Quick answer
A floor joist is one of a series of parallel horizontal framing members, usually lumber or engineered wood I-joists, that span between walls, beams or foundations to carry the subfloor and everything on it down to the supports at each end.
Joists are sized by span, spacing, species and load. Cutting, notching or drilling them is limited by code, and engineered joists follow the maker’s hole rules.
Quick facts
The key facts about Floor Joist, with sources:
Key takeaways
- Floor joists are the horizontal members that hold up a floor and pass its weight to walls, beams and foundations.
- How far a joist can span depends on its size, spacing, species and grade, and the load it carries.
- The code limits notches and holes in lumber joists; engineered I-joists follow the maker’s hole charts.
- Remodels often add weight (tile, stone, bathtubs) or cut joists for pipes; both need a professional check.
- Bouncy, sagging or cracked joists are a structural question for a licensed engineer, not a DIY fix.
What a floor joist does
Picture a floor as a deck of boards laid on edge, side by side, usually 12, 16 or 24 in apart. Each board is a joist.
The subfloor panels sit on top, and the ceiling of the room below often hangs underneath.
Each joist works like a small beam: it bends a little under load and passes that load to its two ends.
Two families dominate. Sawn lumber joists are solid boards such as 2×8, 2×10 and 2×12.
I-joists are engineered members shaped like the letter I. APA, the engineered wood trade association, describes them as flanges of sawn or structural composite lumber and webs of OSB, bonded with exterior-type adhesives 3.
APA’s performance-rated I-joists come in 9 1/2, 11 7/8, 14 and 16 in depths and lengths up to 60 ft from most makers 3.
Joists are part of a load path: floor to joist, joist to beam or wall, wall to foundation. Changing any link, such as removing a wall a joist bears on, changes the whole chain.
How a joist carries a floor
- Load lands on the subfloor. People, furniture, tile and built-ins are “live” and “dead” loads.
- The joist bends. The top edge is squeezed and the bottom edge is stretched. That is why notches on the bottom edge mid-span are so damaging.
- Stiffness limits bounce. Span tables cap deflection, for example span/360 for the lumber table in one city handout 2 and span/480 for APA’s I-joist tables 3.
- Ends bear on supports. The joist hands its load to a wall, beam or rim at each end.
- Floor finish and subfloor
- Floor joistsSpaced 12, 16 or 24 in apart
- Beam, girder or bearing wall
- Posts or foundation wall
- Footing and soil
Removing or cutting any link changes how the rest carries load; that is engineering work. Source: [1]
Types of floor joists
| Type | Make-up | Holes and notches | Notes |
|---|---|---|---|
| Sawn lumber (2×6 to 2×12) | Solid boards, graded by species 2 | Code limits in IRC R502.8.1 1 | Common in older homes and additions |
| Wood I-joist | Lumber or composite flanges, OSB web 3 | Maker’s hole chart only 14 | Longer spans; never notch flanges |
| Open-web floor truss | Lumber chords and webs joined by plates | Not cut or drilled without engineer approval | Room for ducts and pipes through the open web |
| Rim (band) joist | Board or engineered rim at the joist ends | Maker’s rim hole rules for engineered rims 4 | Closes the floor edge and carries walls above |
| Sistered joist | A new joist fastened beside an old one | Designed case by case | A common repair for damaged or weak joists |
Example: why a new addition might use I-joists
A homeowner plans a 19 ft clear span over a new family-room addition, with joists 16 in on center. Here is how two published tables compare for a 40 psf live load and 10 psf dead load. These are illustrations from published tables; a designer or engineer sizes the real floor.
| Option at 16 in on center | Allowable span in the table | Table basis |
|---|---|---|
| 2×10 Douglas fir-larch No. 2 (lumber) | 15 ft 7 in 2 | L/360; 2022 California Residential Code table, City of Sacramento form |
| 2×12 Douglas fir-larch No. 2 (lumber) | 18 ft 1 in 2 | Same table |
| 11 7/8 in APA PRI-40 I-joist | 19 ft 4 in 3 | L/480, glued-nailed subfloor, APA Table 11 |
| 14 in APA PRI-40 I-joist | 21 ft 11 in 3 | Same APA table |
In these tables, a 2×12 lumber joist falls just short of 19 ft at 16 in spacing, while an 11 7/8 in I-joist reaches it with a stiffer L/480 limit.
Heavier finishes such as tile change the dead load and the answer: the Sacramento table drops the 2×10 span to 14 ft 3 in with a 20 psf dead load 2. This is why home additions go through plan review.
Where joists come up in a remodel
Bathroom and kitchen remodels. Drain lines, toilet flanges and new tubs often land on joists. Cutting to route pipes is limited by code for lumber and by maker charts for I-joists 14. A heavy tub or stone floor may need joists checked or reinforced.
Basement finishing. The joists are the basement ceiling. Ducts, wires and recessed lights must respect hole rules 1.
Wall removal. Taking out a wall that joists bear on means a beam is needed; see headers and engineered beams.
On the plans. Framing plans show joist size, spacing and direction, often as “2×10 @ 16 in o.c.” or an I-joist series such as PRI-40 3.
At inspection. Framing is checked at the rough-in inspection, before ceilings close it up.
Lumber rules do not apply to I-joists
ICC’s guidance says the IRC’s notching limits for sawn lumber do not apply to engineered wood products. Holes in I-joists are acceptable only where the maker allows them or a registered design professional designs them 1.
Lumber joists vs I-joists: strengths and trade-offs
Engineered I-joists
Limits: added weight, cuts and moisture
Span tables assume a design load. The tables cited here use 40 psf live load with a 10 or 20 psf dead load 23. A mortar-bed tile floor, a stone island or a cast-iron tub adds dead load, and a floor that met the table when built may need a fresh check.
Cuts weaken joists. ICC’s CodeNotes says that whenever you cut a hole or notch in a joist, that joist is weakened, and that notching should be avoided if possible 1.
Moisture is the slow limit. Wet crawl spaces and plumbing leaks rot joists and the ends where they bear. Joists that stay wet can grow mold and lose strength 5.
What drives joist work cost, and how long joists last
We do not publish framing prices. In a remodel, joist costs come from engineering (a structural check or design), access (opening ceilings or floors), materials (lumber, I-joists, hangers, sister members), permits and inspection, and repair of finishes afterward. Replacing rotted joist ends or a damaged sill is usually more labor than material.
Dry, properly sized joists last as long as the house. What shortens their life is water, insects and unplanned cuts. The cost guide explains how structural surprises fit into a remodeling contingency.
Who handles joist work, and the steps
Any change to floor framing should involve a licensed structural engineer or the joist maker’s design guidance, a licensed contractor, and a building permit where your jurisdiction requires one.
- Identify the joist type, size, spacing and direction from plans or by inspection.
- Have a licensed engineer review new loads, wall removals or damaged joists.
- Pull a permit for structural changes; see the permits guide.
- For pipe or duct routes, follow IRC limits for lumber or the maker’s hole chart for I-joists 14.
- Install repairs or new framing exactly as designed (sistering, blocking, hangers, beams).
- Pass framing inspection before ceilings and floors close.
Keeping floor joists healthy
- Keep crawl spaces and basements dry; fix leaks and drainage first 5.
- Look at joist ends and sill plates for rot or insect damage during any basement or crawl space work.
- Do not let trades cut joists without checking the limits 1.
- Keep a copy of framing plans for future remodels.
- Avoid storing heavy items on attic or floor framing not designed for it.
Warning signs and when to call an engineer
- A floor that sags or slopes noticeably toward the middle of a room.
- Excessive bounce when walking, or dishes rattling.
- Cracked, split or heavily notched joists seen from the basement.
- Soft, dark or crumbly wood at joist ends: rot.
- Cracks in tile or drywall above a single area of the floor.
Never cut or remove a joist without a design
Cutting, notching, boring or removing a joist can weaken the floor and the walls above it.
Have a licensed structural engineer or the I-joist maker’s guidance set what is allowed, use a licensed contractor, and expect a permit and inspection.
ICC notes that alterations to engineered joists must be allowed by the maker or designed by a registered design professional 1.
Codes and standards for floor joists
IRC Chapter 5 (floors). The International Residential Code has floor joist span tables, such as Table R502.3.1(2) for 40 psf living areas, reproduced in local handouts like Sacramento’s for the 2022 California Residential Code 2.
Section R502.8 limits cutting, drilling and notching.
Per ICC’s CodeNotes based on the 2021 IRC, R502.8.1 says notches in solid lumber joists shall not exceed one-sixth of the member’s depth, shall not be longer than one-third of the depth, and shall not be in the middle third of the span; end notches may not exceed one-fourth of the depth 1.
Engineered wood. The lumber limits do not apply to engineered products; alterations must be allowed by the maker or designed by a registered design professional 1. APA PRI I-joists are built to APA Standard PRI-400 and recognized in ICC-ES report ESR-1405 3.
Existing buildings. Alterations to existing homes may be reviewed under the existing building code your jurisdiction adopted. Local adoption and amendments vary.
Floor joists vs related framing members
| Member | Direction | What it carries |
|---|---|---|
| Floor joist | Horizontal, repeated | Floor loads to beams or walls 1 |
| Ceiling joist | Horizontal, repeated | Ceiling and attic loads; often ties rafters |
| Roof truss | Sloped, triangulated | Roof loads to exterior walls |
| Wall stud | Vertical | Loads from floors and roof down to the plate |
| Beam or girder | Horizontal, single member | Joist ends at mid-span supports |
Floor joist myths
- Myth A small notch never matters.
- Reality Location matters as much as size; notches are banned in the middle third of the span 1.
- Myth I-joists can be drilled like lumber.
- Reality Only where the maker’s hole chart allows, or an engineer designs it 14.
- Myth If it held for 50 years, it is fine for new tile.
- Reality Span tables assume a dead load; heavier finishes change the check 2.
- Myth Bouncy floors are just old houses.
- Reality Bounce can mean undersized, damaged or over-cut joists; have it checked.
- Myth Sistering is always the fix.
- Reality Repairs are designed for the specific problem and load.
What changes by state: code edition and design loads
Joist span tables come from the residential code your state or city adopted, and states may amend them; the Sacramento handout, for example, is drawn from the California Residential Code 2.
Snow and seismic design loads that vary by region affect roofs and walls more than floors, but they can change what a floor beam or bearing wall must carry.
Damp crawl spaces in humid regions raise the rot risk at joist ends 5. Check the code edition with your building department before design.
When joists need attention in your project
Joists matter whenever a remodel adds weight, moves plumbing across the floor, removes or moves a wall, adds an opening such as a stair, or uncovers damage. In each case the next step is the same: have a licensed structural engineer or the joist maker’s design guidance set the fix, and permit the work.
If you are hiring, ask how the contractor handles joist conflicts for drains and ducts. The contractor selection guide lists what else to ask.
- If New tile, stone or a heavy tub
- Then Have the floor’s dead load checked
- If Removing a wall the joists rest on
- Then Engineer-designed beam and a permit
- If Sagging, bounce or rot found
- Then Structural engineer evaluation before repairs
Questions about Floor Joist
What is the difference between a floor joist and a beam?
Joists are a series of smaller, evenly spaced members that hold up the floor sheathing. A beam, or girder, is a single larger member that supports the joists partway across their span. The joists sit on or hang from the beam, which then rests on posts or walls.
How far can a floor joist span?
It depends on size, species, grade, spacing and load. In the City of Sacramento’s table for 40 psf live and 10 psf dead load, a Douglas fir-larch No. 2 2×10 at 16 inches on center spans 15 feet 7 inches.
APA’s table lets an 11 7/8 inch PRI-40 I-joist span 19 feet 4 inches at the same spacing.
Can you cut or notch a floor joist?
Only within strict limits, and not without checking. For sawn lumber, ICC’s summary of IRC R502.8.1 limits notches to one-sixth of the depth and bans them in the middle third of the span. Engineered I-joists follow the maker’s hole rules. Have an engineer or the maker’s guidance confirm any cut.
Can you drill holes in I-joists?
Only where the I-joist maker allows. APA publishes web hole specifications and rules for cutting holes in its performance-rated I-joists. ICC notes the IRC’s lumber notching rules do not apply to engineered products. Never cut or notch I-joist flanges.
What spacing are floor joists?
Common spacings are 12, 16, 19.2 and 24 inches on center. Span tables list each spacing separately, and closer spacing allows a longer span or a stiffer floor. Sixteen inches is the most common in houses.
Why is my floor bouncy?
A bouncy floor usually means the joists are deflecting more than you would like under load. Causes include long spans, undersized or damaged joists, missing blocking, or joists weakened by cuts. A licensed structural engineer can find the cause and design a fix such as sistering or a new beam.
What is sistering a floor joist?
Sistering means fastening a new joist alongside an existing one to strengthen or repair it. It is common for cracked, over-notched or rotted joists. The size, length and fastening of the sister should be designed for the specific problem, and the work may need a permit.
Do you need a permit to repair floor joists?
Usually yes. Structural repairs and changes to floor framing generally require a building permit and inspection. Rules vary by jurisdiction, so check with your building department. A permit also creates a record that helps when you sell the house.
Can new tile go on an old floor?
Often, but check first. Tile adds weight and needs a stiff floor to avoid cracking. Span tables assume a design dead load, and the table span drops when dead load goes up. Have the joists and subfloor checked before adding stone or mortar-bed tile.
Sources
- International Code Council, Building Safety Journal, CodeNotes: Cutting, Drilling and Notching (2021 IRC), retrieved .
- City of Sacramento, CDD-0277 Residential Span Tables (2022 CRC, rev. 1-01-2023), retrieved .
- APA – The Engineered Wood Association, I-Joist Selection and Specification (Engineered Wood Construction Guide excerpt, Form E30X, 2019), retrieved .
- APA, Performance Rated I-Joists Design and Construction Guide (document page), retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Notching and engineered-wood rules are quoted from ICC’s CodeNotes based on the 2021 IRC; lumber spans from the City of Sacramento’s 2022 CRC table handout; I-joist spans and description from APA’s 2019 I-joist guide (hosted by a member maker).
The ICC code viewer blocked automated access on the retrieval date, so IRC table numbers should be confirmed against the edition your jurisdiction adopted.
The worked example uses published table values only and is not a design. No prices are given. Not reviewed by a licensed engineer.
Suggest a correction. We fix errors and say what changed.