Structural span
Structural span calculators and IRC 2021 span tables for joists, rafters, beams and headers
Eight calculators that read the prescriptive span tables of the 2021 International Residential Code: floor joists, ceiling joists, rafters, deck joists, deck beams, built-up beams, headers and the load on any of them. Every number is cited to its table, every table was checked cell by cell, and every result prints. Free, no signup.
A span table is a lookup, not a calculation: the code has already done the engineering for the common case, and the whole job is to open the right table and read the right cell. There is no single table for “a 2x8”. The IRC prints 9,637 span cells across six families of tables, and which one applies is decided by what the member does, what the space above it is used for and whether a ceiling hangs from it. The tools below make that choice for you and show the table they read.
Start with the which table do I need router if you are not sure which member you are sizing, or go straight to a calculator. If the question is how much weight is on the beam in the first place, the beam load calculator comes before all of them.
The eight structural span calculators, and the IRC 2021 table each one reads
Each tool reads one family of tables and nothing else. Pick a species, grade, size and spacing and it returns the maximum span; enter the span you have and it returns the smallest member that reaches it. Species, grades and spacings are the ones the code prints, so a combination the table does not list is reported as not tabulated rather than guessed.
Floor joist span calculator: Floor joists from IRC 2021 Tables R502.3.1(1) and R502.3.1(2)
Read at: 30 or 40 psf live, 10 or 20 psf dead, L/360.
Smallest joist that reaches your clear span, or the max span of the joist you have, for four species at four grades and four spacings; every cell of both tables is on the page as a chart.
1,024 cells on the page · every value cited to its table
Beam span calculator: Beams and girders from IRC 2021 Tables R602.7(1), (2) and (3)
Read at: By ground snow load, building width and what the beam carries.
Built-up 2x beams: interior girders under one or two floors, beams in exterior bearing walls and open-porch beams, with plies from double to quadruple and the required jack studs.
773 cells on the page · every value cited to its table
Rafter span calculator: Rafters from IRC 2021 Tables R802.4.1(1) to (8)
Read at: 20 psf roof live or 30, 50 and 70 psf ground snow; L/180 without a ceiling, L/240 with one.
All eight rafter tables, 2x4 to 2x12, with corrected cells flagged in the result and the Table R802.4.1(9) raised-tie factor explained.
5,120 cells on the page · every value cited to its table
Deck joist span calculator: Deck joists from IRC 2021 Table R507.6
Read at: 40 psf live or 50, 60 and 70 psf snow, 10 psf dead, wet service, L/360.
Joist span and the maximum cantilever past the beam for Southern pine, the fir group and the redwood and cedar group, at 12, 16 and 24 in.
528 cells on the page · every value cited to its table
Deck beam span calculator: Deck beams from IRC 2021 Tables R507.5(1) to (4), factor from R507.5(5)
Read at: By effective joist span, 40 psf live or 50 to 70 psf snow.
Single to triple ply 2x6 to 2x12 deck beams, entered with the effective joist span after the cantilever factor, which the tool applies for you.
1,680 cells on the page · every value cited to its table
Beam load calculator: Loads from IRC 2021 Tables R301.5 and R301.6
Read at: Live load by occupancy, roof live load by tributary area, your dead load.
Tributary area of a beam, header, joist or post, the pounds it carries, the load per foot and the end reactions, so you know which line of a span table to read.
loads, not spans on the page · every value cited to its table
Header size calculator: Headers from IRC 2021 Tables R602.7(1), (2) and (3)
Read at: By ground snow load, building width and floors carried.
Door, window and garage openings in exterior or interior bearing walls and porches: the smallest header that clears the opening and the jack studs each end needs.
773 cells on the page · every value cited to its table
Ceiling joist span calculator: Ceiling joists from IRC 2021 Tables R802.5.1(1) and R802.5.1(2)
Read at: 10 psf live and 5 dead without storage, 20 and 10 with limited storage, L/240.
Attic type test from Table R301.5 first, then the smallest ceiling joist for the span, 2x4 to 2x10, with a redirect to the floor tables for a habitable attic.
512 cells on the page · every value cited to its table
Which span table do I need? Member, use of the space above, and the table the IRC sends you to
The table is chosen by the member's job and by Table R301.5, which sets the live load from how the space above is used: 30 psf for sleeping areas, 40 psf for other rooms, 10 or 20 psf for uninhabitable attics depending on the clear height, and 40 psf for decks. A rafter is read from a roof live load or the ground snow load, whichever is larger, and reads a stricter deflection table when a ceiling is attached to it. Deck tables are separate because they include the wet-service factor and are printed for No.2 grade only.
| What you are sizing | Calculator | IRC 2021 table | Live / dead load | Deflection | Species and grades in the table | Spacings |
|---|---|---|---|---|---|---|
| A floor joist under a bedroom or any sleeping area | Floor joist | Table R502.3.1(1) | 30 / 10 (or 20) psf | L/360 | SP, DFL, HF, SPF at SS, No.1, No.2, No.3 | 12, 16, 19.2, 24 |
| A floor joist under living space, kitchen, hall, bath | Floor joist | Table R502.3.1(2) | 40 / 10 (or 20) psf | L/360 | same four species, four grades | 12, 16, 19.2, 24 |
| A joist under a habitable attic, bonus room or attic with fixed stairs | Floor joist | Table R502.3.1(1) at 30 psf | 30 / 10 psf | L/360 | same | same |
| A ceiling joist, uninhabitable attic without storage | Ceiling joist | Table R802.5.1(1) | 10 / 5 psf | L/240 | same four species, four grades | 12, 16, 19.2, 24 |
| A ceiling joist, uninhabitable attic with limited storage | Ceiling joist | Table R802.5.1(2) | 20 / 10 psf | L/240 | same | same |
| A rafter with no ceiling attached | Rafter | Table R802.4.1(1), (3), (5), (7) | 20 psf roof live, or 30, 50, 70 psf ground snow / 10 or 20 psf dead | L/180 | same four species, four grades | 12, 16, 19.2, 24 |
| A rafter with a ceiling attached | Rafter | Table R802.4.1(2), (4), (6), (8) | same loads | L/240 | same | same |
| A deck joist, and how far it can cantilever | Deck joist | Table R507.6 | 40 psf live or 50, 60, 70 psf snow / 10 psf dead, wet service | L/360 (L/180 cantilever) | SP; DFL, HF and SPF as one group; redwood, western cedars, ponderosa and red pine as one group; No.2 only | 12, 16, 24 |
| A deck beam | Deck beam | Table R507.5(1) to (4) | same four loads, by effective joist span 6 to 18 ft | L/360 | same three groups, No.2 only | single to triple ply |
| A header over a door, window or garage opening | Header | Table R602.7(1) exterior, (2) interior, (3) porch | by ground snow 30, 50, 70 psf and building width 12, 24, 36 ft | not stated in the table | one combined value for SP, DFL, HF, SPF at No.2 | single to quadruple ply, 2x4 to 2x12 |
| A beam or girder under a floor, in a wall or on a porch | Beam | Table R602.7(1) to (3), read as a beam | same | same | same | same |
| How much weight is on the member, before you size it | Beam load | Table R301.5, R301.6, R301.2 ground snow | live load by use of the space above | — | — | — |
A habitable attic is a floor in the code's eyes and reads the floor tables. Every calculator above enforces these choices; the ceiling joist tool, for example, stops and sends a habitable attic to the floor joist tool.
One 2x8, seven span tables: why the same board spans 11′ 1″ on a deck and 22′ 4″ over an empty attic
Nothing shows the table choice better than one board read from every table it appears in. A No.2 spruce-pine-fir 2x8 at 16 in on centre spans 12′ 3″ as a living-room floor joist and 22′ 4″ as a ceiling joist under an attic nobody can store anything in, twice the distance, because the ceiling table loads it at 15 psf instead of 50 and allows L/240 deflection instead of L/360. On a deck the same board drops to 11′ 1″, the wet-service penalty. As a rafter it reaches 18′ 2″ at 20 psf with no ceiling, at L/180. And doubled up as a header in a 24 ft wide house it spans only 5′ 9″, because a header carries the roof, not a strip of floor.
| Used as | IRC 2021 table | Load it is read at | Deflection limit | Max span |
|---|---|---|---|---|
| Floor joist, living area | Table R502.3.1(2) | 40 psf live, 10 psf dead | L/360 | 12′ 3″ |
| Floor joist, sleeping area | Table R502.3.1(1) | 30 psf live, 10 psf dead | L/360 | 13′ 6″ |
| Deck joist | Table R507.6 | 40 psf live, 10 psf dead, wet service | L/360 | 11′ 1″ |
| Ceiling joist, attic with limited storage | Table R802.5.1(2) | 20 psf live, 10 psf dead | L/240 | 16′ 3″ |
| Rafter, no ceiling attached | Table R802.4.1(1) | 20 psf roof live, 10 psf dead | L/180 | 18′ 2″ |
| Ceiling joist, attic without storage | Table R802.5.1(1) | 10 psf live, 5 psf dead | L/240 | 22′ 4″ |
| Double 2x8 header, exterior wall, roof and ceiling only, 24 ft wide house | Table R602.7(1) | 30 psf ground snow | — | 5′ 9″ (1 jack stud) |
Every value above is a printed IRC 2021 cell, read by the linked calculator at that setting. The header row is a double 2x8 in an exterior bearing wall supporting roof and ceiling only, 30 psf ground snow.
How a span is measured, and what every IRC span table assumes
- Clear span, face to face. Floor joists, ceiling joists, deck joists and deck beams are read at the clear distance between the faces of their supports, not the length of the board and not centre to centre. Each end also needs bearing, 1½ in on wood or metal and 3 in on masonry, so a board is longer than its span.
- Rafters are measured along the horizontal projection. The rafter tables give the run the rafter covers, not the sloping length. The stick you buy is longer by the pitch factor; the rafter length calculator turns a span and pitch into that length.
- Headers are read at the clear opening. The header tables print the span between the jack studs and, in the same cell, how many jack studs each end needs. The jack stud count changes with the span, not with the header alone.
- Repetitive members. Joist and rafter tables assume at least three members in a row at no more than 24 in on centre, sheathed on the top edge. A single joist, or one at 48 in, is outside the tables.
- Dry service, except decks. Floor, ceiling, rafter and header tables assume lumber at 19% moisture content or less in use. Only the R507 deck tables carry the wet-service factor, which is why a deck joist never out-spans the same joist indoors.
- Grades and species. Floor, ceiling and rafter tables print select structural, No.1, No.2 and No.3 for Southern pine, Douglas fir-larch, hem-fir and spruce-pine-fir. Header and deck tables print No.2 only; the deck tables add redwood, western cedars, ponderosa pine and red pine as a group. No.2 is the grade most yards stock and the tools default to it. What the stamp on the board means is on the lumber size chart.
- Spacing. 12, 16, 19.2 and 24 in are tabulated for floors, ceilings and rafters; the deck tables print 12, 16 and 24 only. 19.2 in is a real column, chosen so five spaces fit an 8 ft sheet, and the tools read it.
- No interpolation on this site. Where the code permits straight-line interpolation between tabulated values, these tools do not do it; a span between two columns is rounded up to the next tabulated member. That is always on the safe side and always checkable against the printed page.
How these span tables were verified, and the 17 defective cells the printed IRC 2021 contains
Every table on this site was entered from the 2021 Seattle Residential Code, which reprints the IRC tables with local amendments marked, and then checked against the same PDF by script, cell by cell, before a calculator was allowed to read it. Where the local text replaces a table, as Seattle does for deck loads, the struck original IRC values are used and the replacement is ignored. The result of each check is below; the methodology page describes the process in full.
| Table family | Printed cells | Check | Result |
|---|---|---|---|
| Floor joists, R502.3.1(1) and (2) | 1,024 | full script diff against the Chapter 5 text layer plus a 20-cell screenshot sample | 0 mismatches |
| Ceiling joists, R802.5.1(1) and (2) | 512 | full script diff against Chapter 8 plus an 18-cell screenshot sample; the 24 in column of table (1) checked against the 12 in column of table (2), 64 pairs | 0 mismatches, 64 of 64 pairs equal |
| Rafters, R802.4.1(1) to (8) | 5,120 | screenshot verification, then every strength-governed cell checked against its twin in the paired deflection table | 12 printed cells defective: 11 corrected from the twin table, 1 withheld |
| Deck joists, R507.6 | 528 | entered from the struck original IRC block of Chapter 5, 18 span and 2 cantilever cells screenshot-checked, monotonicity and cantilever-to-backspan checks | 0 mismatches |
| Deck beams, R507.5(1) to (4) | 1,680 | 18-cell screenshot sample, depth, ply, joist-span and cross-load monotonicity; the fir-group block also compared with the separately typeset Seattle replacement table, 84 cells | 5 printed cells defective, omitted from the tool and listed on the page |
| Headers and beams, R602.7(1) to (3) | 773 | full script diff against Chapter 6 plus an 18-cell screenshot sample of span and jack-stud count | 0 mismatches |
| All six table families | 9,637 | every chart on every page is generated from the same data file the calculator reads | 17 defective printed cells across two table families |
Where the printed code is wrong: 12 rafter cells and 5 deck beam cells
Two table families contain values that cannot be right as printed. In the rafter tables, 12 cells print a span that is longer than the cell for a bigger rafter, or a value such as “12 ft 12 in”. Because a strength-governed span does not depend on the deflection limit, each of those cells has a twin in the paired L/180 or L/240 table that prints the correct number, and the rafter calculator uses the twin and says so in the result; 11 cells were corrected that way and 1 is withheld. In the deck beam tables, two cells in a Douglas fir-larch single-ply 2x12 row repeat the 2x10 row above them, and three redwood-group cells print values that are not feet-and-inches at all; those 5 cells are left blank in the tool and listed on the page. Table R507.5(5) also prints the 1/12 cantilever ratio as “1/12 (0.87)” where 1/12 is 0.083, and the footnote that points to it cites the wrong table number. The full rafter list is in the IRC rafter table errata.
A chart copied from the code without this check inherits the errors, and a chart copied from the 2015 or 2018 edition carries deck beam values the 2021 code replaced. When a number on another site disagrees with these tools, the disagreement is usually one of those two things.
What the IRC span tables do not cover, and where those questions go
The prescriptive tables cover sawn dimension lumber in the common loading cases and nothing else. Everything below is outside them, and a calculator that returns a number for it is not reading the code.
- LVL, PSL, LSL and glulam beams: sized from the manufacturer's span tables or an engineer; the questions people bring are in beam span and size questions
- I-joists and floor trusses: manufacturer span charts only
- Roof trusses: engineered by the truss maker; the roof truss calculator covers count and geometry, not span
- Cold-formed steel joists and rafters (Sections R505 and R804)
- Point loads, a wall or post landing mid-span, hot tubs, pavers or planters on a deck
- Cantilevers beyond the table footnotes, and joist spans over 18 ft on deck beams
- Spans the code marks as exceeding 26 ft, which it does not print
- Species and grades not in the IRC tables, such as No.1 dense or machine-stress-rated lumber: the code sends these to the American Wood Council's span tables and span calculator
- Deck posts and footings: deck post size reads Table R507.4; deck footing size is a separate lookup
- Composite decking: the joist spacing the board needs is the maker's figure, covered on composite decking span
- Local amendments: a jurisdiction can replace a table, as Seattle does for deck live load, and the amended table governs there
- Anything a plan reviewer asks to see engineered
Span table guides: how to read one, the loads on it, and the questions the tables raise
- How to read a joist span table: species, grade, load and deflection, column by column
- Live load vs dead load: what the psf figures on every span table mean and where Table R301.5 gets them
- The IRC rafter table errata: every defective printed cell and its corrected value
- How far can a 2x6, 2x8, 2x10 or 2x12 span: as a joist, a rafter and a beam
- Joist spacing and size: floor, ceiling and deck, 12 to 24 in
- Beam span and size questions: 4x6, LVL and 20 ft spans
- Window and door header sizes: Section R602.7 explained
- Deck footing depth, joist size and framing questions
- Beam deflection and moment of inertia: what L/360 means, without the design
- Rafters vs trusses: span and when to use each
Span table questions from builders and homeowners
Which span table do I use for a bonus room or an attic room?
The floor joist tables. Table R301.5 rates a habitable attic, and any attic served by fixed stairs, at 30 psf, and Section R502.3.1 sizes its joists from Table R502.3.1(1) at L/360, not from the ceiling joist tables. The ceiling joist span calculator stops and redirects you when you choose habitable; the floor joist span calculator has the sleeping-area table it needs.
Why does the same 2x8 span a different distance in every chart?
Because each chart is a different table read at a different load and deflection limit. The IRC prints a floor at 30 or 40 psf live load and L/360, a ceiling at 10 or 20 psf and L/240, a rafter at 20 psf or the ground snow load and L/180 or L/240, and a deck at 40 psf with the wet-service factor. A No.2 spruce-pine-fir 2x8 at 16 in spans 11′ 1″ on a deck, 12′ 3″ under a living room and 22′ 4″ under an empty attic, and all three are correct. A chart that gives one span for a 2x8 has silently picked one of them.
Are the IRC span tables conservative, or is the printed span really the limit?
The printed span is the code minimum, and the margin is not in the span but in the design values behind it. The tables are computed from allowable bending stress and stiffness that already include the safety factors of the wood standard, so a joist at exactly its tabulated span is at its allowable stress, not at failure. Where deflection governs, the cell is set by the L/360 or L/240 limit and will feel firm but not stiff. Meeting the table is compliance; a stiffer floor is a design choice, usually one size deeper or one spacing closer.
Is a rafter span the length of the rafter?
No. The rafter tables give the horizontal projection, the run, and the sloping board is longer by the pitch factor. Size it on the rafter span calculator and get the board length from the rafter length calculator.
My species or grade is not in the IRC tables. What do I use?
The code's own answer. Section R502.3, R802.4 and R802.5 send other species, grades and loads to the American Wood Council's Span Tables for Joists and Rafters, and the AWC span options calculator reads those for every graded species. The IRC tables print four species groups at four grades because that is what most of the country frames with; redwood and the cedars appear only in the deck tables. Mixed loads of lumber are read at the weakest species and grade in the pile, which for the fir group is usually spruce-pine-fir No.2, the default in every tool here.
Does 19.2 inch on centre spacing exist in the span tables?
Yes, for floor joists, ceiling joists and rafters, where it is a printed column between 16 and 24 in. It exists because five spaces of 19.2 in fit an 8 ft sheet of subfloor or sheathing exactly. The deck tables do not print it; Table R507.6 and R507.5 give 12, 16 and 24 in only, and a deck framed at 19.2 in reads the 24 in column. The calculators list 19.2 in where the table has it and leave it out where it does not.
How do I know my live load and dead load?
Live load comes from Table R301.5 by how the space is used, roof live load from Table R301.6 by tributary area, and dead load is the weight of the assembly you are building. The beam load calculator lists the R301.5 values and turns them into pounds; the reasoning is in live load vs dead load.
Can my inspector use the spans from these calculators?
They are the 2021 IRC tables the plan reviewer reads, cited to the table so they can be checked against the printed page. Two things can still change the answer: the edition your jurisdiction has adopted, since the deck beam tables changed between 2018 and 2021, and local amendments, which can replace a table outright. Washington, for example, raises the deck live load from 40 to 60 psf and the tools here use the original 40 psf figure. Confirm both with your building department before you frame. Terms of use are on the disclaimer.
Where are the span tables for LVL, I-joists and trusses?
Not in the IRC. Engineered lumber is proprietary, so its spans come from the manufacturer's tables for that product and series, and trusses are designed by the truss maker for each job. The code accepts them as engineered products built to their own standards, but it prints no spans for them. The questions people bring about LVL beams are answered in beam span and size questions; roof truss count and geometry are on the roof truss calculator.
Why does a span chart on another site disagree with the code book?
Three common reasons. The chart may be from a different edition, and the 2021 deck beam tables replaced the single 2015 and 2018 table with four tables and a joist span factor. It may be read at a different load, such as a 40 psf floor chart presented as a general joist chart. Or it may reproduce one of the 17 cells that the printed 2021 IRC itself gets wrong, listed on this page and in the rafter table errata. The tools here cite the table and the setting for every number, so the source of a disagreement can be found.
How much weight is on my beam, and what size post does it need?
Weight is the beam load calculator: tributary area times the R301.5 load, in pounds, per foot and at each end. A deck post is then sized from Table R507.4 on deck post size; a post inside the house is an engineer's call.
Header, beam or girder: which calculator do I use?
Same tables, two questions. The header size calculator is for an opening in a wall and returns the header and its jack studs; the beam span calculator reads Table R602.7 as a girder under a floor or a beam on a porch.
Source of the span tables behind these calculators
All span values are IRC 2021 Tables R502.3.1, R802.5.1, R802.4.1, R507.6, R507.5 and R602.7, read from the 2021 Seattle Residential Code reprint published by Seattle SDCI, Chapter 5 for floors and decks and Chapter 8 for ceilings and rafters, with Chapter 6 for headers and Chapter 3 for loads. The parent standard for the joist and rafter tables is the American Wood Council's Span Tables for Joists and Rafters, whose span table tutorial explains the loads and deflection limits the tables are built on.