All parts of a floor have to work together for it to function properly. Posts should be spaced so the main beam is not over spanned. Joists need to be deep enough to support their loads without bouncing excessively, and the subflooring must be thick enough to span between the joists without sagging. Mid-span bridging helps to minimize joist flexing.
vapor retarders protect the finish flooring from moisture rising from below. Finish flooring usually runs perpendicular to the joists for better support.
Floors need to be both, but with the materials used in most construction, it’s safe to assume that if the floor is stiff enough, it’s also strong enough.
Stiffness is measured by the floor’s deflection under load.
The standard load designed for most residential floors to support is 50 lb. per sq. ft. Of that, 10 lb. or 12 lb. per sq. ft. is assumed to be the dead load, or the weight of the building materials themselves, and 40 lb. per sq. ft. is the live load, or the weight of the furniture, people, and pets the floor will support.
Commercial and public building floors are designed to carry greater loads. And, of course, we’ve all experienced old houses whose floors were visibly sagging, and which bounced when walked on. These floors either have been weakened with time, or were inadequately designed from the start.
If you find a floor that deflects more than the standards given in the sidebar on the facing page, or that comes very close, there are several things you can do. You might be able to beef it up using one of the methods. Keep in mind that without having an engineer design the fix, you won’t be able to predict the exact effect of the reinforcement.