Joists carry every floor, deck, and ceiling in a wood-framed house, and the distance between supports decides how deep those joists must be. An I-joist is an engineered wood member with flanges made from machine stress-rated lumber or laminated veneer lumber and a web of plywood or oriented strand board. The I-shaped section concentrates material where bending stress is highest and removes it where stress is low, so an I-joist is lighter than a solid beam of equal capacity yet straighter, stiffer, and far less likely to split, bow, or twist. A floor that must clear 16, 18, 20, 24, 26, or 28 feet without an intermediate wall follows the same logic engineers use for long-span residential framing, scaled down to a single member. Choosing the right depth and spacing is a structural decision, and the sections below walk through the sizing process step by step.
How an I-Joist Carries a Floor Load
An I-joist works as two parts doing two jobs. The top and bottom flanges resist the bending that tries to fold the member, while the web resists the shear that acts across the section. Each part supports the other: the flanges hold the web in place, and the web keeps the flanges apart at their working distance. Because the web is thin and the flanges are wide, the section reaches a strength-to-weight ratio that dimensional lumber cannot match. In the span-by-span casting method of bridge construction, each deck segment is engineered for the bay it crosses and the supports that define its load path; a floor joist follows the same rule at a much smaller scale.
Flanges and Webs Share the Work
The flanges are the wide pieces at the top and bottom of the member. They absorb the compression and tension that bending creates, and their width gives floor sheathing a broad surface for nailing and bearing. The web is the thin panel between them, and it carries the shear that builds near the supports. Typical I-joist webs are plywood or OSB, while flanges are usually MSR lumber or LVL, materials with tight grading and predictable strength.
Bending and Shear in Practice
Bending stress peaks at midspan, where the member wants to sag, and shear peaks near the ends, where the load transfers into the supports. That split explains the design rules that protect I-joists: never notch or cut through the flanges, because that removes the material fighting bending, and keep web holes inside the manufacturer’s chart, because oversized holes reduce shear capacity.
Joist Span and Spacing Work Together
Span is the clear distance between supports, measured from beam to beam or wall to wall. Spacing is the center-to-center distance between parallel joists, and the two values are always read together. A joist spaced 16 inches on center carries the strip of floor between it and the next joist, so doubling the spacing doubles the load on each member. Standard deck joist spans shift with member size, spacing, deck board thickness, and framing material, and residential floors are usually framed at 12, 16, or 24 inches on center. Space joists too far apart and the floor feels bouncy, because each member flexes under a wider share of the load. Engineers compare prestress layouts for span-by-span construction when they decide how loads transfer across intermediate supports, and the same continuous-versus-simple logic applies when joists lap over a center beam.
Reading a Spacing Plan
A floor plan rarely shows every joist, but the spacing note does the work. Framing at 12 inches on center uses more members and allows a shallower depth for the same span; framing at 24 inches on center halves the member count and usually demands a deeper joist. Most builders frame floors at 16 inches on center as the default and switch to 24 inches for decks and some second-floor layouts.
Recommended I-Joist Sizes for 16 to 28 Foot Spans
The table below gives practical starting points for residential floors under typical loads of 40 pounds per square foot live load plus 10 to 15 pounds per square foot dead load, with deflection limited to L/360. Manufacturers publish their own span tables, and local codes can impose stricter limits, so treat the table as a first pass and confirm the final size with the manufacturer’s data or an engineered layout.
| Span | 16 in. On Center | 24 in. On Center | Typical Use |
|---|---|---|---|
| 16 ft | 9-1/2 in. | 11-7/8 in. | Bedroom floors, small rooms |
| 18 ft | 11-7/8 in. | 11-7/8 in. | Living areas, dens |
| 20 ft | 11-7/8 in. | 14 in. | Great rooms, open kitchens |
| 24 ft | 14 in. | 16 in. | Double-height spaces |
| 26 ft | 16 in. | 18 in. | Cathedral ceilings |
| 28 ft | 18 in. | 20 in. | Long clear-span rooms |
How to Read the Table
Each row assumes a single clear span between supports at the spacing shown. The depths are nominal I-joist sizes such as 11-7/8 inches, and the actual required depth depends on the load, the joist series, and the deflection limit. Floors that carry tile or stone should be designed to L/480, which usually moves the selection one depth larger. A continuous multiple-span deck behaves differently from a row of simple spans, and the same principle lets a joist that runs across an intermediate beam carry more than the single-span table suggests.
When to Move Up a Depth
Choose the larger depth when the floor carries point loads from bearing walls above, masonry veneer, heavy tubs, or any finish that cracks under movement. When in doubt, the deeper member costs a little more material and buys a stiffer, quieter floor.
Loads, Deflection, and the Checks That Set the Size
Two load types govern joist sizing. Dead load is the weight of the framing, subfloor, underlayment, finishes, and fixed fixtures; live load is the moving weight of people, furniture, and stored goods. Residential floors are commonly designed for 40 psf live load, sleeping areas for 30 psf, and decks for 40 psf or more in snow country. Deflection, the amount the joist bends under load, is limited to a fraction of the span: L/360 for general floors and L/480 where brittle finishes are installed.
Working Through a Load Check
Run the check in the same order every time:
- Add the dead load and live load for the floor.
- Multiply the total by the tributary width to find the load per joist.
- Find the span and spacing in the manufacturer’s span table.
- Verify the deflection limit against the finish type.
- Confirm bearing length and any web openings.
Deflection Limits at a Glance
The limit is a fraction of the span, so longer spans are allowed to move more in absolute terms while the ratio stays the same. At L/360 a 20-foot span may deflect about 0.67 inch; at L/480 the same span may deflect about 0.5 inch. Tile installers and cabinetmakers notice the difference, which is why low-deflection floors specify the tighter limit.
The same verification habit extends beyond the framing. On a project with a concrete slab over the new floor system, the grain size analysis of aggregates verifies that the sand and stone in the mix meet specification before the slab is placed.
Installation Practices That Protect Span Performance
A correctly sized joist fails the job if it is installed wrong. Bearing length, hangers, blocking, and hole placement all change how the member carries load, and the engineering behind the span table assumes the framing follows the rules below.
Field Rules That Keep the Engineering Valid
- Bear on wood or steel for at least 1-1/2 inches, and use joist hangers where joists frame into a ledger or beam.
- Keep all holes in the web inside the manufacturer’s hole chart, and never cut or notch the flanges.
- Install blocking, squash blocks, or full-depth blocking at bearing points and around openings.
- Set the rim board around the perimeter to lock the joists in place and distribute lateral loads.
- Check each joist for straightness and correct any twist before fastening the subfloor.
These details are part of the larger topic of floor framing systems, where the span tables, subfloor installation, and structural design rules for residential floors all assume the pieces are done in sequence rather than skipped.
Costs and a Practical Sizing Workflow
Engineered joists cost more per linear foot than dimensional lumber, typically $2 to $4 per linear foot depending on depth and market, against roughly $1.50 to $3 for a 2×10 or 2×12. The total usually favors the I-joist on long spans because fewer members are needed, waste is lower, and the floor is flatter and quieter. Prices vary by region and availability, so compare delivered quotes rather than list prices.
A Step-by-Step Sizing Workflow
- Establish the span and spacing from the floor plan.
- Add the dead and live loads required by code.
- Select the deflection limit from the finish type.
- Pick a trial depth from the span table.
- Check bearing, continuity, and web openings.
- Confirm the size with the manufacturer’s software or a structural engineer.
For decks the sequence is identical, and most builders start by calculating deck joist options, then move through span tables, load requirements, and lumber selection before ordering a single board.