How to Build a Chimney

Planning a Chimney

Consult your local construction codes when designing your chimney. While each community has its own codes for building chimneys, most codes are based on the National Fire Protection Agencies 211 standard. Go to the NFPA website to review national fire code associated with chimneys. The code is available online for free. This code should guide all aspects of the function of your chimney. Two general requirements of the 211 standard are that chimneys be at least 3 feet (90 centimeters) taller than the highest point where it goes through the roof of a single structure and 2 feet (60 centimeters) taller than any portion of the building or any adjacent building within 10 feet (3 meters) of the chimney. The taller the chimney, the better it drafts. A chimney also needs at least a 2-inch (5-centimeter) clearance from combustible material if it is built in or through the walls of a house and a 1-inch (2.5-centimeter) clearance if built alongside the house.

Decide what material the chimney will be made of. The most common material for a chimney is brick or masonry, but prefabricated metal chimneys are also widely available. If building a masonry or brick chimney, you will need to figure out how big it will be to estimate how many bricks to order. If using a prefab metal chimney, you will need to measure the distance from your stove, or other appliance, to the location where it is exiting the house. Masonry chimneys are usually required to have walls at least 4-inches (10-centimeters) thick. Standard chimney bricks should come in this thickness.

Determine what waste products the chimney is going to vent. This will determine whether your chimney can vent multiple appliances or if it will vent only one. It is possible to build chimneys for several appliances and connect them to a main discharge flue, sized to handle the combined output, provided the subordinate flues for the individual chimneys are not angled more than 30 degrees off vertical and they vent the same type of by-product. In other words, a chimney for a gas range can be combined with one for a gas furnace, but not a wood-burning fireplace. Flues also must not change in shape or size within 6 inches (15 centimeters) above or below a floor, ceiling or roof made of combustible materials.

Determine the width of the chimney flue. This is mostly determined by what appliance(s) the chimney is built to vent. Metal chimneys are usually round flue pipes, while masonry chimneys usually have square or rectangular flues. Each type has different diameter requirements depending on what they are venting. Consult local and national fire code to determine the diameter you should use. Masonry chimneys are typically lined with a clay or ceramic tile flue, which goes up the center of the brick chimney with a gap between the outside brick and the inside flue, so that the ceramic or clay flue is able to expand and contract without moving the brick. However, stainless steel pipe or concrete may also be used in some cases, depending on what the chimney is designed to vent. Look at local building code to determine the size flue that is required for the appliance you are venting. These differ, so make sure to check for your specific need.

Building a Chimney

Buy all supplies needed for building your chimney. While this will vary a bit, depending on the type and location of your chimney, you should have all supplies on hand when starting your project. When installing a metal chimney you will need flashing, prefab chimney box, chimney cap, prefab chimney sections, and chimney roof braces. When passing through a wall or ceiling, you should use a section of chimney that is rated as a wall pass through, so that there is no risk of your wall or roof catching on fire. When building a masonry chimney you will need to buy the brick and masonry, mortar, support pieces (such as rebar), flashing, a cap, and specialty masonry tools.

Build the smoke chamber. You will need to start building your chimney from the ground up. For a masonry chimney, this usually means that you will be building your chimney right off the top of a fireplace (if the chimney is designed to vent a fireplace), using a brick that is similar to that used to build the hearth. You will need to mix your mortar mix with water, making it the consistency of thick peanut butter. Once it is the right consistency, you will use a mason's trowel to load mortar onto each brick before you set it in place. You need to put enough mortar on each brick so that it oozes out the joints a bit, which guarantees that the entire area between the bricks is filled with mortar. As you build up the walls of the chimney with bricks, make sure that it is level on all planes. Take a level and check that all walls are level on their vertical and horizontal planes, and that in each line of bricks all the bricks are lined up with the other. For a prefabricated metal fireplace, the smoke chamber is almost indistinguishable from the rest of the chimney. The chimney should be built on a very solid base. If you are not building on an existing fireplace or base, you may need to pour a reinforced concrete pad for the chimney to be built upon. Consult local code to determine if it is required and what specifications it should be built to.

Construct the flue and outside of chimney. The flue connects the top of the smoke chamber to the roof through the middle of the chimney. As noted in the previous section, it may be made of either brick or metal pipe. If clay or ceramic tiles are used to create the flue in a brick chimney, they are laid as the exterior of the chimney is put up. Metal liners can be dropped into place, while concrete liners are cast in place by pouring concrete through a flexible hose. For more information on how to install a chimney liner see: How to Install a Chimney Liner.

Surround the chimney with flashing where it goes through any walls. You'll want to put metal flashing around the chimney in 2 places: around the chimney where it meets the roof and as a lining underneath the chimney cap. Use a waterproof silicone caulk or polyurethane caulk underneath the flashing to keep water from leaking into the house around the chimney.

Cap the chimney. The chimney cap goes over the top of the brick flue housing. It should extend past the chimney wall by at least 2 inches (5 centimeters) and have a drip edge to channel water away from the chimney. Recommended chimney cap materials include stone and concrete either pre-cast or cast in place. For more information on how to cap a chimney see: How to Cap a Chimney. A chimney cap is important because it will reduce the amount of moisture that can get down your chimney, keep out animals, block downdrafts, stop sparks from leaving your chimney and also block debris, like leaves and branches, from going down your chimney.