As houses settle, floors often become unlevel. Sometimes there are bowed or sagging areas in flooring, or the entire floor could be slanted.

First, determine whether the floor is sagging in certain areas or whether there is a gradual sloping from one side of the room to the other. This can be done by using a level in various parts of the room.

A floor will slope for any number of reasons. The sill, or wooden structure that rests on the foundation, could be damaged or rotting. The foundation itself could be cracked or crumbling. Both of these problems can be quite costly to repair. Then again, the entire floor can slant or slope because the house foundation had a lot of settling.

If the floor has a considerable difference in height, it is a good idea to hire a structural engineer to examine the foundation. When serious foundation problems are found and repaired early, homeowners can save themselves a lot of money and stress.

Unfortunately, if there is structural damage, there is little that a homeowner can do themselves other than hire a professional to repair the damage. If the floors have slanted or shifted due to settling, there are many different ways a homeowner can level the floors.

Most options require the removal of the existing flooring (carpet, wood floors, etc) and plywood cover to gain access to the subfloor. The subfloor is the main strength of a floor or level, usually composed of many beams and joists. The subfloor of a basement or garage is usually cement or concrete.

After exposing the subfloor, homeowners can level joists by either sanding down the high areas or shimming the low areas. Shimming is an engineering term that refers to using wedges of material to fill in gaps or spaces between objects. Shimming floors means to use wedges of plywood or many layer of roofing felt to level out the flooring. The plywood is then reattached to the subfloor, and the flooring is reinstalled.

Sagging floors can be caused by joist damage. Incredible amounts of weight or strain can cause these beams to bow or warp. Water damage can also easily weaken important structural beams.
Sometimes joists are spaced too far apart to properly support the floor. Over time, this will also make the floor sag in certain areas.

Luckily, sagging wooden or carpeted floors can be much easier to fix than sloping. By removing the flooring, a do-it-yourselfer can replace damaged joists and insert extra beams as necessary. It is especially important to add extra joists to any floor that will be tiled. The weight of the tile and grout can cause regular floors to gradually bow and sag.

With severe sagging, there might be serious water damage or other complications. A professional might be needed to figure out the exact problem. After examining the floor, he or she can state the problem and offer advice on what steps the homeowner should take next.