When installing a Class A chimney in a multi-story home, you need to consider the ceiling height of each level. Is your ceiling flat, is there an attic above it, or is it a sloped ceiling with dead air space above?

The next aspect to consider is the pitch of your roof. The pitch is inches of rise per 12″ of run. For example: For every 12″ of horizontal, how many inches vertical? The most common roof pitch in North America is 6/12, which can be walked on easily. An 8/12 pitch is much more difficult to walk on, while a 12/12 pitch requires special equipment.

You will also need a floor to ceiling height where the appliance and chimney will be installed.

You will also need to take into account framing dimensions. What is the structure framed with? Are the rafters 2×4, 2×6, 2×8, 2×10, 2×12, or are you using trusses? Generally, roof rafters or trusses are installed vertically and rest on the sill plate of the exterior wall, reaching up at an angle where they meet their twin at the ridge. These are generally spaced 12″, 16″, 24″, or 36″ on center.

When it comes to framing, find the answers to these questions:

• What is your fafter spacing?
• What is your Joist spacing?
• Is the framing lining up or will you have to cut into the support?

Chimney Chase. When passing through a living area in a multi-story application, the living area must be sealed off from the area below and above it to prevent what is known as the “chimney effect” in the event of a structure fire. You can effectively seal off a living area using firestop radiation shields when passing through floors and ceilings. The chimney pipe should be sealed off if it is in a living space by installing it inside a chimney chase. This prevents possible burns or structure fires if flammable material is stored in contact with a Class A chimney, which requires a minimum of 2″ clearance to combustibles.

Offsets are important too. Sometimes, your framing just doesn’t line up in a way that allows you to get a straight shot up and out the roof without having a rafter or other structure member in the way. In many cases, you can simply cut out the obstacle and re-frame using bridging. Other times, the structure member is critical and cannot be removed, in which case you will need an offset. Offsets allow you to zig when you have a zag; in other words, you can go around the obstacle. A simple Plumb Bob will help you find the offset measurements.

Rules for offsets: You can only use 15 and 30 degree elbows with Class A chimney systems. The stove pipe can use 90, 45, 30, or 15 degree elbows in double wall stove pipe. Restricting the smoke path in any way will hinder the draft and create creosote buildup. With that in mind, try to use two 45 degree or three 30 degree elbows to create a sweeping effect in your offset. Avoid using 90 degree angles if possible, and do your offsets below the ceiling as it saves money, is easier to install, and doesn’t require additional support. If your offset is above any structural pass-through, such as a floor or ceiling, you must use a Class A chimney to prevent a fire from the radiant heat transfer. You must also properly support the Class A chimney from above the offset to prevent chimney separation, which results in hazardous conditions.

You must properly support the Class A chimney from above the offset in a multi-story home to prevent chimney separation and resulting hazardous conditions. In multi-story applications you must utilize firestops, attic insulation shields, and support brackets to meet UL listings per manufacture specifications and to insure you are in cooperation with your warranty.

Roof design. Most homes have “gable roofs” which look triangular in shape. Some roofs use hips instead of gables, making them “hip roofs”. Barn-style roofs are called “gambrels”. There are other styles as well, but these are the most common.

Your attic space matters, as well. Sometimes you will pass through an attic space as you extend the chimney out through the roof. Is there access to get into this space? If so, measure the area where the Class A chimney will pass through.

Finally, what is your roofing material? Most multi-story structures have shingled roofs. Shingles are easy to work with, especially when it comes to cutting a hole in them for fitting the roof flashing on (flashing prevents leaks). Some roofs are metal, and many people are afraid to cut a hole in a metal roof and properly flash it so it doesn’t leak. It’s actually easy than it looks, and a much better option than going up the side of your structure. We hope to have a video demonstrating this process soon, so keep checking this page to find that. Other roofs have slate or tile, and in those cases you will need special roof flashings that we highly recommend a professional install for you.

One final note: Pay careful attention to where your Class A chimney will penetrate the roof. If your roof has a valley near where you plan to put a chimney, we recommend finding another location. Valleys often cause leaking and are rarely worth the trouble to install a chimney around. Dormers are another issue to watch out for: You must be two feet higher than anything within 10′ of your Class A chimney, so please keep that in mind.