Metal flashing is installed around the base of the chimney to prevent roof leaks. Water runs down the slope of the roof and it will find a way to get into small cracks and gaps between the chimney and the roof. If caulking seals are broken around the flashing, water will leak through them and eventually drip down on your ceiling. Repairing these leaks takes a few hours and a diligent approach.
Inspect the flashing around your chimney for cracks, breaks or gaps between the flashing pieces. The gaps are not always easy to see. Even small gaps and cracks will let water in.
Inspect the corners of the flashing near the roof side edge of the flashing material. At this point the flashing is bent backward to form a corner and fit around the chimney securely. Here the flashing is extra susceptible to leaks, as the caulking often fails, shrinks or weathers in the sun.
Remove damaged and weathered caulking around the edge of the flashing at the roof seam and around the chimney with a razor. Peel away the flashing by hand and brush the surface area clean with a wire brush.
Apply a new bead of roof caulking around the edge of the flashing. Cut the caulking cartridge nipple with a knife so the opening is at least 1/2 inch or larger to allow the largest possible bead. Use a continuous bead of caulking for a tighter, more waterproof seal. Allow the caulking to dry overnight, and inspect your repair for signs of cracks, air bubbles or holes that may eventually lead to leaks.
Re-caulk the flashing again if necessary to seal it properly. Alternatively, apply a roof flashing sealant over the flashing with a paintbrush to seal out water.
Copper flashing is more expensive but lasts longer and accepts a weld, so the corners are more watertight.