How to Seal Roof Flashing Leaks

Flashing is the unsung hero of your roof, acting as the critical transition point where shingles meet chimneys, vents, or walls. When these metal strips pull away or the caulk dries out, water finds an immediate path into your attic or wall cavities. Because these areas see constant expansion and contraction from temperature changes, the seal is prone to cracking over time. Repairing flashing isn't about replacing the entire assembly, but rather maintaining the integrity of the barrier. A successful repair leaves the flashing tight against the structure with a continuous, bead-free bond that sheds water away from the junction. If you catch these leaks early, you can prevent significant water damage and wood rot in your roof deck.

  1. Strip debris and prime. Use a stiff brush to remove all moss, loose shingle granules, and dried-up old caulk from the flashing edge. Ensure the surface is completely dry and free of debris to allow the new sealant to stick.
  2. Assess flashing integrity. Look for rusted-through metal, lifted nails, or deep gaps where the flashing has pulled away from the wall. If the metal is punctured or severely rusted, it must be patched with a flashing membrane rather than just sealant.
  3. Resecure separated flashing. If the metal flashing is flapping, secure it back against the wall using roofing screws with rubber washers. Drive the screws into the existing nail holes if possible, or drill new holes slightly offset from the old ones.
  4. Lay continuous sealant bead. Load a tube of high-performance polyurethane roofing sealant into your caulk gun and cut the tip for a medium bead. Apply a steady, consistent line along the top edge of the flashing where it meets the building material.
  5. Shape and smooth sealant. Use a small plastic trowel or a gloved finger to press the sealant firmly into the gap and smooth it out. You want a concave profile that directs water away from the joint rather than letting it pool.
  6. Verify cure and seal bond. Allow the sealant to cure for at least 24 hours before any anticipated rain. Conduct a final visual inspection to ensure there are no gaps where the sealant failed to bond to either surface.