For purposes of foundation repairs, chimneys can be divided into two groups: 1) masonry chimneys, and 2) non-masonry chimneys. The distinction is important because masonry fireplaces with masonry chimneys are very heavy. A typical three- by six-foot fireplace with a 20-foot masonry chimney can weigh 10,000 to 15,000 pounds. A non-masonry chimney of the same dimensions will not weigh any more than a few hundred pounds.
Why is there such a difference? Masonry (usually brick) chimneys rest on masonry fireboxes. The firebox being the area where the fire burns. The brick that lines a firebox can get so hot that it will set on fire any wood that touches the backside of the brick. To keep your fireplace from burning down your house, there is a second brick wall that surrounds the firebox. Between the inner wall and the outer wall, there is an air space. The same is true for your chimney. So there are a lot of bricks and a great deal of concrete in a typical masonry chimney.
Masonry Fireplaces & Chimneys
Masonry fireplaces and chimneys are so heavy that house foundations require extra reinforcing under chimneys so that the foundations will not break or sag. Typically, under a chimney a foundation will be 12 to 24 inches thick, versus four inches for most foundations. In addition, the thickened portion of a foundation under a chimney (the pad) is often connected to two- to four-grade beams. Grade beams are like buried walls that are part of your foundation. Grade beams are typically 18 to 36 inches deep and 10 to 12 inches wide.
Fireplaces that are located on the interior of a home, that is they do not touch an exterior wall, tend to settle by going straight down. As such fireplaces settle, they usually create a roughly circular depression with the fireplace at the center. To lift interior fireplaces, it is necessary to place two to six pilings under a fireplace. The number of pilings required depends on the size of a fireplace, the thickness of the pad under a fireplace, and the number of beams connected to the pad under a fireplace.
Lifting a Fireplace
When a fireplace is lifted, it is extremely important to lift it evenly. If one side is lifted more than the opposite side, even by a small amount, the chimney will tilt. Because chimneys are often so tall, a small amount of tilt at ground level can become a great deal of tilt at the top of a chimney. As an example, for a 24-foot-tall chimney that is three feet wide lifting the front ¼ of an inch more on the front than the back will tilt the chimney a full two inches at the top. The real problem comes at the point where the chimney goes through the roof. As the chimney tilts it compresses the roof on one side and pulls it on the other side. With enough pulling, the roof can leak. Sometimes flashing will prevent leaks and sometimes it won’t.
Chimneys on Exterior Walls
For chimneys on exterior walls, it is advisable to always use 4 pilings. With a piling on each outside corner and one on each inside corner, the chimney can be tilted back to a vertical position and then lifted evenly. With just two pilings, one on each side, it will not be possible to take a chimney that is tilted away from a home and move it back into a vertical position. Sometimes, to save a piling, a contractor will only put one piling on the back of a chimney. The problem with this approach is what happens in the rare event that the back side piling slips or fails. There have been instances where the chimney actually fell over!
Load-Bearing Chimneys
Another problem occurs when a chimney is load bearing. In some cases when a house is framed, some of structural supports, such as roof rafters and floor raters for second floors, rest on or are connected to a chimney. When a chimney that is a structural support tilts, supports can slip off of the supporting chimney, creating damage. Drops in roof and in ceilings are examples of the type of damage that can occur.
If you are having foundation problems and you have a masonry chimney, call an experienced contractor. An inexperienced contractor will assure that he can handle your chimney, and then leave you to pay for the damage.