What is a shear wall? What is a load baring wall? How do they differ?

Answer 1

A shear wall is one that is designed to resist a twisting motion that may result from an earthquake or wind load. Modern wood framing practices use ⅜" plywood on all exterior walls, as well as some of the interior walls, so they will resist the shear forces that might otherwise cause the structure to collapse. The nailing schedule is usually 4" or 6" on the edges, and 12" in the center.

A load bearing wall is designed to carry the weight of an upper floor or the roof. Most exterior walls are load bearing walls, as well as some interior walls. Usually load bearing walls are also shear walls.

Answer 2

These walls are designed to carry wind loads (side loads) on the building and are the same as the X bracing you often see. The resulting load on these wall creates a shear load on the wall where the upper half of the wall and the lower try to move in opposite directions. A structural engineer needs to get involved if you want to cut into one of those wall.

A load bearing wall actually carrys part of the roof structure load on the top of the wall. Instead of columns supporting the roof, the wall is used.