What Is Floor Joist Deflection

All structural members will deflect or flex under load.

What is floor joist deflection.

Typically for better performing floors minimal defection is desired. In many countries the fabrication and installation of all framing members including joists must meet building code standards. For example a floor joist spanning 20ft 240 inches a deflection limit of 2 3inch for live loads and 1inch for total loads. 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.

Deflection is the bending or sag caused by loading. This is the maximum deflection the designer should meet but it is only a minimum requirement. 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. For floor joists a live load deflection limit of l 360 and a total load deflection limit of l 240 must be met.

Live load is weight of furniture wind snow and more. See the image below for an example of joists spaced 16 on center 16 o c. Dead load weight of structure and fixed loads 10 lbs ft 2. The amount of flex depends on the magnitude of the load applied span of the member and stiffness of the member.

Allowable deflection is generally expressed as a fraction of the span in inches. 1 psf lb f ft 2 47 88 n m 2. 1 ft 0 3048 m. Ft plus any long term deflection due to the weight of the floor.

The l 360 standard means that the floor should not deflect more than the span divided by 360. 2 grade of douglas fir are indicated below. 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. 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. The most common cause of deflection in a floor joist system is undersized joists that span too great a distance for their thickness and width. For example the maximum deflection for a joist span of 15 is 15 12 360 1 2. Floor joist deflection help you ll get a much stiffer floor by installing bridging between the joists and doubling up on your plywood than by just sistering the joists with 2x4.

Joists must exhibit the strength to support the anticipated load over a long period of time. The floor joist spacing is the distance between the centers of any two installed joists. Frequently there is misunderstanding regarding deflection between joists.