Some putting in a new chimney or even having an inspection may wonder about height of chimney, this is actually one that comes up quite often as many are simply not done properly.
How tall does a chimney need to be? Well in Canada it is a bit more complicated as many things are, which factory built stainless steel we need to follow the manufacturers directions which are often given in imperial measurements, however for masonry chimneys we follow the Building Code which is metric.
But they are still pretty close in their numbers so not that difficult.
Lets start with a stainless steel chimney. Now before we get started one more small thing, it has to be certified to the 650C standards which is ULC-S629. Anything else does not work and wood burning systems are not grandfathered – they either work or they do not.
For chimneys which are factory built and use imperial measurements we use the 3-2-10 foot rule, which you likely have seen before if you have researched chimney installation. Essentially the chimney must be a minimum of 3 feet above the roof at the shortest point where it penetrates the roof surface. And it must be 2 feet above any part of the structure which is within a 10 foot radius.
Calculation of this is fairly easy if you know the roof pitch – which is not hard to measure – or there are even phone apps which can get the pitch (if I use an app it is always best to round up and be on safe side for accuracy). If the pitch is 4/12, that means it rises four inches for every 12 inches of run or movement toward the centre of the building. Next you need to know how far you are from the highest point nearby. If you measure ten feet and get that point then it is a start; however sometimes you do not even need to do that. If the chimney is near or at the peak you may be okay with the 3 feet above penetration point and leave at that. However when I say near it needs to be potentially quite close depending on the pitch – for 4/12 given it would need to be no more than six feet away and for a 12/12 no more than one foot away! Many however do not fall into that so you do need the radius measurement. Say the peak is 12 feet away from chimney, well you only need to go to the 10 to meet the code .. so take 10 time the 4 (from pitch) and you get 40 which is in inches. Add 24 to that (for two feet above that point) and you get 64 inches which is minimum chimney height at the low side of the chimney. Note some try to measure the high side and cheat!
The image on the right shows an example of how to measure a chimney and determine if it meets minimum requirements. Note that you can always go higher but can not go lower! Generally speaking the taller you make the chimeny the better the draft. Keep in mind that if you do go above 60 inches above the roof you will need a roof brace installed to support the chimney.
Masonry chimneys are similar but use metric measurements and when we do inspections reports for masonry are always in metric to match the code. The Ontario Building Code section 184.108.40.206 Chimney Height above Roof states: (1) A chimney flue shall extend not less than (a) 900 mm above the highest point at which the chimney comes in contact with the roof, and (b) 600 mm above the highest roof surface or structure within 3 m of the chimney.
This is essentially the same as the previous with metric measurements as 900mm equals 35.43 inches (three feet equals 36 inches), 600mm equals 23.62 inches (two feet equals 24 inches) and 3 metres is 118.11 inches (ten feet is 120 inches).
If you have questions ask the WETT certified inspector who is onsite.