Can you fix a chimney without tearing it down?
Ask a Registered Professional Engineer Series
By: Donald J Clayton, PE – Registration No. TX37112
“The roofer said that my chimney is tipping away from the house.”
“He tells me that he can put his hand in the space between roof and chimney.”
“Can the chimney be fixed without tearing it down?”
What you are describing is not uncommon for two-story houses with an exterior masonry chimney. If the chimney is located on the gable end of the house, the chimney could be as tall as 30 feet. When you consider all chimney weight is concentrated on a small concrete pad that extends away from the house approximately 2 to 2-1/2 feet and about 5 to 6 feet wide. Compared to the structural weight along an exterior wall of the house, the load that the chimney places on the concrete pad is huge! The movement of the chimney that you describe suggests that the concrete foundation for the chimney was not built upon sufficiently robust soil to support it. As a result settlement of the outer edge of the chimney foundation occurred while the connection to the house foundation suffered little or no movement.
Chimney foundations are usually attached to the house’s foundation beam along one side. The backside of the chimney is cantilevered out from the perimeter of the house. As a result settlement of the outer edge of the chimney foundation occurred while the connection to the house foundation suffered little or no movement. The area where the chimney foundation attaches to the perimeter of the house added additional support to the chimney foundation because the perimeter beam of the house is supporting part of the chimney weight. There is no additional support available at the outer side of the chimney foundation. As a result, there is an imbalance of foundation support under the chimney footing. The connection between the chimney foundation and the house footing becomes much like a door hinge. This hinge action allows the outer side of the chimney footing to rotate downward, resulting in the chimney tipping away from the house.
The problem with downward movement at the outer side of the chimney is that a slight amount of foundation settlement will cause a large separation between the chimney and the roof. As an example, if the concrete chimney footing drops only 1/4 inch relative to where the chimney connects at the house, a two-story chimney can rotate over three inches!
The question is, “Can this be fixed?” The answer is, “Absolutely!” REPAIR WITH FOUR PIERS.
There are different types of masonry chimneys and different chimney foundation configurations. We will discuss the differences to better understand pier placement recommendations.
• Heavy Chimney Construction: In older homes, the fireplace and chimney are constructed totally from masonry. To direct the smoke up the chimney, some chimneys use a tile flue and others have an interior fire brick flue. This type of construction creates the heaviest chimneys. The weight of a full masonry chimney is evenly distributed on the chimney foundation. Engineering evaluations of double-wall masonry chimneys usually require that four piers be installed to lift and stabilize this very heavy and tall structure.
Many times a foundation repair company will propose only two piers on the outside corners of the chimney footing. This might seem like a good idea since the cost to repair will be roughly one-half of a four pier installation. Here is the problem with installing only underpinning at the outer corners of a heavy chimney.
Remember that the chimney foundation is acting as a hinge at the connection with the house. If you install only two engineered piers to verified load-bearing or rock, the outer edge of the chimney will be lifted and strongly supported. The effect of this repair at the connection between the chimney foundation and the house foundation must be considered because these foundations are still relying on the weak soil for support. As the chimney is righted, much of the chimney weight is shifted back to where the chimney foundation connects to the house.
This weight transfer due to lifting the outer edge of the chimney can cause the perimeter wall of the house was connected to the chimney foundation to settle. The result is that the chimney will now begin to tip inward toward the house. The house structure will resist this movement and the chimney will develop horizontal separations in the brick and flue. This renders the chimney and fireplace unsafe. A serious situation can develop where heat and flames can enter a space with combustible materials.
- Light Masonry Chimney Construction: Many newer homes have a metal fireplace insert with triple wall stainless steel flue pipe hidden inside a three-sided decorative masonry enclosure. This construction puts considerably less weight on the chimney foundation. The main difference between this construction and the full masonry chimney is the concentration of weight appears along the ends and the outer face of the chimney foundation. There is not as much weight where the chimney attaches to the house because the metal fireplace does not require masonry to be installed above the front of the firebox. Because of the lower total weight of the decorative masonry surrounding a metal fireplace and flue, it is often possible to successfully install two piers on the outer corners of the chimney foundation to raise the footing and align the chimney without causing much load transfer at the chimney foundation to the house connection. Caution is needed when dealing with existing construction, one is never sure of the exact construction configuration of the chimney foundation, the durability of the connection to the house foundation and the total amount of weight to be lifted. Engineers will generally recommend four piers when lifting and stabilizing any very tall, masonry chimney. Even though the load of a metal insert fireplace chimney is lower where the chimney foundation connects to the house, installing uniform support at all four corners of the chimney footing is simply a good conservative design for the accurate restoration and long term success of the repair.
- Separate Chimney Footing: Sometimes the chimney is constructed on a separate footing that is not connected to the house foundation. The chimney foundation was cast at some time after the concrete foundation for the house was completed. This happens when there are design changes or with house renovations. This situation is easily confirmed by looking for a cold joint (disconnect) between the house and chimney foundations. In this situation, without question, four piers are required regardless of the chimney height or flue construction. REPAIR WITH TWO PIERS When the tilting chimney is on a single-story house, it is usually possible to lift and rotate the chimney in place using only two piers. The reasoning here is that the weight, while significant compared to the structural weight of the house, is not so great as to overpower the exterior structural foundation beam of the house. Keep in mind; however, if there is significant damage to the structure of the chimney, excessive chimney rotation, a settled floor in front of the fireplace, or if there is evidence that the chimney footing was constructed separate from the house foundation, then four piers are required. LET’S TALK ABOUT FOUNDATION REPAIR Because there are heavyweights involved when restoring chimney foundations and the underpinning piers will be mounted onto a very small foundation, the piers will be installed in close proximity. Excavations are necessary under the edge of the chimney foundation to gain access for pier installation. Keep in mind that these excavations will be removing some of the weak soil that is currently supporting the tilting chimney. Because access excavations are destabilizing to the chimney footing, small excavations and rapid the pier installation is the safest repair strategy. Consider the following:
- A full masonry fireplace and chimney on a two-story house can weigh around 30,000 pounds. Recall that this weight is concentrated on a very small concrete footing. To get an idea of how much weight this is, consider that seven 2020 Model Ford F150 XL pickups stacked on top of each other weigh a little over 29,000 pounds according to the manufacturer. In this example, you need a quality, engineered underpinning pier that is capable of supporting an ultimate load in excess of 15,000 pounds at each of four piers. Remember the soil under the chimney has proven insufficient to support the chimney, so the piers must be driven deep into the soil, beyond the layers of soft soil, until encountering suitable load-bearing stratum or rock. The chimney foundation repair cannot rely on support from a pier that is designed to be installed to a depth that is insufficient to penetrate through the existing weak soil and water table to reach suitable load-bearing.
- An engineered pier system designed to support the chimney and a pier with a verified load capacity above the force simply to lift the chimney is required. You should consult and rely on a reputable foundation repair company to do this repair and restoration. It is imperative that the ultimate capacity of the chosen pier system can support a significantly greater load than the force that is required to lift and restore the chimney. (Usually, a pier that you should consider to be suitable needs be rated at twice the working load (or lifting force) necessary to stabilize, restore and support your chimney.) This extra capacity is called a “Factor of Safety”. It is extremely important to have a “Factor of Safety” to ensure long term stability of your chimney restoration. You must ask and receive technical information certifying the ultimate capacity of proposed piers.
- In the example given here, the factored pier capacity for each pier should exceed 15,000 pounds. Your chimney could weight more or less depending on the specifics of the construction. This is why you need a competent foundation professional or registered professional engineer to analyze and determine the pier capacity required to restore the chimney. This repair project is highly specialized and possibly dangerous. It is not a repair project that you talk to a salesperson about. Additional pier capacity (Factor of Safety) provides peace of mind and long term success. A high capacity pier will cost you more than buying a marginal capacity pier with barely enough load capacity to lift and rotate your chimney. You must ask for specification and proof that the proposed underpinning is more than adequate. You must see technical data to be convinced that the installed load capacity of the proposed pier system exceeds the force needed to lift the chimney. Why is this so important? This is because you get only one chance to properly restore and support your failing chimney foundation. TYPES OF FOUNDATION REPAIR SYSTEMS – WHICH IS BEST TO REPAIR A CHIMNEY? The discussions offered here should help you quickly eliminate some underpinning products and methods and narrow the field of foundation repair companies. Keep in mind that the chimney is tipping away from the house because the chimney foundation is resting on soft, failing soil. If the failing soil situation is not bad enough, the foundation contractor must excavate access holes under the footing to permit pier installation, which decreases the amount of soil under the footing and increases the load pushing on the settling soil.
- Dismiss any kind of cast concrete piers for this project because cast concrete piers require a large excavation, which can destabilize the chimney. In addition, cast concrete needs at least seven days to cure before the concrete can support the chimney foundation. This means that for at least a week there will be less soil under the foundation supporting the tilting chimney. In addition, there is a chance that rainwater might enter the open excavations and soften the soil even more. This could result in additional settlement or failure.
- Helical Screw Piles are problematic for this restoration because Helical Screw Piles are installed at a downward angle rather than vertically. Four helical screw piles installed at a downward incline in such close proximity will experience interference with proper load transfer from the helical plates to the load-bearing soil. As a result, the actual load capacity developed by the helical pile will be less than indicated by the shaft torsion measurements made during installation. This means that the Helical Screw Piles will not be able to support as much weight as when the Helical Screw Piles have an adequate distance from nearby piles. Therefore the total load capacity of the screw piles cannot be accurately verified. The only alternative might be to increase the Factor of Safety of the Helical Screw Pile design, but that is still taking a chance that the installed pile capacity may not be sufficient.
Inexpensive underpinning systems like pressed piles and similar inexpensive products are never installed to a sufficient soil depth to reach suitable load-bearing soil. Underpinning piers must be founded within sufficiently robust soil that is capable of supporting the heavy chimney load plus a Factor of Safety. These low-cost repair products have no way to verify load capacity and the installer cannot guarantee the piers will not settle in the future.
The suggestions to avoid certain types of foundation repair products are serious. The foundation repair company working on your project will be dealing with a concentrated load on a very small and unstable foundation. This heavy structural load is resting on weak soil that can be dangerous when destabilized by pier access excavations or moisture seeping under the footing. Therefore, in the interest in safety, the piers must be installed quickly and the chimney load transferred to the piers immediately.
Remember that the pier must support the weight of the chimney and possess a Factor of Safety to ensure that no future settlement can occur. If a pier design is installed that cannot restore the chimney to vertical, or if the pier settles after installation due to the pier having insufficient support capability; it is not possible to install an alternate system to correct the problem. Trying to install another piering system after a failed chimney foundation repair is impossible. There is no place to install an alternate pier system on the chimney foundation because the critical locations needed for the new pier installations are blocked by the previous inferior pier placements.
IMPORTANT NOTE: If the first pier installation fails, There is not a chance for a “do-over”.
STEEL RESISTANCE PIERS
From an engineering perspective, the best pier product to use for repairing a tilting chimney is Steel Resistance Piers. This product is manufactured by several companies and offered by many factory trained and certified installers. Steel Resistance Piers eliminate the concerns discussed above.
- Steel Resistance Piers are installed in a small hand-dug excavation, which is very important when excavating under the small chimney foundation resting on unstable soil.
- Steel Resistance Piers use a compact foundation bracket, which is why the excavation is small and can be rapidly dug by hand.
- Steel Resistance Piers are quickly installed and have superior support capacity for this application compared to other underpinning products. Steel piers are driven through the soil until the pier encounters resistance to additional advancement.
- After reaching resistance where the pier pipe can no longer advance, the load capacity of each Steel Resistance Pier is field load tested to ensure there is a suitable Factor of Safety. Each pier is verified to support more than the weight of the chimney before proceeding with the restoration, future settlement and instability are highly unlikely.
- The chimney load is immediately transferred to the steel piers after the installation and load testing. There is no danger of rainwater entering excavations. There is no additional chimney foundation downward creep because the excavations under the footing are not left open long term. The excavations to permit steel pier installation are open for only a few hours, not days.
- The entire steel pier installation project and restoration is normally completed in one day.
- Steel Resistance Piers are proven foundation repair products; they are installed by trained and certified technicians. The manufacturer requires rigid installation procedures and installation records to ensure quality control, verified capacity, and proper long term support. RESTORATION After installing foundation underpinning, the contractor will lift the chimney foundation. The lifting should allow the chimney to rotate back in place. Most times this restoration process is very successful and the gap between the roof and the masonry will close or nearly close. One must realize that over time, as the chimney rotated away from the exterior of the house, debris may have fallen into the open space between the wall of the house and the chimney. This debris usually consists of loose mortar or caulking or roofing materials. The gap at the roof can easily be three or more inches wide and will taper down to nothing as it nears the fireplace. Any debris that falls will wedge near the bottom. Trapped debris can hinder the rotation of the chimney back to vertical. The foundation contractor must carefully check for, and remove, all trapped debris before attempting to rotate the chimney to vertical.
At the completion of the repair project, you should receive an installation report containing logs of the pier installations indicating the depth to end bearing support and a load test result for each pier. In addition, you will be provided with the amount of force that was applied to each pier in order to support and restore the chimney. All of this data proves that the pier installation is verified to have more than sufficient capacity for the long term, stable support.
One needs to always verify that the floor near the fireplace is level. This is to determine if settlement of the perimeter of the house. This can occur as a result of the weight of the chimney causing the floor to slope toward the fireplace. If the chimney has caused settlement at the perimeter beam on the house, additional piers are necessary on the foundation of the house to lift and level the interior floor near the fireplace. When in doubt, or if a serious chimney problem is evident, please consult a licensed professional engineer.
If you are having problems with salespersons coming to your house and confusing you with different kinds of foundation repair, or concern about the number of piers being proposed, or with high pressure and negative talk; you should consult a local licensed professional engineer for referrals to reputable companies. Better yet, hire the engineer to evaluate your dangerous chimney situation. The engineer will provide you with a step by step repair plan and give you product and company recommendations. These structural engineers work with really good foundation repair companies and have to deal with the bad ones on a daily basis. This repair needs to be done right the first time.