I-Joist Span Calculator

I-joist span tables

The longest clear span of each APA Performance Rated I-Joist (PRI) on a simple span, under 40 psf live load and 10 psf dead load, on 1-3/4 in end bearings with no bearing stiffeners. Worked out by the calculator from APA's design values for the bare joist, as for a subfloor that's nailed but not glued. APA's own tables, for a subfloor glued and nailed to the joists, run a foot or so longer.

Live load deflection to span/480 (the PRI rating)

Live load deflection to span/360 (the IRC's limit for floors)

For other loads, spacings and bearings, use the calculator above.

How the calculator checks an I-joist

APA, the Engineered Wood Association, rates I-joists by depth and series, from PRI-20 to PRI-90. A joist stamped with a PRI rating meets that series' design values in APA's PRI-400 standard, whoever made it. The calculator checks one joist on a simple span with those values, for normal load duration, against:

• Bending and shear under the dead and live load, and under the dead load alone at 0.9 of the joist's capacity, the load duration factor for loads that stay on for good.
• The end reaction: the load each end puts on its support, against what the joist can take on its bearing length. A 4 in bearing, or bearing stiffeners nailed to the web, lets some joists take more.
• Deflection under live load to the span over 480, the stiffness every PRI is rated for, or over 360, the least the International Residential Code (IRC) allows for floors; and under the total load to the span over 240. An I-joist's thin web lets it sag more in shear than a solid joist does, so the sag counts that too, as APA's guide works it out.

The span checked runs center to center of the bearings: the clear span plus one bearing length. The subfloor isn't counted on to stiffen the joist, so the results suit a nailed subfloor and run a little short for a glued one.

I-joist questions

How far can an I-joist span?

At 16 in apart under 40 psf live load and span/480, a 9-1/2 in PRI-20 spans 13′ 6″, an 11-7/8 in PRI-40 18′, a 14 in PRI-60 21′ 8″ and a 16 in PRI-90 27′ 7″, with a nailed subfloor.

What about TJI, BCI and other brands?

This calculator covers joists stamped with an APA PRI rating, which several makers produce. Brand-name joists such as these have their own design values in the maker's evaluation report, often higher than a PRI series of the same depth, and their own span tables. Use the maker's tables or software for them.

What hanger does an I-joist need?

One made for an I-joist of that depth and flange width, and rated for at least the end reaction the calculator gives. Hanger makers list their hangers by joist size and load, and some need bearing stiffeners in the joist.

Can I cut holes in an I-joist?

Only in the web, and only where and as large as the joist maker's hole chart allows. Never cut or notch the flanges.

Should I use I-joists or 2x joists?

I-joists span farther for their depth, stay straight and come in long lengths. For short spans, 2x joists are often cheaper: compare with the floor joist span calculator.