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. However, when subjected to constant moisture, 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. 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, this is one of the simpler structural problems to solve. A power planer makes quick work of planing away the high area, but you can also plane it by hand with a manual planer. 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 plane the area to remove any high spots. 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.
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. 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. All foundation problems should be inspected and diagnosed by a structural engineer. Some of the common reasons why foundation footings can sink and basement walls can shift are due to the following:.
Depending on the cause of the uneven floor, leveling subfloors successfully can be a relatively inexpensive project that many experienced DIYers can undertake. We will share some builder's tricks to level a subfloor depending on the cause and condition. Wood shims are the remedy to fix most floors that are out-of-level or out-of-plane.
Have you heard of the "Marble Method" to determine if the floor is out-of-plane? It's a fool-proof method and works by dropping a few marbles around the floor to see where they roll. You can quickly tell if you have a slope or if there are high and low spots called "heaves" and "dips" in the flooring trade within the flooring area. The other method is to use a 6 or 8-foot carpenter's level to check the floor's slope. If you see a gap under the level, that's a dip.
If the level rocks slightly, that means there is a heave under part of it. You'll see a slight gap between the level and the floor at the high point. Before you can fix the level problem, you need to find the root cause of it. Typically, age and settling cause an unlevel floor. Other causes of an uneven floor include:.
To find the exact cause, you may need to get under the floor if there is a crawlspace or basement. Otherwise, you'll need to take up part of the subfloor to examine the structure below. If you find termite damage, cracks in the foundation, or rotted joists, call in an expert to repair them. It's critical to fix these issues before attempting to level the subfloor. Occasionally, shoddy construction or a weak foundation can be the cause of an unlevel floor. Before there were uniform building codes, some builders saved money using undersized lumber or spaced the joists too far apart when building the floor joist system.
These shortcuts resulted in weak, sagging, or bouncy floors. The best way to fix the spacing issue is to have a flooring professional "sister" the joists. It's a technique of bolting the new joists onto the old ones and using the same sill plate. Especially in older homes, the foundation footings can sink, or the basement walls can crack and shift due to soil and water pressure.
It's best to have a specialist inspect the foundation if you think there may be a problem. Even after repairing the foundation, there's no guarantee that the floors will be level again, but at that point, shimming the joists may be a feasible option. In one existing structure I worked on, the floor was extremely out of level. If we had done everything possible to shim the floor up, it would have needed another layer of plywood, which would have created problems with the doorways, radiant heat and cabinetry.
After discussion with the home owner, we shimmed the floor just slightly to take out some of the unevenness without affecting other areas. As in all cases, educating the homeowner is important to prevent further headaches created by false expectations.
When done correctly, shimming a floor can produce an acceptable installation job. A flat floor sands better, finishes better and is cosmetically more appealing. For a nail-down floor, asphalt or fiberglass shingles work best because they are cheap, easy to work with and are a relatively quick fix for small flattening problems. In addition, these non-wood products will not create the squeaks that wood shims do when they are not glued down.
To shim the floor, take a straightedge or a level and look for imperfections in the flatness of the floor and determine if it needs to be raised or lowered.
This may require either sanding high spots or shimming with shingles in the low spots to build them up. When you are using more than one layer of shingles, stagger or stair-step the shingles. You may also incorporate and pound felt paper to help in the leveling process. Always recheck with a straightedge to make sure you haven't created a high spot in the floor. Remember that pieces cannot overhang a joist, so if you come to a joist where the end of your sheet terminates in open space, cut it to fit so that it splits half the joist width and shares the other half with the adjacent piece of subflooring.
Screw the subfloor material into place on top of the joists. For pieces that share a joist, angle the screw in slightly from the top so it sinks in slightly inwards into the joist. Tim Anderson has been freelance writing since He spent more than 15 years as a third-generation tile and stone contractor before transitioning into freelance writing. By Tim Anderson. Related Articles.
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