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Flooring - Deflection and Vibration

Deformation of Floor Joists - 1

Problem: Deformation of flooring joists - 1


Under-designed floor joists.

Floor joists will deflect or deform to some degree, no matter how large or small the load. When joists are installed with the crown or camber up, the deformation from small loads is probably not even noticeable. When the loads are large, the deformations can become excessive to the point where the floor may not be acceptable.


Decrease spans:

  • The span tables available in most building codes should be used to determine the joist size, spacing, and maximum spans permitted. As with beams, the selection of the joist size is a function of the loading, the length or span the type and grade of lumber.
  • If unusually large loads are to be applied, decrease the span of the joists. Unusual loading conditions may include the use of concrete toppings, large hot tubs, masonry fireplaces, etc. Professional design is required when such conditions are expected.

Decrease joist spacing or use a stronger species of wood.

  • Another way to accommodate large loads and reduce deflections is to strengthen the floor system by spacing the joists closer, or using a stronger species of wood.
  • Spacing joists at 12″ (300 mm) o.c. rather than 16″ (400 mm) o.c. will reduce the load carried by each joist by about 25%. There should be a corresponding reduction in the amount of deflection that will occur.
  • Using a stronger species of wood will also reduce the amount of deflection. A property of the wood that affects the amount of deflection is the modulus of elasticity. S-P-F (No. 1) has a modulus of 9,300 while Douglas fir (No. 1) has a modulus of 12,400. The higher the modulus, the stronger the wood, resulting in smaller deflections.
  • Using stronger species of wood or decreasing the joist spacing will reduce vibrations as well as deflections.
  • Stress-rated lumber is much less variable than standard sawn lumber, resulting in stronger members overall. Use stronger lumber to accommodate heavier loads.