Structural Strength
The combination of dimension lumber or engineered wood and wood panel sheathing gives wall, floor and roof assemblies the strength to resist vertical loads (snow, occupants and contents) and horizontal loads (wind and earthquake). Partitions, closets and finishes such as gypsum board (drywall) also add rigidity.
Where additional strength is required because of a high risk of exposure to earthquake or wind, the floors, walls and roofs can be made stronger by using thicker sheathing panels and by placing the fasteners closer together.
In addition, the floors, walls and roofs of wood-frame buildings are meant to act as a structural unit. This is why the floor must be bolted to the concrete foundation. Additional connections are also required between the roof or floors and walls and between the walls and the foundations to provide a continuous and complete load path, especially in high exposure category areas for wind or earthquake.
Source : Canada Mortgage and Housing Corporation (CMHC)