Flooring & Stairs

8 Things to Know Before Leveling a Floor

8 Things to Know Before Leveling a Floor

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Slightly sloping floors, which are common in older houses, often worry homeowners who want to install new rigid floorings, such as laminate, ceramic tile, or hardwood. A common misconception, however, is that the floor must be level (horizontal) in order to successfully put in rigid flooring. In fact, all that's required is that the floor be “in plane,” which simply means “flat.”

The difference between “plane” and “level” is subtle but not arbitrary. Level is the horizontal variance between two points. For example, if you use a laser level to determine the elevation of the floor on both sides of a room, you may find that both sides are pretty close to being the same elevation (level). But if there's a sag somewhere in the middle, the floor is not in plane even though it's technically level. Plane refers to the flatness of the entire surface, and any high and low spots must be remedied before installing rigid flooring. The process of fixing both an out-of-level and an out-of-plane floor is simply known as leveling a floor.

When inflexible flooring is installed over a substrate that dips or heaves, various things can go wrong. Laminate planks can pull apart, and gaps can appear between hardwood planks. Ceramic tile can suffer from “lippage”-since tile won't bend to fit a wavy floor, the edge of one tile will often sit higher than the edge of the adjacent tile, forming a “lip.” The undesirable effect is more pronounced with larger tiles. While you might not notice lippage on two-inch-square tiles, large format tiles, such as the eight-inch-by-48-inch planks designed to mimic real wood, can develop a sizeable lip at one end of a plank if the floor is out of plane.

To avoid unsatisfactory results with rigid flooring, read on to get clear about if, where, and why your substrate might be uneven-and how to deal with the issues.

1. Pinpoint Uneven Areas

The easiest way to find out if your floor is out-of-plane is to use the time-honored marble method. Drop a few marbles onto the floor in various areas around the room and then watch them roll. If they consistently roll to the same side, that side is lower than the rest of the floor. If they roll to other areas of the floor and stop, those areas are likely dips that you probably can't see with the naked eye. High areas in the floor are called “heaves” and low areas are called “dips.”

If you don't have any marbles handy, you can check for dips and heaves with an eight-foot carpenter's level. Place the level on different areas of the floor, and notice if it lays flat in all spots. If a gap appears under the center of the level, it indicates a low spot-if the level rocks back and forth, it's a sign of a high spot.

2. Determine the Underlying Problem

Before taking steps to level a floor, you must find out why it's not level. Leveling a floor can be either a do-it-yourself project or a job for the pros, depending on what's causing the problem. Often, age-related settling results in an uneven floor, which is likely nothing to worry about. But in some cases, problematic structural issues, including foundation problems, a cracked floor joist, a rotted sill plate (what the joist sit on), or delaminated subflooring cause the floor to be uneven.

The only way to tell for sure is to get under the floor and check the condition of the joists and beams beneath. If joists appear rotted, cracked, or broken, if you find termite damage, or if you discover cracks in the foundation walls, you're best advised to have a structural engineer determine the type and extent of the structural problem, which will have to be fixed before any floor-leveling can take place.

When it comes to concrete floors, height discrepancies are often due to the original concrete finishers not leveling the floor adequately. This is a minor issue and can be fixed using the method discussed next.

3. Understand Self-Leveling Underlayment

If an engineer found no structural issues, you may be able to level the floor by applying a cement based self-leveling floor product, such as Henry Level-Pro Self-Leveling Underlayment (available from The Home Depot). A dry substance is mixed with water to form a liquid slurry which is then spread over the floor with a gauge rake. Because it's a liquid, the slurry will naturally settle into low spots, filling them in and creating a level surface.

Self-leveling underlayment products can be used on concrete floors, wood subfloors that are uneven but still in decent shape, and even over ceramic tile floors-eliminating the task of tearing out the old tiles. Check to ensure that the product you purchase is compatible with your floor. This type of floor-leveling is DIY-friendly and costs approximately $1.50 per square foot when applied 1/8-inch thick. Your cost could be more or less, depending on the size of the floor area to be treated and the depth of the low areas.

4. Deal with a Delaminated Subfloor

When a subfloor becomes warped or delaminated (swells and deteriorates), it is no longer level or structurally sound. The best remedy is replacing damaged subfloor panels with new ones. The two types of subfloor material, oriented strand board (OSB) and plywood, are both constructed of wood fibers combined with glue and pressed into durable panels that are more than adequate for constructing a strong floor. When subjected to constant moisture, however, such as a steady drip from leaky plumbing, they will eventually delaminate. Once this occurs, you must correct the cause of the moisture condition, and then the affected section of the subfloor can be replaced.

If you're an enthusiastic DIYer with basic construction skills, you should be able to replace damaged panels to even out the floor. A four-foot-by-eight-foot sheet of OSB subflooring costs $18 to $22 and the same size sheet of plywood subflooring costs $20 to $24. When replacing delaminated subfloor, it's best to remove and replace the entire sheet that's damaged, rather than trying to piece in smaller sections.

5. Plane a Heaving Joist

Occasionally, a floor joist will bow upward, creating a heaved area on the floor above. If your marble test indicated a high area where marbles always roll away, it could be that a floor joist has bowed upward-fortunately, one of the simpler structural problems to solve. It requires removing the subfloor over the bowed joist and then planing the high part of the joist down until it's level across the top. By popping a chalk line along the side of the bowed joist from end to end (hold it at the top end of the joist), you'll be able to see the portion of the joist that is too high. A power planer makes quick work of planing away the high area, but you can also plane it by hand with a manual planer. When the subfloor is reinstalled, you'll have an even floor.

6. Use Shims to Correct Other Joist Issues

If the uneven floor is due to floor joists that have warped or twisted over time or were incorrectly installed, the only way to level the floor is to remove the subflooring and shim the joists. Shimming involves attaching thin, wedge-shaped pieces of wood on top of the low areas of the joists to make the tops of the joists even. This requires using a laser level (to pinpoint the low areas) and attaching the shims (working on bare joists above a basement or crawlspace), by gluing and screwing them to the joists; then the area is planed to remove any high spots.

Many homeowners call the pros if shimming the joists is required, which could run between $1,000 and $5,000, depending on the extent of the job. But if you're experienced with the tools and techniques described above and wish to DIY, a bundle of 42 shims runs about $5, and you may need one or more bundles, depending on the number of joists you're shimming.

7. Sister an Inadequate Joist System

Before uniform building codes were common, some builders used undersized lumber or spaced the joists too far apart when constructing the floor joist system. The result was a weak, saggy, or bouncy floor. A structural engineer may suggest “sistering” the joists, a process whereby new joists are attached to the existing joists to add strength to the floor. Sistering joists is a job for the pros, who will bolt the new joists to the old joists and position the ends of the joists on the same sill plate that supports the original joists. Sistering is a major project that could cost in excess of $8,000, depending on the number of joists that require sistering and whether plumbing and wiring have to be relocated in order to attach the sister joists.

8. Figure out Foundation Factors

While the above factors commonly result in uneven floors, a number of other structural problems can occur that result in foundation movement, and subsequently, everything above the foundation moves. This can create uneven floors, cracks in walls, and doors that won't close. All foundation problems should be inspected and diagnosed by a structural engineer. Foundation footings can sink and basement walls can shift due to lateral pressure from the soil, but whatever the cause, the fix is typically expensive-usually no less than $10,000 to have a foundation contractor stabilize the foundation. Even when the foundation is repaired, there's no guarantee that the floors will be level again, but at that point, shimming the joists may be a feasible option.


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