A load bearing wall gives a building structural integrity. It carries and distributes weight from the roof and top floors down to the foundation. Damage to a load bearing wall can cause floors to sag, finishes to crack and the entire structure to collapse.
How do you determine whether a wall is load-bearing?
This can be difficult to determine even for an experienced carpenter. Analyzing the wall loads in large, complicated homes is tough enough to understand. When there have been renovations, this determination can become nearly impossible. Even building inspectors rely on the "when in doubt" principle... when in doubt, assume the wall is load bearing and act accordingly.
It is easy to understand how renovations can cause weight to be transferred onto formerly non-load bearing partition walls. For example, the addition of exhaust fans and attic stairways often requires cutting of ceiling joists, which can also transfer loads from the original walls... the main (center of the house) beam and the outside wall, onto non-load bearing walls that are in between them. Adding a room in an attic can change the entire load bearing status of the walls below.
To confuse matters further, some types of construction, such as post and beam or steel girder, may not have any bearing walls at all except for the outside walls. What's a Contractor to do?
Look at the structure of the house and ask the following questions:
(1) Is there a significant load above, such a built-up (multi-board) carrying beam or another wall? Is there a full floor above it, or just an empty attic?
(2) If you can view the joists in the attic, is the wall parallel or perpendicular to them? Generally, load bearing walls are perpendicular to the joists they support. If two separate floor joists or ceiling joists intersect over a wall, that wall should be considered load bearing.
(3) Is it an outside wall? You should consider all outside walls load bearing. If the house has been remodeled, a former outside wall could now be an inside wall. Examine the foundation to find these "stealth" outside walls.
(4) Look at the beams and posts in the basement. In multi-floor dwellings, posts and beams in the basement indicate bearing walls above them, even up two floors. Be aware that these multi-floor bearing walls may not be directly above each other.
In complex, large homes, the basement can be a jungle of carrying beams and posts, crisscrossed and interlocked. Careful inspection is necessary to determine how this maze of beams supports the house, and its effect on the walls above. To play it safe the best solution is to hire a Structural Engineer.