The fact that your chimney protrudes from the roof, makes it vulnerable in many ways. Homeowners often have issues with moisture inside their chimney flue. This can start off in a number of places and eventually reach the bottom of your fireplace. Moisture itself usually isn't an issue except when it starts to cause corrosion to the interior walls of your flue. It can even become so bad that you end up with structural damage if left untreated. The first step in fixing the problem is finding the source. This article explains how to best locate where the moisture is entering your chimney flue.
Check the Flue Cover
Unfortunately, you will probably need to get up on the roof to thoroughly check your chimney. Everything might look great when you look at it from the ground, but you need to get up close. The first and most obvious thing to check is the chimney cover. The entire purpose of the chimney cover it is to prevent debris and moisture from falling into the flue. Since the cover is so visible, it is often decorative. Also, covers are commonly made out of thin metal sheets, which can rust over time. Whether your cover is damaged or just knocked out of place, it is a good idea to immediately change it to fix your problem. Chimney covers are usually inexpensive and easy to replace.
Checking the Flashing
You also need to check the flashing where the chimney meets the roof. The flashing is particularly important on slanted groups, especially along the top edge because of the angle. The angle between the roof and the chimney can cause water to pool up. Standing water is never good when it comes to the integrity of your flashing. The most common problem with flashing around the chimney is the fact that it peels away from the roofing material. This is often due to the fact that the roofing material is rough and the glue bond deteriorates over time. Changing flashing is a little more complicated than replacing a chimney cover. However, it is affordable, and with a few basic tools, you can probably fix it yourself. Otherwise, you should call a roofing professional to fix your issue.
At the very least, getting up on your roof and inspecting your chimney should enable you to find the source of the problem. If it doesn't seem to be the flashing or the chimney cover, you should call a chimney repair expert such as Clean Sweep for a professional opinion.