Masonry Chimney Construction

Masonry Chimney Concerns:

Masonry Chimney Construction can be done by yourself but you need to follow a few guide lines. Virtually every building or home has at least one chimney.

They have the potential to provide warmth and heat to our families yet can also be destructive to our possessions and loved ones. During Chimney Construction,


Be Careful, make sure you build it properly you could burn your house down!

chimney fireplace fire

During Masonry Chimney Construction keep your eye open for combustible material close to the; through wall thimble. For the most part, they are one of the most overlooked parts of our structures, and rarely are they built according to the local building codes. Masonrworktools.com offers this article, and it takes a look at two major categories of chimneys, and provides some quick guidance and notes for concerned professionals or homeowners on what to look for when assessing the condition of the chimney during Masonry Chimney Construction. The purpose is not to make experts out of you - lets leave that to the properly certified, full time masonry chimney contractors. However, this Masonry Chimney Construction information may help raise your awareness of possible problems associated with chimneys.

Purpose of a Chimney:

The purpose of the different types of chimneys is to allow combustion, by-products to be vented away from the structure safely without causing damage to the structure they are attached to. These types of chimneys have many similarities. They are constructed of similar materials and both are subject to many of the same building code requirements. Even though chimneys serving fireplaces, and chimneys serving appliances may be venting different combustion by-products (for instance, a fireplace chimney may be venting solid fuel by-products), they should all be constructed to discharge these by-products at a rate that does not adversely affect the combustion process and to release the discharged material at a height and location that provides fire safety. Masonry Chimney Construction is quite simple if these guidlines are followed.

Common Building Code Requirements:

chim con

It is important to note that Masonry Chimney Construction building code requirements often vary on a local basis depending on what your city requires. Typically, the reference used for local building codes is based upon the National Fire Protection Agencies 211 standard for chimneys, fireplaces, vents, and solid fuel burning appliances. (The latest version was updated in 1996 and may be obtained by calling 1-800-344-3555). A few of the codes which are typically universally adopted include the following:

1. The minimum chimney height for fire safety is the greater of 3 ft. above the highest point where the chimney penetrates the roof line, or 2 ft. higher than any portion of the structure or adjoining structures within 10 ft. of the chimney.

2. A chimney flue shall not change in size or shape within 6 in. above or below any point where the chimney passes through a combustible floor, ceiling, or roof component.

thru wall

3. Chimney penetrating through the wall thimble shall have a minimum of 8 inches of masonry and 12 inches total distance away from anything combustible.

4. Residential masonry chimney wall thickness should be a nominal 4 inches.

5. Masonry chimneys should be lined. The selection of the lining material should be appropriate for the class of chimney service and the type of appliance to it in accordance with the terms of the appliance listing and the manufacturers instructions.

6. Chimney clearance from combustible material is a minimum of 2 in. except where the chimney is located outside the structure, in which case 1 inch is acceptable. "Do not misinterpret this as being by the thimble" The through wall thimble is exceptionally important, it gets very hot at that location and needs to be addressed carefully. Refer to figure 3.

7. According to most building codes, flues may be constructed to discharge their by-products with other flues as long as the following conditions are met:

8. The flues do not slope more than 30 degrees from vertical.

9. The flues are discharging by-products of similar fuels (for instance, you can't combine a flue venting gas by-products with one venting wood by-products).

10. When combining flues, the main discharge flue should be sized for the maximum combined flow of both smaller flues.

Metal chimneys, also called prefab, class A and Double or triple wall are usually made in three diameters as show below. Larger diameters which may apply to prefab fireplaces. Round flue tiles or single wall stainless chimney liner are also shown. Each diameter has a specific usable area, which is also called it's cross section. The capacity of a chimney to remove smoke and/or BTU's from an appliance is directly related to this area.

Chimney Size Square Inches Common Venting Uses
6 Inch Inside Diameter 28.3 Stoves, Central Heaters
7 Inch Inside Diameter 38.5 Stoves, Central Heaters
8 Inch Inside Diameter 50.3 Stoves, Small Fireplaces, Central Heaters
10 Inch Inside Dia. 78.6 Stoves*, Fireplaces, Central Heaters
12 Inch Inside Dia. 113 Fireplaces, Central Heaters

*only rare older stoves use 10" chimneys

One thing that we notice from this table is that a 6" flue is almost 1/2 the size of an 8" flue. The natural tendency might be to think that a 6" flue is "only" two inches smaller that an 8" one, but in truth you can see the difference is much larger.

Masonry Flues

Flue Size (O.D.) Square Inches Common Venting Uses
7 1/2 X 7 1/2 31 Stoves, Central Heaters
8 1/2 x 8 1/2 41 Stoves, Central Heaters
8 1/2 X 13 70 Stoves, Fireplaces, Central Heaters
13 X 13 99 Fireplaces
13 X 18 156 Fireplaces

Effective Area

Since smoke rises in a circular motion, only a certain area of a square flue tile is actually being used effectively. The listings in the table below refer to the usable area.

The benefit of a square flue is it takes a little longer for creosote to build up, thus prolonging the need for a chimney sweep.

flue area

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  • Chimney Flashing Details


    Chimney Flashing Detail

    The chimney cap should be designed to keep moisture from entering the system. Caps should not be constructed as a mortar wash, which is simply parging the surface with mortar. If the masonry chimney cap is constructed with mortar only it will crack severely because mortar is designed to hold at a maximum of three quarters of an inch and not in mass amounts, it has no aggregate to hold the cement together, only sand which is not substantial enough to get any strength out of it. The chimney cap should be constructed with cast in place concrete, precast concrete or stone. The chimney cap should be sloped away from the flue to direct water out of harms way. If water gets between the flue and the structural masonry, this can cause efflorescence in high humidity areas and it can also freeze, simply prying your cap off your chimney. The cap should overhang the chimney wall at least two inches and should have a drip edge cast into the overhang. Rain water is now directed off of the top of the chimney and drips off of the overhang.

    Because the concrete cap and clay flue liner are different materials having different thermal expansion characteristics, there should be a void left between the cap and the flue. This void should be filled with a compressible material and caulked. This allows both materials to move independently, while the sealant keeps moisture out.

    All of the measures discussed above are focused on keeping water out of the chimney. Although these measures can protect the chimney from moisture, they are not foolproof. Some water will still find it's way past the cap. For this reason, through wall flashing should be placed under the cap as a second line of defense against moisture penetration. The flashing is adhered to the flue liner, extends horizontally under the cap and is extended beyond the face of the wall. If necessary provide weep vents at base of chimney weeping out on the roof using a # 10 cotton wick, laying it in the bed joint, you can get this at most any hardware store. (Technical Notes on Masonry Chimney Construction Construction, by masonryworktools.com an ever growing masonry information site to help you succeed in your masonry venture.)

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