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Flooring - Deflection and Vibration

Floor Vibration 1

Problem: Floor Vibration - 1


Vibration-sensitive areas, such as kitchens and dining rooms, may require special attention beyond the minimum spans specified by building codes for floor joists.

Even though most building codes use vibration as a criterion for determining allowable spans, there may be instances where the allowable vibration is more than homeowners would like.


Decrease joist spans:

  • Reducing the distance between or adding additional joist supports will reduce the span of the structural members and hence stiffen the flooring system. In a typical flooring system utilizing a beam or girder support system - at mid span, adding an additional joist support beam or girder system will result in sizable reductions in floor vibration as shown in Figure 13a and 13b.
Reducing Joist Spans - Conventional Design
Figure 13a - Reducing Joist Spans - Conventional Design
Reducing Joist Spans - Alternative Design
Figure 13b - Reducing Joist Spans - Alternative Design

Even if there is currently no support columns, or if it would appear, by desired, the homeowner can add a beam with appropriate support columns in the area where the excessive vibration is occurring.

It is important to install the support columns on footings. If the support columns are not placed on proper footings and only rest on the concrete floor it is likely, that over time the concrete floor will crack.

The support columns used in floor support systems have multiple terms used to identify them. They can be found as lally posts, lally columns, adjustable floor jacks (Figure 13c) or adjustable columns.

Increase depth of joists:

adjustable floor jack
Figure 13c Adjustable Floor Jack

This fix is not realistic if the house is already constructed. As removing floor joists to increase their physical dimensions would require the complete dismantling of the home.

The value of this fix or construction method, is to prevent the excessive floor vibration problem from occurring in the first place.

  • Increasing the depth of joists, while retaining the same spacing between joists can dramatically increase the stiffness of the floor. Since the allowable span, controlled by vibration is determined by deflection at mid span, deeper joists are significantly better.
  • If a floor, that is typically constructed with 2″ × 8″ (38 × 184 mm) joists is instead constructed using 2″ × 10″ (38 × 235 mm) joists, the increase in the maximum span controlled vibration increases by approximately 18%. Switching from a 2″ × 10″ (38 × 235 mm) construction to a 2″ × 12″ (38 × 286 mm) construction results in an increase in the maximum span controlled by vibration by about 14% as shown in Figure 14.
Depth Of Floor Joist versus Deflection
Figure 14 - Depth Of Floor Joist versus Deflection
  • If you plan to use deeper joists, think about the amount of shrinkage that will take place. Joist shrinkage can cause other problems, such as drywall cracking. The deeper the members are, the greater the shrinkage that may occur.