CRC · California Residential Code

How do I size rafters and ceiling joists using the CRC span tables?

Use the rafter span table in § R802.4 that matches your loads and spacing, then—if your ceiling joists or rafter ties sit above the top of the wall—multiply that tabulated span by the HC/HR adjustment factor in Table R802.4.1(9); confirm bearing, ridge, nailing and lateral‑support rules in § R802.4 before building.

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Last reviewed: July 6, 2026

What the code requires

In plain English: use the CRC tabulated rafter span tables in § R802.4 and apply the table notes and adjustment factors (for example when ceiling joists or rafter ties are not at the bottom of the attic). The tables are conditioned on loads (dead/live/snow), member spacing and L/Δ limits; if the table conditions do not match your situation you must adjust or use engineered design. See § R802 and § R802.4 for the rules and the required adjustment table.

The most important rule: read the rafter span table in § R802.4 for your load, spacing, species/grade and L/Δ; if the ceiling joists or rafter ties are higher than the top of the support walls, multiply the tabulated rafter span by the correct HC/HR adjustment factor from Table R802.4.1(9).

Requirements in detail

Which tables to use

  • Use the rafter span tables located under § R802.4 (tables R802.4.1(3)–R802.4.1(8) and related tables), selecting the table that matches the roof load condition (e.g., ground snow load, ceiling attached or not, and L/Δ deflection limit). § R802.4 contains the tables and the table notes.

  • The tabulated spans assume the ceiling joists (or equivalent rafter ties) are located at the bottom of the attic space (i.e., at the top of the rafter support walls). If the ties are higher, use the adjustment factors in Table R802.4.1(9).

  • The tables specify the L/Δ (deflection) limit used for the tabulation. For example, some tables use L/Δ = 240 (ceiling attached) and others L/Δ = 180 (ceiling not attached). Pick the correct table before reading a span.

Decision-relevant dimensions / values (quick reference)

Decision item Typical values / thresholds Where it comes from Code Reference
Deflection limit when ceiling is attached L/Δ = 240 Table headings in R802.4 tables § R802.4
Deflection limit when ceiling not attached L/Δ = 180 Table headings in R802.4 tables § R802.4
Adjustment for ceiling/rafter tie height HC/HR ratios → factors 0.67, 0.76, 0.83, 0.90, 1.00 Table R802.4.1(9) § R802.4.1(9)
Maximum tabulated span availability note Span exceeds 26 feet (check lumber availability) Table notes § R802.4
Required ridge board minimum thickness 1‑inch nominal and depth not less than rafter cut Framing detail § R802.4.2

(See the table notes in § R802.4 for full header conditions — species/grade, spacing, dead/live/snow loads, and whether ceiling is attached; those determine which row/column you read.)

How the HC/HR adjustment works (explicit)

  • HC = vertical distance from top of the rafter support wall to the top of the ceiling joist or rafter tie.
  • HR = vertical distance from top of the rafter support wall to the roof ridge.
  • Compute HC/HR and find the matching adjustment factor in Table R802.4.1(9); multiply the tabulated rafter span by that factor to get the allowable span for your condition. The adjustment factors are listed in § R802.4.1(9).

Bearing, ties and framing details you must check

  • Rafters must be framed opposite at the ridge, and a ridge board not less than 1‑inch nominal thickness and not less deep than the cut rafter required. § R802.4.2 covers these framing details.
  • Where ceiling joists are not parallel to rafters, an equivalent continuous rafter tie is required. Rafter ties spaced and connected per the tables/notes are required to resist outward thrust. (See R802.4 notes and rafter tie connection tables.)

Exceptions & special cases

  • If the attic space is intended to be occupied (i.e., living space), ceiling joists must be designed per the applicable floor/ceiling rules (see related sections) — the simple span tables may not apply.
  • If the rafter support members (ridges, hips, valleys) are carrying significant load (roof pitch < 3:12), they must be designed as beams and bearing provided in accordance with § R802.4.4.
  • Where the tabulated span would exceed practical lumber length or 26 feet, check material availability or use engineered framing — the table notes warn about spans exceeding 26 feet.

Common mistakes

  • Reading the wrong table: mixing up tables for ceiling attached (L/Δ = 240) vs not attached (L/Δ = 180). The deflection limit matters — pick the correct table header.
  • Forgetting the HC/HR adjustment when ceiling joists or ties are not at the top of the bearing wall — this can significantly reduce allowable rafter span if ties are high. Always apply Table R802.4.1(9) when HC > 0.
  • Assuming tabulated spans cover all conditions — the tables are for specific species/grades, spacings and load combinations; if your load, spacing or member grade differ, you must adjust or use engineered design.
  • Neglecting to provide required bearing length (minimum 1½ inches for ceiling joists at top plate) or correct rafter-to-top‑plate nailing; these are framing requirements in R802.4 and related tables.

Worked example — step‑by‑step (illustrative)

Scenario: roof with 24‑inch o.c. rafters, ceiling joists (or rafter ties) located 2 ft above top of wall, roof ridge 8 ft above top of wall, you read a tabulated allowable rafter span of 16 ft‑6 in from the appropriate R802.4 table for your species/grade and loads (pick the correct table based on L/Δ and loads).

  1. Compute HC/HR = 2 ft / 8 ft = 0.25 → nearest ratio = 1/4.
  2. From Table R802.4.1(9) the adjustment factor for 1/4 is 0.76. § R802.4.1(9).
  3. Adjust tabulated span: 16 ft‑6 in × 0.76 = 12 ft‑6 in (approx). That adjusted span is the maximum allowable rafter span for this geometry.
  4. Confirm bearing, ridge board, and rafter tie connections per § R802.4.2 and the table notes (nailing, ties, bearing lengths).

Notes: the numeric tabulated span (16‑6) was taken from the appropriate rafter table for the roof load/spacing/species — you must read the exact span for your species/grade and load from § R802.4 tables before applying the factor.

Related provisions

  • § R802.4.1(9) — Rafter span adjustment factors (HC/HR) (Table).
  • § R802.4.2 — Framing details (ridge, nailing, collar ties).
  • § R802.4.5 — Purlins and braced rafter construction (sizing purlins to reduce rafter span).
  • § R802.7 / R802.8 — Ceiling joist details, taper cuts, lateral support and bridging rules that affect joist sizing and restraint.
  • Ceiling joist span tables (design of ceiling joists) are in § R804.3.1.1 (ceiling joist spans, lateral support/bracing rules) — consult when sizing ceiling joists specifically.

Code references

Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:

  • CRC § 2308.11.2 High relevance — show source text

    The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b.

  • CRC § 25.4 High relevance — show source text

    Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location.

  • CRC § 25.4 High relevance — show source text

    sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table 2308.11.2(7).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location.

  • CRC § 23-75 High relevance — show source text

    2025 CALIFORNIA BUILDING CODE 23-75

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    WOOD

    TABLE 2308.11.2(7)—RAFTER SPAN ADJUSTMENT FACTOR Col2
    HC/****HR
    a    		 RAFTER SPAN ADJUSTMENT FACTOR
    1/3 0.67
    1/4 0.76
    1/5 0.83
    1/6 0.90
    1/7.5 or less 1.00
    a._ HC_ = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls;HR = Height of roof ridge measured vertically above the top of the rafter
    support walls.    		 a._ HC_ = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls;HR = Height of roof ridge measured vertically above the top of the rafter
    support walls.

    2308.11.3 Ceiling joist and rafter framing. Rafters shall be framed directly opposite each other at the ridge. There shall be a ridge board not less than 1-inch (25 mm) nominal thickness at ridges and not less in depth than the cut end of the rafter. At valleys and hips, there shall be a single valley or hip rafter not less than 2-inch (51 mm) nominal thickness and not less in depth than the cut end of the rafter.

    2308.11.3.1 Ceiling joist and rafter connections. Ceiling joists and rafters shall be nailed to each other and the assembly shall be nailed to the top wall plate in accordance with Tables 2304.10.2 and 2308.11.4. Ceiling joists shall be continuous or securely joined where they meet over interior partitions and be fastened to adjacent rafters in accordance with Tables 2304.10.2 and 2308.11.3.1 to provide a continuous rafter tie across the building where such joists are parallel to the rafters. Ceiling joists shall have a bearing surface of not less than 1 [1] / 2 inches (38 mm) on the top plate at each end.

    Where ceiling joists are not parallel to rafters, an equivalent rafter tie shall be installed in a manner to provide a continuous tie across the building, at a spacing of not more than 4 feet (1219 mm) on center. The connections shall be in accordance with Tables 2308.11.3.1 and 2304.10.2, or connections of equivalent capacities shall be provided. Where ceiling joists or rafter ties are not provided at the top of the rafter support walls, the ridge formed by these rafters shall be supported by a girder conforming to Sections 2308.3 and 2308.4. Rafter ties shall be spaced not more than 4 feet (1219 mm) on center.

    Rafter tie connections shall be based on the equivalent rafter spacing in Table 2308.11.3.1. Rafter-to-ceiling joist connections and rafter tie connections shall be of sufficient size and number to prevent splitting from nailing.

    Roof framing member connection to braced wall lines shall be in accordance with Section 2308.10.7.2.

    |TABLE 2308.11.3.

  • CRC § 5-8 High relevance — show source text

    pine|#3|5-8|8-4|10-6|12-9|15-1|4-11|7-3|9-1|11-0|13-1| |24|Spruce-pine-fir|SS|8-5|13-3|17-5|21-8|25-2|8-4|12-2|15-4|18-9|21-9| |24|Spruce-pine-fir|#1|8-0|11-9|14-10|18-2|21-0|6-11|10-2|12-10|15-8|18-3| |24|Spruce-pine-fir|#2|8-0|11-9|14-10|18-2|21-0|6-11|10-2|12-10|15-8|18-3| |24|Spruce-pine-fir|#3|6-1|8-10|11-3|13-8|15-11|5-3|7-8|9-9|11-10|13-9| |Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b.

  • CRC § 3.1.3. Medium relevance — show source text

    When determining the size of ceiling joists, the lateral support of the top flange shall be classified as unbraced, braced at midspan or braced at third points in accordance with Section R804.3.1.3. Where sheathing material is attached to the top flange of ceiling joists or where the bracing is spaced closer than at third points of the joists, the “third point” values from Tables R804.3.1.1(1) and R804.3.1.1(2) shall be used. Ceiling joists shall have a bearing support length of not less than 1 [1] / 2 inches (38 mm) and shall be connected to roof rafters (heel joint) with No. 10 screws in accordance with Figure R804.3.1.1 and Table R804.3.1.1(3).

    Where continuous joists are framed across interior bearing supports, the interior bearing supports shall be located within 24 inches (610 mm) of midspan of the ceiling joist, and the individual spans shall not exceed the applicable spans in Tables R804.3.1.1(1) and R804.3.1.1(2).

    Where the attic is to be used as an occupied space, the ceiling joists shall be designed in accordance with Section R505.

    TABLE R804.3.1.1(1)—CEILING JOIST SPANS 10 PSF LIVE LOAD (NO ATTIC STORAGE)a, b, c, d Col2 Col3 Col4 Col5 Col6 Col7
    MEMBER DESIGNATION ALLOWABLE SPAN
    (feet-inches)    		 ALLOWABLE SPAN
    (feet-inches)    		 ALLOWABLE SPAN
    (feet-inches)    		 ALLOWABLE SPAN
    (feet-inches)    		 ALLOWABLE SPAN
    (feet-inches)    		 ALLOWABLE SPAN
    (feet-inches)
    MEMBER DESIGNATION ** Lateral support of top (compression) flange** ** Lateral support of top (compression) flange** ** Lateral support of top (compression) flange** ** Lateral support of top (compression) flange** ** Lateral support of top (compression) flange** ** Lateral support of top (compression) flange**
    MEMBER DESIGNATION Unbraced Unbraced ** Midspan bracing** ** Midspan bracing** ** Third-point bracing** ** Third-point bracing**
    MEMBER DESIGNATION ** Ceiling joist spacing**
    (inches)    		 ** Ceiling joist spacing**
    (inches)    		 ** Ceiling joist spacing**
    (inches)    		 ** Ceiling joist spacing**
    (inches)    		 ** Ceiling joist spacing**
    (inches)    		 ** Ceiling joist spacing**
    (inches)
    MEMBER DESIGNATION 16 ** 24** 16 24 16 24
    350S162-33 9′-6″ 8′-6″ 11′-10″ 9′-10″ 11′-10″ 10′-4″
    35
  • CRC § 4.1 Medium relevance — show source text

    Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b. Span exceeds 26 feet in length.|Check sources for availability of lumber in lengths greater than 20 feet.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa.
    a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the
    bearing walls, such as rafter ties, is provided at that location. Where ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the
    adjustment factors in Table R802.4.1(9).
    b. Span exceeds 26 feet in length.|

    2025 CALIFORNIA RESIDENTIAL CODE 8-9

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    ROOF-CEILING CONSTRUCTION

    TABLE R802.4.1(4)—RAFTER SPANS FOR COMMON LUMBER SPECIES
    (Ground snow load = 30 psf, ceiling attached to rafters, L/Δ = 240)    		 Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 Col11 Col12 Col13
    ** RAFTER**
    SPACING
    (inches)    		 ** SPECIES AND GRADE** ** SPECIES AND GRADE** ** DEAD LOAD = 10 psf** ** DEAD LOAD = 10 psf** ** DEAD LOAD = 10 psf** ** DEAD LOAD = 10 psf** ** DEAD LOAD = 10 psf** **

Frequently asked questions

Do I always have to apply the HC/HR adjustment factor?

Yes — if your ceiling joists or rafter ties are not located at the top of the rafter support walls (i.e., HC > 0), the tabulated rafter spans must be multiplied by the factor in Table R802.4.1(9) (see § R802.4.1(9)).

Which table do I use if the ceiling is attached to rafters?

Use the rafter span table that states “ceiling attached to rafters” — those tables use L/Δ = 240. Confirm the table header matches your load and spacing. § R802.4 tables indicate whether the ceiling is attached.

What if my desired span is longer than the table shows?

If the tabulated span is exceeded (or the table warns about spans exceeding 26 feet), check lumber availability and consider engineered members, purlins or a girder per § R802.4.4/R802.4.5.

Where are ceiling joist spans found?

Ceiling joist span tables and lateral support rules are found in § R804.3.1.1 (ceiling joist spans, bracing and bearing requirements). Use those tables when sizing ceiling joists.

Do I need a registered design professional for unusual cases?

Yes — when conditions fall outside the tabulated assumptions (unusual loads, non-standard species/grades, large spans, or changes to trusses/engineered members), you must use engineered design by a registered design professional. The CRC tables are for the specific tabulated conditions only.

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