How to Repair a Leaking Flue Pipe

What You'll Need
High-temperature silicone sealant
New rain cap, if damaged
New vent stack, if damaged
Tin snips
Utility knife

In a gas-heated home, a flue pipe vents the gases out through the roof. Like all installations that poke through the roof, sometimes a flue pipe develops leaks that result in water leakage into the attic or roof space when it rains. A number of points may fail in the flue, but they are mostly easy to detect and fix. For more difficult jobs, you will need to employ a roofer or licensed contractor. 

Step 1 – Find the Cause of the Leak

Occasionally the water vapor from the gas combustion causes condensation within the pipe, but this is a small problem that is rarely noticed. Most often the fault lies with faulty installation or corrosion above the roofline.

The best way to find a leak is to go into the attic or roof space when it rains and see where the leak comes from. You will probably find a rivulet coming down one side of the flue pipe, or you may discover drips coming off an elbow joint. If this is the case, a leak may exist at the flashing.

After the roof is dry, get on top of the roof and check for rust and corrosion. This should be easy to see if the problem is not at the flashing join. Then the problem will often be in the storm collar. 

Step 2 – If the Problem Is in the Storm Collar

The storm collar is placed immediately above the roof flashing. Scrape off the old sealant with your utility knife and raise the storm collar. Inspect the pipe junction to discover any damage there and scrape off any remaining sealant. Sometimes you can remove this sealant easily, and sometimes it is difficult.

Slide down the collar and reseal with the sealant. 

Step 3 – If the Problem Is in the Rain Cap

If you see significant rust at the rain cap, you will need to replace it. Take off the rain cap and inspect the pipe underneath. If the pipe is rusted, you should also replace that section.

Remove the cap by cutting the sealant/caulk and cutting the clips. You will probably have to disconnect the pipe at a joint in the attic. To disconnect it, slide the collar up and jiggle the pipe until it comes free. Cut the sealant at the flashing and storm collar and remove the lot. You will probably have to pull the old pipe down, not up.

Reinstall the new flue pipe the opposite way that you removed it. Remember to seal all the joints. 

Step 4 – Check the Flue Pipe Again

Enter the ceiling just after rain to check that the new flue pipe is not leaking. 

If you do find a leak, the causes could vary. Often it is the slope of the pipe that is at fault. Or the flue pipe may be too wide, a factor which causes water to condense and leak at a joint. Wind can cause the vents to bend and the joints to come loose, another condition which allows water in.