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Chimney Leaks: Many Roof Leaks Start Here

Chimney leaks are caused by problems with the flashing around the chimney. It is estimated that improper flashing around things that penetrate roofs, like chimneys and skylights, cause 99% of roof leaks!

Poor Installation

It’s a well-known fact that the cause of a chimney leak can often be traced back to the initial installation. There are a number of reasons that the installation may not have been done properly:

  • A failure to understand that tar and underlayments are only secondary sources of protection and should never be relied on as a primary waterproofing solution.  Only properly installed flashings and correct roofing materials will create a long-term water shedding system that will provide adequate protection from chimney leaks.
  • A lack of technical experience or the hands-on skills to deal with the rest of the vertical exterior coverings, such as stucco, vinyl siding, wood board, etc.
  • The Alberta Building Code is silent on the issue of flashing, so many contractors do not have the knowledge or the training to install the flashing properly.

Checking for a Chimney Leak

If you have a roof leak, it can be a challenge to locate the exact source of the problem. If you suspect you have a chimney leak, here are a few things to look for:

  • Each piece of flashing must extend a bare minimum of 4 inches away from the penetration onto the roof deck. If this isn’t the case, you may have chimney leaks.
  • The back pan, which is the flashing on the back of the penetration, is by far the most vulnerable part of the chimney flashing because it creates an area where water and snow accumulates for long periods of time. A proper back pan exit is essential to chimney leak prevention because it draws water away from danger zones.
  • Chimney flashing must be tied in properly with the exterior finishing on the chimney. It’s extra expense and effort, but without this step, you will not have a functioning waterproofing system. For the most part there are 5 types of chimney exteriors that roof flashing must be tied into:
    • Brick Chimney: It is very important that brick chimneys have a mortared-in counter-flashing on the outside of the brick where the brick can be serviced, repaired, or replaced.
    • Vinyl Siding Chimney: Vinyl has holes drilled into it for ventilation, so it is crucial that your installer understands how the flashing ties into the building envelope.
    • Wood Board Chimney: Wood board tends to crack and split, so again, the building envelope is essential.
    • Stucco Chimney: It is very important to understand that stucco does not waterproof a home. It is basically sand. When re-flashing a stucco chimney, the stucco will have to be redone at an extra cost.
    • Cinderblock Chimney: These are a special problem because a complex counterflashing called a gumlip must be used to create water shedding.

See before and after photos of chimney flashing here.