How to read IRC span tables: joists, rafters and headers

This guide gives the inputs each table needs, a fixed lookup order and three worked lookups. It cites section and table numbers only. Every number in the examples is invented for this page and is not taken from any IRC table, so read the real values in your own book.

Why span tables matter on the B1

ICC's B1 outline describes a 60-question, two-hour, open-book exam. Three of its tasks name spans directly:

• 0402 Floor Joist Framing (3%), under floor construction: spans, bearing and connections, plus pre-engineered floor framing checked against the drawings and the manufacturer's specifications.
• 0503 Header Spans (2%), under wall construction and coverings: span, bearing and connections.
• 0603 Roof Framing (4%), under roof/ceiling construction: rafters, collar beams and ceiling joists for span, grade, type, connections and bearing, plus trusses checked against the drawings.

That is 9% of the outline, and each of those questions is usually a table lookup with conditions attached. Read the table quickly and those questions come down to finding the right page.

Where the tables are

ICC offers the B1 on the 2021 and 2024 IRC (see 2021 or 2024 IRC). The span sections below are in both editions. Use the numbers for the edition you booked.

For 2024 rafters, the 2024 IRC index lists R802.4.1, so start there and read the table numbers from your own book. Section R602.7 also covers single-member headers (R602.7.1), wood structural panel box headers (R602.7.3) and jack studs (R602.7.5). Tab these pages using the IRC tab list, and see IRC chapters for the B1 for where Chapters 5, 6 and 8 sit in the outline.

The inputs every span table needs

Before you open a table, write down these inputs. If one is missing, the lookup can't be finished.

1. Use of the space (live load). For floor joists, the 2024 IRC sends sleeping areas, and attics reached by a fixed stair, to Table R502.3.1(1), with a lower live load. Other areas go to Table R502.3.1(2) (ICC, 2024 Chapter 5). The room's use picks the table before you touch a number.

2. Dead load. The weight of the building materials themselves. The floor joist tables have separate span columns for two dead-load levels, and in the 2024 edition a footnote sends some seismic cases to R301.2.2.2 for their dead-load limits (ICC, 2024 Chapter 5). If the plans don't state a dead load, find out which column applies before reading a span.

3. Snow load or roof live load. The 2021 rafter tables are split by roof live load and by ground snow load (ICC, 2021 R802.4.1). Ground snow load is a local value. It comes from the design criteria your jurisdiction sets under R301.2, not from the span table.

4. Deflection limit. Written as L/Δ: the span divided by the allowed sag. A bigger number means a stiffer floor or roof and a shorter allowed span. The 2021 rafter tables use a stricter limit where the ceiling is attached to the rafters than where it isn't (ICC, 2021 Chapter 8). So "is the ceiling attached?" is a table-choice question.

5. Species and grade. Read them from the grade stamp on the lumber (outline task 0401 covers lumber quality). Each table lists rows for several common species and grades. For species, grades or loads the tables don't cover, the code points to the American Wood Council span tables (ICC, 2021 R802.4.1).

6. Spacing. The on-center distance between members. It's a column heading or row group in the joist and rafter tables. Measure it on site, because it often differs from the plans.

7. Size. The nominal member depth, and for headers the number of plies.

Headers add two more. The 2024 header tables are organized by building width and what the wall carries, and they also tell you how many jack studs are needed at each end. Spans may be interpolated between the building widths shown, and a reduction applies where the top of the header isn't laterally braced (ICC, 2024 R602.7). In the 2021 edition, Table R602.7(1) covers exterior bearing walls, R602.7(2) interior bearing walls and R602.7(3) open porches (ICC, 2021 R602.7).

The lookup order

Use the same order every time. It stops you reading a correct number from the wrong table.

1. Confirm the edition and any local amendments. Some jurisdictions amend these tables.
2. Name the member and its job. Floor joist, rafter, ceiling joist or header, and for a header, exterior wall, interior wall or porch.
3. Pick the table from the use of the space, the load, and (for rafters) whether the ceiling is attached.
4. Pick the row by species and grade.
5. Pick the column by spacing, size and dead load.
6. Read the cell. Feet-and-inches entries are written with a dash, so convert both numbers to inches before you compare them.
7. Measure the real span the way the table means it. Rafter spans are measured along the horizontal projection, not the sloped length (ICC, 2021 R802.4.1).
8. Read every footnote on that table and apply any adjustment factor.
9. Check the rest: bearing, connections, jack studs and notching. A span that passes doesn't make the member compliant on its own.

Worked lookup 1: a floor joist

All numbers below are invented for this example. "Species A, Grade 1" is a made-up row, not a real one.

The plans show a bedroom with joists of Species A, Grade 1 at the spacing drawn. The measured span is 14 ft 7 in (175 in).

• Step 2 and 3: a bedroom is a sleeping area, so the sleeping-area table applies, Table R502.3.1(1).
• Step 4 and 5: find the Species A, Grade 1 row, then the column for the spacing, the joist depth and the dead load on the plans.
• Step 6: suppose the cell reads 15-3 (183 in).
• Compare: 183 in is at least 175 in, so the span passes.

Now change one input. The owner turns the bedroom into a family room. That area is no longer a sleeping area, so the lookup moves to Table R502.3.1(2). Suppose the same row and column there read 13-11 (167 in). 167 in is less than 175 in, so the same joists now fail. The fixes are closer spacing, a deeper joist, a better grade or an engineered design. The use of the room changed the answer while the lumber stayed the same.

Worked lookup 2: a rafter (2021 numbering)

All numbers below are invented for this example.

The jurisdiction's ground snow load is 40 psf. The ceiling is not attached to the rafters. The rafter's sloped length is about 15 ft, but its horizontal projection is 12 ft 0 in (144 in). Rafter ties are installed partway up the attic rather than at the plate.

• Step 3: pick the rafter table for "ceiling not attached" at the right snow load. Here the load falls between two tabulated loads.
• Snow loads between tabulated values: Connecticut's 2021 IRC portion, hosted on ICC Digital Codes, adds a note that permits linear interpolation for ground snow loads that aren't tabulated (ICC, CT 2022 Chapter 8). The base 2021 IRC text has no such note, so on a base IRC question use the table for the next higher snow load.
• Step 6: suppose the next-higher-load table gives 14-6 (174 in) for the row and column that match.
• Step 8: where ceiling joists or rafter ties sit higher in the attic, the 2021 IRC adjusts the rafter span using Table R802.4.1(9) (ICC, 2021 R802.4.1). Suppose the factor for this tie height is 0.85. Then 174 × 0.85 = 147.9 in.
• Compare with the horizontal projection, not the sloped length: 147.9 in is at least 144 in, so it passes. Comparing against the 15 ft (180 in) sloped length would have failed a rafter that passes. Skipping the factor can do the opposite and pass a rafter that fails.

Worked lookup 3: a header with interpolation

All numbers below are invented for this example. Real tables use their own building widths.

An exterior bearing wall has a rough opening of 6 ft 1 in (73 in). The proposed header is two plies of one size of Species A, Grade 1. The building is 31 ft wide. Suppose the table's two nearest widths are 20 ft and 40 ft.

• Step 3: an exterior bearing wall points to Table R602.7(1). Find the group of rows for what this wall supports.
• Step 6: suppose the cell reads 6-4 (76 in) at the 20 ft width and 5-8 (68 in) at the 40 ft width.
• Interpolate: (31 − 20) ÷ (40 − 20) = 0.55. The span drops 76 − 68 = 8 in across that range, so 8 × 0.55 = 4.4 in. The allowed span is 76 − 4.4 = 71.6 in.
• Compare: 71.6 in is less than 73 in, so the header fails. Rounding the building down to the narrower width would have passed it wrongly.
• Step 8 and 9: if the header did pass, you would still read the jack-stud count from the same cell, check R602.7.5, and apply the reduction if the top isn't laterally braced.

Common slips

• Using the living-area table for a bedroom, or the other way round.
• Comparing a rafter's sloped length with a table built on horizontal projection.
• Reading the spacing column that matches the plans when the site spacing differs.
• Forgetting that a raised rafter tie changes the allowed rafter span.
• Treating feet-and-inches entries as decimals (14-6 is 14.5 ft, not 14.6 ft).
• Stopping at the span and skipping bearing, connections and jack studs.

Practice the method

Run each lookup in your own book, timed. The B1 study plan gives floors their own week and roofs another, so put these lookups into those weeks. For the same method applied to stairs, the IRC stair riser and tread check shows each limit by section for both editions. To turn a roof pitch and a level run into the sloped length, rise and angle before you compare, the rafter length calculator shows each step.

This page gives section and table numbers only and never reproduces code text or table values. Use your own licensed IRC.

See how fast your lookups are

Take the free 10-question check to see which framing and lookup areas to drill first. It takes minutes, needs no signup and explains every answer. The Rafter Tie course adds timed practice exams and a lesson for each outline area.

FAQ

How much of the B1 is span table work?

Three outline tasks name spans: floor joist framing, header spans and roof framing. Together they are 9% of the outline, and each question is usually a table lookup with conditions attached.

Why do I get span table lookups wrong?

Most wrong answers come from picking the wrong table or the wrong column, not from misreading a number. Use the same order every time: edition, member, table, row, column, cell, real span, footnotes, then bearing and connections.

Is a rafter span measured along the slope?

No. Rafter spans are measured along the horizontal projection, so comparing the sloped length with the table can fail a rafter that passes.

Are the numbers in this guide real IRC table values?

No. Every number in the worked lookups is invented for this page. Read the real values in your own licensed IRC.

Sources

Show the list

• B1 Residential Building Inspector Content Outline, International Code Council.
• 2021 IRC Chapter 5 Floors and 2024 IRC Chapter 5 Floors, International Code Council.
• 2021 IRC R602.7 Headers and 2024 IRC R602.7 Headers, International Code Council.
• 2021 IRC R802.4.1 Rafter size and 2024 IRC R802.5.1 Ceiling joist size, International Code Council.