What Is Floor Joist Deflection

Considering the cross section of a typical joist the overall depth of the joist is critical in establishing a safe and stable floor or ceiling system.

What is floor joist deflection. Dead load weight of structure and fixed loads 10 lbs ft 2. Divide the total span of the floor joists in inches by 360 to determine the maximum amount the floor can give in the middleunder a live load of 40 lb sq. Maximum floor joist span for no. Allowable deflection is generally expressed as a fraction of the span in inches.

1 ft 0 3048 m. All structural members will deflect or flex under load. Live load is weight of furniture wind snow and more. This is the maximum deflection the designer should meet but it is only a minimum requirement.

The l 360 standard means that the floor should not deflect more than the span divided by 360. Joists must exhibit the strength to support the anticipated load over a long period of time. The amount of flex depends on the magnitude of the load applied span of the member and stiffness of the member. For example the maximum deflection for a joist span of 15 is 15 12 360 1 2.

See the image below for an example of joists spaced 16 on center 16 o c. 1 psf lb f ft 2 47 88 n m 2. For floor joists a live load deflection limit of l 360 and a total load deflection limit of l 240 must be met. Floors built in the 1950s or earlier usually feature subfloors of 1x slats fastened diagonally across the joists.

Like the framing members these slats and their connections can weaken over time causing creaks and squeaks and definitely deflecting with the joists. Ft plus any long term deflection due to the weight of the floor. If the span of the joists is 10 feet between supports then the deflection should not be more than 1 3 between the center and the end. In many countries the fabrication and installation of all framing members including joists must meet building code standards.

Typically for better performing floors minimal defection is desired. You should also check the ditra specs i seem to remember my tile guy telling me that when you re using natural stone ditra requires a thicker 1 1 8th i think subfloor anyway. Frequently there is misunderstanding regarding deflection between joists. The floor joist spacing is the distance between the centers of any two installed joists.

For example a floor joist spanning 20ft 240 inches a deflection limit of 2 3inch for live loads and 1inch for total loads. Since the example house we are designing for is 12 feet wide we need to find in the floor joist span table a joist size and centering that can span 12 or wider. 2 grade of douglas fir are indicated below.