Tips for a Leaky Chimney from your Local Chimney Sweep
Do you have a chimney that leaks but, does not leak all of the time?
Once the roof has been ruled out - if it leaks all of the time,every time it rains- then you have a seal broken on the flashing itself or the 'storm collar'. If it leaks right on the edge of the chimney itself and drips to the wood stove- then you know it is the storm collar seal. If it leaks on the area surrounding the chimney support box- it is the flashing seal or the surrounding shingles.
Then, it gets a little more complicated. If it leaks from the support box, but not necessarily when it rains then you probably have a condensation problem. The temperature difference between the outside and the inside of the house is causing the moisture in the home to condensate on the surface of the support box. If that is the case, it can happen in summer or winter and is especially aggravated by air conditioning.
Some chimney manufacturers allow insulation to be added to the support box to help prevent this. Those that do -allow only their insulating products to be used on their chimney. Manufacturers never allow insulation from another brand to be used with their chimney. There are good reasons for this. The most important is that their chimney has not been tested with that insulation and a fire could be caused by its use. This is the basic principal that installers must adhere to: Do not 'Mix & Match brands'. It is questionable how effective the added insulation is at preventing condensation ( with the models that allow its use ). Most people just learn to live with this, usually minor leaking. In fact, some people don't even notice that it is happening.
Then, there is the last type of leak. The most allusive and confusing, even baffling. I have had many long-winded emails about this phenomenon. This is my inspiration for writing this piece actually. I have seen homeowners replace their flashing only to have the mysterious leak return! You see, chimneys today are required to be ventilated at the support box / roof line area. This is the purpose of the 'storm collar', the little 'cone' sitting just above the top of the flashing. The gap under the storm collar allows air passage into the top of the flashing. What can happen is if you live beside a lake or any large open field where a wind gust can carry rain side-ways toward the chimney. Rain gets up, under the storm collar and into the top of the flashing. It can also happen it the roof is steel and very steep. In that case, during heavy rain the rain drops will actually bounce off the roof and up, under the storm collar. You know for sure if you have this kind of leak because it only happens during heavy rain and / or wind. If it does not occur each and every time it rains then this is the cause.
When I was figuring these things out I just had to ask. I emailed the inventors of modern factory built chimneys to see what they would say on the topic.
I will end with my email to the Selkirk Technical dept. And their response.
I install your products and like them very much.
I have had occasions were customers have said that their chimney flashing have leaked.
It has only happened during heavy rains (not mild rainfall) and really hard winds at lakeside cottages. I have come to the conclusion in these cases that wind must carry the rain under the storm collar contacting the chimney. Have you ever heard of this or do you have literature that I could refer to my customers in this situation? I am sure that no chimney systems are guaranteed- no leaks in every situation.
Regards, Ian Myers
Thank you for contacting Selkirk. Yes there are circumstances when rain water will find it's way under the storm collar and into the flashing. Nothing can be done regarding these situations. The vents must be kept clear to let air circulate around the chimney. Thank you.