House floors are very often not level, especially in houses more than 40 years old. Before any attempts to "fix" the floor are made, the following two basic issues must be addressed;
1- Any structural damage to floor framing members or supporting elements
2- Whether the floor, or any part of the floor, can be leveled at an acceptable cost and without damaging other building elements.
As a structural engineer, I inspect and evaluate sloped and sagging floor conditions regularly. In some cases, the owners have me develop plans for remedial work. In other cases, after I determine that there is no major structural damage, the owners decide that the cost of trying to level the floor is just too great.
Many long-time owners learn to live with sloping or sagging floors. Real estate agents might even describe sloped floors to be part of the "character" or "charm" of an older home. However, for new or prospective owners, sloped floors are more often considered a defect to be repaired.
Leveling a sloped floor is always technically possible. However, major work may be required, especially if the floor has structural defects. If you are buying a house with sloped floors, do not automatically believe any real estate agent or contractor who suggests that there is an "easy fix".
Before investing time, effort and money to repair and level a sloped floor, a complete evaluation should be performed to determine; (1) The reason or reasons for sloping, and (2) The most effective method of making permanent improvements that will not cause other costly problems. This evaluation can sometimes be performed by a qualified contractor or an owner with working knowledge of basic construction. However, consultation with a qualified engineer should be considered if there are indications of structural distress for which the cause is not obvious and simple to repair.
If floor framing is covered (from underneath) by a ceiling, it will be necessary to remove at least some ceiling finish materials to allow for thorough inspection and evaluation.
Considering cost and disruption, it is often not practical to completely level a floor that is not level (out-of-level). For some conditions, even attempting to level the floor may not be advisable.
A summary of basic methods used to level floors is provided below, followed by detailed discussion of the evaluation to be performed before any work is started.
BASIC METHODS TO LEVEL WOOD-FRAMED FLOOR
After any structural repairs are made, as described below, one or more of the following methods can be considered to level a wood-framed floor.
1 Lift or raise ("jack") the low part of the floor using appropriate equipment such as hydraulic jacks or adjustable steel-post jacks ("screw" jack). A sturdy wood (or steel) beam is usually required under the floor framing, to distribute upward forces from the jacks. Lifting must be performed slowly and carefully to avoid damaging other building elements, including water pipes and electric wiring throughout the building.
2 Install new plywood or OSB subflooring (1/2-inch or 3/4-inch), on top of the existing subfloor, using wood shims as necessary to provide a level surface. Shims (or "furring" strips) must be spaced no further apart than 16 inches. Shims will almost certainly have to be different thicknesses, making this method somewhat difficult. Very thin shims (less than 1/4-inch) may split and become loose over time.
3- Remove (and replace) existing subfloor to allow for installation of new wood joist sections, nailed to the side of existing floor joists. New joist sections are installed level, at the required elevation.
Before raising any floor, it is essential to perform a careful evaluation to determine if and how other parts of the building will be affected. For example, lifting a floor that supports interior walls can cause damage to upper building elements, including the roof.
Required lifting capacity must also be carefully assessed to prevent potentially catastrophic collapse due to overload of the jacking assembly. Supports for the jacks (including a basement floor slab) must be strong enough (and stiff enough) to prevent failure or excessive movement.
Installation of a new, permanent beam (girder) is often required to support the floor in the level position. New beams may require support from new columns and footings.
BASIC METHODS TO LEVEL CONCRETE FLOOR (SOIL-SUPPORTED)
After determining the cause of floor movement, one of the following methods can be considered to level a concrete floor that is soil-supported;
1 - Remove and replace the out-of-level floor area if feasible considering constraints of other building elements.
2 If there are no voids under the slab, and any settlement has stopped, install a new concrete or polymer topping on the existing floor.
3 If any settlement has stopped, raise the slab by pumping "grout" underneath. This work must be performed by a qualified contractor with experience (and insurance).
CAUSES FOR OUT-OF-LEVEL FLOORS
The vast majority of house floors are built with wood framing. Primary reasons for out-of-level wood-framed floors are discussed below. However, one common reason is that wood has the well-known property of "creep", which results in sagging (deflection) over time. Due to sustained weight ("load"), wood takes on a permanent curvature that can be very difficult (if not practically impossible) to reverse.
For residential use, concrete floors are almost always supported by soil ("subgrade"). Therefore, movement of the floor is due to movement of the underlying soil.
Before a repair plan is developed, the cause of sloping or sagging must be determined. The first step is to determine if the floor is sloping or sagging (or both).
SLOPING, SAGGING OR BOTH
A sloped floor can be visualized by tilting a flat board that had been level. The surface of the board remains flat.
In general, a sloping floor is high at one side of a house or room and low at the other side. Sloped floors are caused by downward movement at the low side.
To visualize a sagging floor, picture a heavy weight placed at the center of a long, flat board supported only at each end. The board sags downward, with curvature. The surface of the board does not remain flat. The sagging board has a low point between the ends.
A sagging floor has a low area between sides of a house or room. Sagging floors are caused by downward movement at the low area, relative to the sides of the room.
In some cases, sloping and sagging has occurred, complicating the evaluation. However, either the sloping or sagging is usually the predominant problem.
For example, the floors on each side of a center bearing wall (midway between front and back walls) may be sloping down towards the center of the house. Although there is a low area at center of the house, the problem can be identified as sloping (from each outside wall towards the center), not sagging, since the floor surfaces remain flat, without curvature.
Sloping and sagging can be detected and measured using a standard carpenter level, at least four feet long. More exotic measurements using a tripod-mounted construction level, or laser-type tools, may be warranted, especially for concrete floors. However, relatively simple "detective" methods should always be used first.
Sloping is easily detected by placing the standard level on the floor, at various locations and oriented in several directions. The rate of slope is determined with the level held (or propped up) in the level position. One end of the level will be above the floor. Rate of slope (inches per foot) is then the vertical gap (between the floor and the end of the level) divided by the length of the level. For example, if the gap is 1 inch, and the level is four feet long, the rate of slope is 1 inch per 4 feet, or 1/4-inch per foot.
Sagging can be detected by placing a standard carpenter level (at least four feet long) on the floor surface, at several locations and oriented in various directions. If the floor is sagging, there will be a gap under the level, between the ends. Such a gap will be most noticeable when the level is placed over ("straddles") the low area.
SLOPED FLOOR WITHOUT SAGGING
When a floor is sloped only, without sagging, foundation settlement is often found to be the cause. As long as settlement has stopped, it may be practical to level the floor. However, if there is any valid reason to believe settlement has not stopped, work on the floor should be put off until the foundation settlement problem is evaluated and corrected.
Another common reason that a floor is sloped is simply because that is the way it was built. For various reasons, the tops of foundation walls (or main support beams) are often not constructed at the same elevation. The floor is constructed before anyone notices (or cares) that the floor is sloped.
For the example previously described, with floor areas sloping down from outside walls towards the center of a house, the sloping condition is usually due to some problem with support at center of the house. This type of condition can be much more costly to repair than sloping caused by foundation settlement that has stopped.
SAGGING (DEFLECTED) FLOOR
As indicated by the previous description, sagging (deflection) generally indicates a problem with the load-carrying (structural) capacity of the floor. Other than decay or termite damage,the main reason is inadequate capacity to support heavy weight (load) on the floor. Excessive load may have been applied only once in the past, causing structural damage.
Due to the laws of physics, all floors over open space must sag (deflect), even if we do not "see" the sagging, which is also known as "deflection". Some small amount of deflection is expected to occur due to the weight of permanent elements, such as the floor itself and "partition" walls on the floor. Heavy furniture that remains in place for many years might also be considered "permanent".
Due to the weight of persons and items moved across a floor, sagging (deflection) is "elastic", unless the structural parts of the floor are permanently damaged by the weight. An elastic floor regains (bounces back to) the original shape after the load is removed.
Without significant weight on a floor, there should not be any noticeable sagging, except for "vintage" wood-framed floors, as previously discussed ("creep" deflection). Therefore, if a floor is sagging noticeably, without any significant weight, there could be some structural damage that must be repaired.
STRUCTURAL ASSESSMENT OF WOOD-FRAMED FLOOR
Complete description of how to assess the structural condition of a wood-framed floor could take up many pages, including illustrations.
Of course, there is no way to completely evaluate the condition of wood framing that is covered by a ceiling. Therefore, ceiling finish must be removed to allow for inspection and assessment.
The following key elements and conditions should be evaluated;
1 - Condition of floor joists (such as 2x10s) and beams (girders) that support floor joists. Any member with split or crack extending through the bottom edge is severely damaged and should be repaired.
2 - Check for decay or termite damage, especially above a crawlspace and along the outside walls. Decay of wood (commonly termed "dry-rot") is caused by long term exposure to water, which allows fungus to grow and destroy wood. To evaluate extent of termite damage, you must probe wood with an awl or narrow-point screwdriver.
3 - Obvious sagging (deflection) of beams that support floor joists. Hold a carpenter's level up against bottom edge of beams, at various locations, to determine magnitude of deflection. For long beams, some deflection is expected. Deflection will show up mostly near the ends of a beam, at supports. If there is no obvious damage to the wood, excessive deflection indicates an overload condition that must be corrected.
4 For floors sloped towards outside walls, inspect foundation walls at the low side.
EVALUATION OF SOIL-SUPPORTED CONCRETE FLOOR
This discussion is limited to concrete floors that were originally installed on soil "subgrade".
Concrete floors move downward when the soil subgrade moves downward.
The following are primary causes of soil movement;
1 Consolidation of fill soil due to the lack of proper (or any) compaction.
2 Settlement (consolidation) of underlying soils. This type of settlement is most often caused by the placement of thick fill soil (during construction) on top of "native" soils. Fill soil is often placed to level a building site. If variable-thickness (tapered) fill is placed, one side of the building can settle much more than the other side.
3 Voids caused by water. This problem is most often caused by leaking drain pipes, which may crack due to consolidation of fill soil or settlement of underlying soil.
Voids under concrete slabs can exist for many years without any obvious indication. The slab can span over (bridge across) the void as long as the void is relatively small. However, if the void increases, the slab can collapse suddenly, especially if there is a bearing wall or column on the slab above the void.
Unless there is a way to look under the slab from the edge, part of the slab must be removed to allow for inspection of soil conditions underneath. Use of special video equipment should be considered if available. This would allow for inspection by drilling one or more small holes.