How to Easily Calculate Allowable Deflection

Authored by David Micnhimer, Professional Engineer with Weyerhaeuser

Calculating how much a member will deflect and if that member meets the code minimums is a complicated engineering process. ForteWEB®, our member sizing software solution, can help simplify this process without the use of beam deflection formulas. But first, what is deflection? 

What is Deflection?

Deflection is the bending or "sag" caused by loading. All structural members will deflect or flex under load. The amount of flex depends on the magnitude of the load applied, span of the member, and stiffness of the member. Typically, minimal deflection is desired for better performing floors.  

Deflection illustration

Maximum Allowable Deflection

Chapter 3 of the International Residential Code (IRC) provides the maximum allowable deflection for a given structural member (floor, roof, wall, etc.). See the table below. Allowable deflection is generally expressed as a fraction of the span. A larger number in the bottom of the fraction represents a more stringent limitation. For example, the allowable deflection of a 12 ft span floor joist with plaster (L/360) is 0.4 inches (12 ft divided by 360). If that same joist had gypsum ceiling (L/240), the allowable deflection is 0.6 inches.

Table R301.7 Allowable Deflection of Structural Members

Structural Member

Allowable Deflection

Rafters having slopes greater than 3:12 with no finished ceiling attached to rafters

L / 180

Interior walls and partitions

H / 180

Floors and plastered ceilings

L / 360

All other structural members

L / 240

Exterior walls with plaster or stucco finish

H / 360

Exterior walls-wind loads with brittle finishes

H / 240

Exterior walls-wind loads with flexible finishes

L / 120

Lintels supporting masonry veneer walls

L / 600

L: span length      H: span height

Calculating Allowable Deflection Without Deflection Equations

ForteWEB software makes calculating allowable deflection a breeze. You can specify a custom deflection limit for your project or use the code minimums. Here you can see a 24-foot long, 5 1/4” x 16” 2.0E Parallam® PSL floor flush beam with 5 feet of tributary width. When loaded with 12 psf dead load and 40 psf live load, this beam has a live load deflection of L/640 and a total load deflection of L/447. These deflections easily meet the code minimum requirements of the IRC.

ForteWEB member report


For more questions about Parallam® PSL members, consult Weyerhaeuser Technical Support or our specifier’s guides.


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