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To begin determining which walls in your house are load-bearing ones, it's best to start at the most basic load-bearing feature of any home - the foundation. If your house has a basement, start here. If not, try to start wherever on the first floor you can locate your house's lower concrete "slab."  Once you've reached your house's lowest point, look for walls whose beams go directly into the concrete foundation. Your house's load bearing walls transfer their structural strain into a sturdy concrete foundation, so any walls that interface directly with the foundation should be assumed to be load bearing walls and should not be removed. Additionally, most home's exterior walls are load bearing. You should see this at the foundation level - whether wood, stone, or brick, nearly all exterior walls will extend right into the concrete. Begin to look for thick, sturdy pieces of wood or metal called beams. These account for the majority of your house's load, which they transfer into the foundation. Beams often stretch through multiple floors and thus can be parts of multiple walls. If your beam spans from the foundation through any wall above it, the wall is load bearing and should not be removed. Except for in unfinished rooms, most beams will be behind drywall, so be ready to consult construction documents or contact the builder if you cannot find them. Beams are often easiest to find in an unfinished basement (or attic) where portions of the structure are exposed. Look at the point where a beam meets the ceiling (if you're in the basement, this will be the underside of the first floor of your home, while if you're on the first floor, this will be the underside of the second floor). You should see long supports spanning the length of the ceiling which are called floor joists because they support the floor of the room above. If any of these joists meet a wall or a main support beam at a perpendicular angle, they are transferring the weight of the floor above into the wall and, thus, the wall is load bearing and should not be removed. Again, because most walls' supports are behind drywall, they can't be seen. To determine whether certain floor joists in your house run perpendicular to a given wall, you may need to remove a number of floorboards in the floor above the wall so you have an unimpeded view to look down at the supports. Starting at the basement (or, if you don't have one, the first floor), locate your internal walls, which, as you can probably guess, are the walls inside your four external walls. Follow each internal wall up through the floors of your home - in other words, locate exactly where a wall is on a lower floor, then go to the floor above that spot to see whether the wall stretches through two floors. Pay attention to what is directly above the wall. If there is another wall, a floor with perpendicular joists,  or other heavy construction above it, it is probably a load bearing wall. However, if there is an unfinished space like an empty attic without a full floor, the wall probably is not bearing a load. The bigger a house is, the farther apart its load bearing exterior walls will be and, thus, the more load bearing internal walls there will need to be to support the floor. Often, these load bearing walls are roughly near the center of the house because the center of the house is the farthest point from any of the exterior walls. Look for an internal wall that's near the relative center of your house. There's a good chance this wall is load bearing, especially if it runs parallel to a central basement support beam. Internal load bearing walls can incorporate the house's main support beams into the construction of the wall itself. However, because these support beams are relatively large compared to non-load bearing studs, often, the wall itself will be designed to accommodate the extra size of the beam. If an internal wall has a large boxy section or an enlarged column at its end, this may be concealing a main structural support beam, a sign that the wall is load bearing. Some of these structural features may appear decorative, but be skeptical - often, painted columns or narrow, embellished wooden structures can conceal beams that are highly important for a building's structural integrity. Sometimes, rather than rely on load bearing interior walls, builders use special load-bearing structures like steel support girders and and post and beam constructions to transfer part or all of a building's weight to the exterior walls. In these cases, there is a chance (but not a guarantee) that nearby interior walls may not be load bearing. Look for the signs of big, sturdy wooden or metal structures crossing a room's ceiling and intersecting a wall that you know is load bearing or an external wall, like boxy horizontal protrusions crossing the ceiling. If you see these, nearby internal walls may not be load bearing. This method can give you a clue of where non-load bearing walls might be, but you can't be sure without checking the walls themselves. If you're unsure, check with the builder to be sure that this was the type of construction used. Many houses, especially old ones, have been modified, expanded, and remodeled several times. If this is the case with your house, a former external wall may now be an internal wall. If so, this innocuous-looking internal wall can be load bearing for the original structure. If you have any reason to believe your house has been significantly modified, it's best to contact the original builder, just to be sure that your external walls are your real external walls.
Start at the lowest point in your house. Locate the beams. Look for floor joists. Follow internal walls up through your structure. Check for internal walls near the center of the house. Look for internal walls with large ends. Look for steel girders or post and beam construction. Look for evidence that the house has been modified.