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Flooring - Deflection and Vibration

Deformation of Floor Joists - 3

Problem: Deformation of flooring joists - 3


Notching and drilling of joists.

When joists are notched or drilled improperly, the effective depth of the joist is decreased. A 2″ × 10″ (38 × 235 mm) joist may perform like a 2″ × 8″ (38 × 184 mm) joist if it is incorrectly notched or drilled. This weakening of the joist can result in excessive deflection or failure.


Keep holes away from the top and bottom edges and from mid-span areas:

    Notches, chases, holes and other penetrations through the joist are often required for plumbing, heating or electrical services. These can be accommodated if done correctly.

    Drill holes no larger than one-quarter the depth of the member. Locate them not less than 2″ (50 mm) from the edges unless the depth of the member is increased by the size of the hole.

    Floor joists can be notched, provided the notch is located on the too of the member within half the joist depth form the edge of the bearing. Do not notch deeper than one-third the joist depth unless you increase the depth of the member by the size of the notch as shown in Figure 25.

Correct Notching And Drilling Of Joists
Figure 25 - Correct Notching And Drilling Of Joists