Finding a load bearing wall in a residential home is actually quite easy, but to find a non-load bearing wall can be a difficult thing to do. You will seldom find that a wall is not load bearing. This is because modern framing uses the stick framing method. Each level of a building is built on the previous level. Each level is held up by the studs or "sticks." All exterior walls are load bearing. Most interior walls are load bearing, even if only pertaining to the strength of the building's ability to withstand sway under a load. Another type of framing, balloon framing will typically have non-load bearing interior partitions. In this type of framing, the exterior walls will span the height of the structure. The second sub floor and every one thereafter is connected to the exterior walls; although, this is not an economical style of framing and is rarely used. No matter the type of structure of your home, here are the steps to take when searching for load bearing (or non-load bearing) walls.
To find out if a wall is load bearing, first check the substructure. If your home has a monolithic foundation, there is no substructure to check. A monolithic foundation is referred to as a slab foundation, which means a poured slab is the first level's subfloor. On the other hand, if the first level's subfloor is wood, it will have a crawl space. Check the crawl space for girders holding up the floor joists. The girders will run wall to wall and usually have pillars supporting them. These girders support the bulk of the home's weight. Any wall that crosses the girder is definitely load bearing.
Next, check the framing of the roof. There are two types of framing used in the roofs of residential homes: trusses and rafter framing. You might be in luck if you want to remove an interior partition and your home has trusses. Trusses usually have only three to five points that carry the load of the roof. The truss will have a web shape construction of triangles. These triangles give the building strength to resist sway. Where these triangles meet the base of the truss is where the truss distributes the downward force. Between these points is where you might find non-load bearing walls. On ridge and rafter framing, you will see a large ridge beam under every peak in the roof. The rafters will extend from that ridge to a wall (usually exterior). These walls are definitely load bearing. In this example, the rafter has two points of support. When the rafters get longer or extra framing is required to resist sway, the rafters will be supported by three or more points. Any interior walls that cross under these points will be load bearing.
Finally, to be completely safe, it is recommended that you get a structural engineer involved to sign off on any structural plans before moving partitions. Even architects do not trust their own structural work until an engineer signs off on it.