides. Whether to smooth it differs by product, so follow your tube's label.

Give it time. On OSI's sheet, the sealant skins over in about 30 minutes and reaches full cure in 7 to 14 days, and low temperatures and high humidity stretch both.^[1] A few dry days in the forecast help.

Sealant does not fix a slope problem. Bostik's sheet notes that its product is not for constant water-immersion applications,^[2] which is what a seam at the bottom of a sag lives in. If water stands at the joint, see how to adjust gutters so they drain properly before or after you reseal.

Staying safe on the ladder

Gutter seams are ladder work. Virginia Commonwealth University's ladder program says to pull the base out so its distance from the wall is one-fourth the height to the point of support,^[5] and the top should extend at least 3 feet above the eave, gutter or roofline.^[5] It also calls for no ladder work outdoors in winds over 20 miles per hour.^[5] Lean the ladder on the wall, not on the gutter, and move it rather than reaching.

Look up before raising a ladder. Virginia's Overhead High Voltage Line Safety Act forbids work that puts a person or a ladder within 10 feet of any overhead high voltage line,^[6] and the service drop to the house is often near a corner.

Gutter sealants are solvent products. OSI labels its white and aluminum versions flammable and says not to use or store them near heat, sparks or open flame.^[1] Wear gloves and keep the tube away from anything lit.

Stop and call a pro if

• The gutter is above the first storey, or the only way to reach the seam is from the roof.
• The joint has pulled apart or the sections have shifted, so it needs new rivets, screws or a connector rather than sealant. That often goes with a gutter pulling away from the house.
• The metal at the seam is rusted through, split or badly dented.
• Water runs behind the gutter onto the fascia, or the fascia is soft, which involves the roof edge.
• Most of the seams are leaking, which is a sign to weigh repairing or replacing the gutters. In Virginia, a Class C licence covers construction, removal, repair or improvement jobs with a single contract over $1,000 and under $30,000,^[7] so check the licence of anyone quoting a job of that size.

For more on gutters, roofs and eaves, browse the roofing answers, gutter repair on Loclis, or exterior pros around Blacksburg. If the seam is out of reach or keeps failing, describe the gutter repair once and licensed local businesses quote it.