How to Seal Chimney Flashing

What You'll Need
Extension ladder
Masonry Trowel
Silicone caulk w/caulking gun
Flashing cement

Chimney flashing is the layer of sheet metal that connects from the roof to the chimney, designed to be the seal that prevents water from entering your home. Most often, when a chimney leaks, the leak originates in that spot. If you find water leaking through your ceiling, you'll need to repair it quickly before more serious damage occurs. 

Step 1 - Setting Up

Since you're going to be going up on your roof, you'll want to wait for a clear day. If your ladder doesn't have hooks at the top that can attach to the roof and hold itself in place, you'll need someone to stand at the base and keep the ladder steady. It may be a good idea to wear a safety harness attached to a secure spot on the roof, depending on how high in the air you'll be. It's easier to attach a rope to a bucket or toolbox and pull all your supplies up to you, rather than make multiple trips. 

Step 2 - Stripping the Old Seal

Before you can begin applying the new sealant, you'll need to strip off all the old sealant, if there is any. Peel and scrape off as much as you can so you can get an accurate view of the flashing underneath. 

Step 3 - Refitting the Flashing

Before you apply any caulk or flashing cement, you want to make sure the flashing is secure and none of the pieces are loose. Flashing is basically pieces of sheet metal layered over each other to create a seal, identical to shingles, so they need to be tightly layered. Use your hammer to tap any loose pieces back into place, being careful not to hit too hard; it is only sheet metal, after all. 

Step 4 - Applying Caulk

Once each piece of flashing is as secure as it can be, apply the silicone caulk. You want to squeeze it into all the nooks and crannies you can. Put a layer on the edge of each piece of flashing. You can't use too much, so get as much as you can in there. 

Step 5 - Applying Flashing Cement

You'll need to give the caulk a little time to dry so it and the flashing cement don't smear into each other. Caulk doesn't take very long to dry, so you'll be able to apply the cement in short order. Once the caulk has dried enough on the surface to maintain some solidity, apply the flashing cement with the masonry trowel. How much you spread depends on how thorough you want to be. You can spread a layer over the edges of the flashing, where each edge meets the roof and chimney, or you can coat the entire area. Either way will be effective as long as the flashing cement is thick enough. You can place a tarp over your chimney until the cement is dry, butr it's not really necessary.