Detailed Answer
Leaking gutter joints are among the most repairable gutter problems. Most arise from original sealant that has dried, cracked, or pulled away after years of thermal cycling — not from any structural failure. Correct repair seals the leak for 5–10 years.
The repair sequence: first, identify all leaking points by running a hose through the gutter and watching from below. Mark each leak location. On the same day as repair or the day after, make sure the area is completely dry — no moisture in the seam or adhesion will fail within weeks. Use compressed air or allow 24–48 hours of dry weather before proceeding.
Remove all old sealant from the interior face of the joint. A plastic scraper or stiff brush removes most dried material; isopropyl alcohol wipes clean any residue. The metal surface must be bare for new sealant to bond. Apply gutter-grade butyl rubber sealant (not ordinary silicone caulk, which doesn't adhere well to metal) to the full interior seam length. Tool it flat with a gloved finger or putty knife and allow it to cure before running water through the gutter.
If the joint gap is wider than 1/4 inch — typically from a section of gutter that has shifted or pulled away at the connector — sealant alone won't hold long-term. The connector clip or sleeve joint needs to be reset and re-secured before resealing. On sectional (non-seamless) gutters, this is straightforward; on homes with sectional gutters over 20 years old, repeated joint failure across multiple locations is a sign that replacement with seamless gutters is the practical long-term fix.
Seamless aluminum gutters eliminate most joint leak risk. Joints only exist at inside corners, outside corners, and end caps — not along the gutter run itself. Homes with older sectional gutters that keep developing new joint leaks are often better served by full replacement than ongoing patching.