A leaky chimney is mistaken for a leaky roof so often that even roofers recommend chimney inspections. While it can be very difficult to determine the source of a leak, the chances improve when professionals with lots of experience look for them. Homeowners willing to get up on their roofs — who can safely do so — can definitely try, however.
By closely looking at a few things, you may be able to spot the obvious. The place to start is with the components intended to keep water out. The chimney cap and crown, and the mortar between bricks or roof and flashing, are the places to start. They exist to prevent water from entering the chimney, so if it got in, one or more of them is probably implicated.
Along with those, some of you will also need to check the waterproof seal applied to your chimney because nothing is impervious to water and time. Once you start to really look, a few things might jump out at you, like a missing chimney cap. It happens more often than you probably imagine, as wind, birds, and animals can all take a lightweight or poorly designed chimney cap.
If there is no cap on your flue and your damper is open, you may have just found the source of the “leak”, but do not stop there. Really examine the chimney crown, which is the cover for the chase around the flue. Look for cracks, crumbling, chips, stains, mold, — anything which permits water to enter or is evidence of it.
The crown is supposed to send water away from its own center as well as the chimney. If mold is on it, it is not doing its job right. So, even if you cannot see significant cracking in it, something is wrong. If you do see new cracks, you may have found another explanation for a leaky chimney. However, this is probably the time to call in a chimney sweep to take a look and repair it properly.