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Walls can supported only by a beam or another wall on a floor below that is collinear in plan view with the wall for its entire length of the wall. If a wall created on an upper floor does not find a support or an obstruction, it extends to the ground.
If the wall encounters a beam or a wall on a floor below that overlaps with the wall for any distance but does not fully support the wall, the lower beam or wall is regarded as an obstruction and the upper wall cannot be created.
However, If there is a legitimate support on a floor above the obstruction, the wall can be created.
If, in plan view, the wall intrudes into the interior of a joist area on a floor below, the joist area is regarded as an obstruction and the wall cannot be created unless the joist area supports the wall as described below.
Wall Supported By Beams and Other Walls
Important - A wall cannot be supported by two or more discontinuous beams. In this case, the wall must be split up into separate walls over each beam. The same applies to walls supported by other walls.
A wall can, however, be supported by only part of a beam or wall below it, and will create a partial line load on its support.
Wall Supported By Joist Area
A wall can be supported by a 4-sided joist area on the floor below if the wall extends exactly from one edge joist to the other. This ensures that it is fully supported and that it loads all of the joists.
Walls cannot be supported by 3 sided joist areas.
If the wall spans the interior of the joist area, it must be parallel to all but possibly one of the underlying supports of the joist area. Thus if there are four supports to a multi-span joist area, the wall must be parallel to at least three of them. This ensures that the outermost joists in the joist area are the critical ones.
There is an exception to the above rule if the wall sits on the cantilevered end of the joist area. In this case, the wall does not have to be parallel to any of the supports. The outermost joists will still be the critical ones.
Line loads applied to the top of the wall appear as point loads on the joists in the joist area.
A wall that is collinear with the side of a joist area which runs in the same direction as the joists will continue past that joist area down to the next floor.
A wall that is collinear with one of the underlying supports for a joist area on the floor will be supported by the beam or wall support, not by the joist area.
A legitimate wall can be disallowed in one unusual circumstance. It involves a wall on top of a sloping joist area that has a beam or wall on the same level but different gridpoint elevation beneath it for part of its span. It is difficult to imagine such a building.
Warning messages
In Sizer, a detailed message is supplied as to the reason the wall cannot be supported, and the identity of any member causing an obstruction.