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Deck Joist Span Calculator (Floor Joists and Rafters Too)

Member and lumber

Maximum allowable span
11'-10"Deck joist · Southern pine No. 2 · 2x8 @ 16 in o.c. · 40 psf live load, 10 psf dead load
Table cell
11-10 (ft-in)
In inches / metres
142 in · 3.61 m
Cited table
2021 IRC Table R507.6

Spans are clear spans, per the cited table; local amendments and actual loads govern — check with your building department.

Deck joists — 2021 IRC Table R507.6, Maximum Deck Joist Spans

Allowable joist span, ft-in. Notes a–c and g of the table: dead load 10 psf, snow load not concurrent with live load; No. 2 grade, wet service factor included; L/Δ = 360 at main span; interpolation allowed, extrapolation not. Note e: Douglas fir-larch, hem-fir and spruce-pine-fir include the incising factor; note f: the redwood group does not. The table’s maximum-cantilever columns are not reproduced here. Source: ICC, 2021 IRC Chapter 5.

Load caseSpecies, No. 2Size12 in o.c.16 in o.c.24 in o.c.
40 psf liveSouthern pine2x69‑119‑07‑7
40 psf liveSouthern pine2x813‑111‑109‑8
40 psf liveSouthern pine2x1016‑214‑011‑5
40 psf liveSouthern pine2x1218‑016‑613‑6
40 psf liveDouglas fir-larch, hem-fir, SPF2x69‑68‑46‑10
40 psf liveDouglas fir-larch, hem-fir, SPF2x812‑611‑19‑1
40 psf liveDouglas fir-larch, hem-fir, SPF2x1015‑813‑711‑1
40 psf liveDouglas fir-larch, hem-fir, SPF2x1218‑015‑912‑10
40 psf liveRedwood, W. cedars, ponderosa, red pine2x68‑108‑06‑10
40 psf liveRedwood, W. cedars, ponderosa, red pine2x811‑810‑78‑8
40 psf liveRedwood, W. cedars, ponderosa, red pine2x1014‑1113‑010‑7
40 psf liveRedwood, W. cedars, ponderosa, red pine2x1217‑515‑112‑4
50 psf snowSouthern pine2x69‑28‑47‑4
50 psf snowSouthern pine2x812‑111‑09‑5
50 psf snowSouthern pine2x1015‑513‑911‑3
50 psf snowSouthern pine2x1218‑016‑213‑2
50 psf snowDouglas fir-larch, hem-fir, SPF2x68‑108‑06‑8
50 psf snowDouglas fir-larch, hem-fir, SPF2x811‑710‑78‑11
50 psf snowDouglas fir-larch, hem-fir, SPF2x1014‑1013‑310‑10
50 psf snowDouglas fir-larch, hem-fir, SPF2x1217‑915‑512‑7
50 psf snowRedwood, W. cedars, ponderosa, red pine2x68‑37‑66‑6
50 psf snowRedwood, W. cedars, ponderosa, red pine2x810‑109‑108‑6
50 psf snowRedwood, W. cedars, ponderosa, red pine2x1013‑1012‑710‑5
50 psf snowRedwood, W. cedars, ponderosa, red pine2x1216‑1014‑912‑1
60 psf snowSouthern pine2x68‑87‑106‑10
60 psf snowSouthern pine2x811‑510‑48‑9
60 psf snowSouthern pine2x1014‑712‑910‑5
60 psf snowSouthern pine2x1217‑315‑012‑3
60 psf snowDouglas fir-larch, hem-fir, SPF2x68‑47‑66‑2
60 psf snowDouglas fir-larch, hem-fir, SPF2x810‑119‑118‑3
60 psf snowDouglas fir-larch, hem-fir, SPF2x1013‑1112‑410‑0
60 psf snowDouglas fir-larch, hem-fir, SPF2x1216‑614‑311‑8
60 psf snowRedwood, W. cedars, ponderosa, red pine2x67‑97‑06‑2
60 psf snowRedwood, W. cedars, ponderosa, red pine2x810‑29‑37‑11
60 psf snowRedwood, W. cedars, ponderosa, red pine2x1013‑011‑99‑7
60 psf snowRedwood, W. cedars, ponderosa, red pine2x1215‑913‑811‑2
70 psf snowSouthern pine2x68‑37‑66‑5
70 psf snowSouthern pine2x810‑109‑108‑2
70 psf snowSouthern pine2x1013‑911‑119‑9
70 psf snowSouthern pine2x1216‑214‑011‑5
70 psf snowDouglas fir-larch, hem-fir, SPF2x67‑117‑15‑9
70 psf snowDouglas fir-larch, hem-fir, SPF2x810‑59‑57‑8
70 psf snowDouglas fir-larch, hem-fir, SPF2x1013‑311‑69‑5
70 psf snowDouglas fir-larch, hem-fir, SPF2x1215‑513‑410‑11
70 psf snowRedwood, W. cedars, ponderosa, red pine2x67‑46‑85‑10
70 psf snowRedwood, W. cedars, ponderosa, red pine2x89‑88‑107‑4
70 psf snowRedwood, W. cedars, ponderosa, red pine2x1012‑411‑09‑0
70 psf snowRedwood, W. cedars, ponderosa, red pine2x1214‑912‑910‑5

Floor joists — 2021 IRC Table R502.3.1(2), residential living areas, live load 40 psf, L/Δ = 360

Maximum floor joist spans, ft-in, No. 2 grade rows only (the table also lists SS, No. 1 and No. 3 grades). Note a of the table: end bearing length shall be increased to 2 inches; note b: dead load limits for townhouses in Seismic Design Category C and all structures in D0, D1 and D2 per Section R301.2.2.2. Source: ICC, 2021 IRC Chapter 5.

Spacing (in o.c.)Species, No. 2Dead load 10 psfDead load 20 psf
2x62x82x102x122x62x82x102x12
12Douglas fir-larch10‑914‑218‑020‑1110‑813‑616‑519‑1
12Hem-fir10‑013‑216‑1020‑410‑013‑116‑018‑6
12Southern pine10‑313‑616‑219‑19‑1012‑614‑917‑5
12Spruce-pine-fir10‑313‑617‑320‑710‑313‑316‑318‑10
16Douglas fir-larch9‑912‑915‑718‑19‑311‑814‑316‑6
16Hem-fir9‑112‑015‑217‑78‑1111‑413‑1016‑1
16Southern pine9‑411‑1014‑016‑68‑610‑1012‑1015‑1
16Spruce-pine-fir9‑412‑315‑517‑109‑111‑614‑116‑3
19.2Douglas fir-larch9‑211‑814‑316‑68‑510‑813‑015‑1
19.2Hem-fir8‑711‑313‑1016‑18‑210‑412‑814‑8
19.2Southern pine8‑610‑1012‑1015‑17‑99‑1011‑813‑9
19.2Spruce-pine-fir8‑911‑614‑116‑38‑310‑612‑1014‑10
24Douglas fir-larch8‑310‑512‑914‑97‑69‑611‑813‑6
24Hem-fir7‑1110‑212‑514‑47‑49‑311‑413‑1
24Southern pine7‑79‑811‑513‑67‑08‑1010‑512‑4
24Spruce-pine-fir8‑110‑312‑714‑77‑59‑511‑613‑4

Rafters — 2021 IRC Table R802.4.1(1), roof live load 20 psf, ceiling not attached to rafters, L/Δ = 180

Maximum rafter spans, ft-in, No. 2 grade rows, 2x6 to 2x12 (the table also lists 2x4 and grades SS, No. 1 and No. 3). “Note b” means the span exceeds 26 feet and the table gives no figure. Note a: spans assume ceiling joists or rafter ties at the bottom of the attic; if higher, multiply by Table R802.4.1(9). Source: ICC, 2021 IRC Chapter 8.

Spacing (in o.c.)Species, No. 2Dead load 10 psfDead load 20 psf
2x62x82x102x122x62x82x102x12
12Douglas fir-larch16‑1021‑426‑0Note b14‑718‑522‑626‑0
12Hem-fir15‑1120‑825‑3Note b14‑217‑1121‑1125‑5
12Southern pine15‑719‑823‑5Note b13‑617‑120‑323‑10
12Spruce-pine-fir16‑321‑025‑8Note b14‑418‑222‑325‑9
16Douglas fir-larch14‑718‑522‑626‑012‑716‑019‑622‑7
16Hem-fir14‑217‑1121‑1125‑512‑315‑618‑1122‑0
16Southern pine13‑617‑120‑323‑1011‑814‑917‑620‑8
16Spruce-pine-fir14‑418‑222‑325‑912‑515‑919‑322‑4
19.2Douglas fir-larch13‑316‑1020‑723‑1011‑614‑717‑1020‑8
19.2Hem-fir12‑1116‑420‑023‑211‑214‑217‑420‑1
19.2Southern pine12‑315‑718‑621‑910‑813‑616‑018‑10
19.2Spruce-pine-fir13‑116‑720‑323‑611‑414‑417‑720‑4
24Douglas fir-larch11‑1115‑118‑521‑410‑413‑015‑1118‑6
24Hem-fir11‑714‑817‑1020‑910‑012‑815‑617‑11
24Southern pine11‑013‑1116‑619‑69‑612‑114‑416‑10
24Spruce-pine-fir11‑914‑1018‑221‑010‑212‑1015‑818‑3

Rafters — 2021 IRC Table R802.4.1(3), ground snow load 30 psf, ceiling not attached to rafters, L/Δ = 180

Same layout and notes as Table R802.4.1(1). Source: ICC, 2021 IRC Chapter 8.

Spacing (in o.c.)Species, No. 2Dead load 10 psfDead load 20 psf
2x62x82x102x122x62x82x102x12
12Douglas fir-larch14‑017‑821‑725‑112‑615‑1019‑422‑5
12Hem-fir13‑717‑221‑024‑412‑215‑418‑921‑9
12Southern pine12‑1116‑419‑522‑1011‑714‑817‑420‑5
12Spruce-pine-fir13‑917‑521‑424‑812‑415‑719‑122‑1
16Douglas fir-larch12‑115‑418‑921‑810‑1013‑816‑919‑5
16Hem-fir11‑914‑1118‑221‑110‑613‑416‑318‑10
16Southern pine11‑214‑216‑1019‑1010‑012‑815‑117‑9
16Spruce-pine-fir11‑1115‑118‑521‑510‑813‑616‑619‑2
19.2Douglas fir-larch11‑014‑017‑119‑109‑1012‑615‑317‑9
19.2Hem-fir10‑913‑716‑719‑39‑712‑214‑1017‑3
19.2Southern pine10‑212‑1115‑418‑19‑211‑713‑916‑2
19.2Spruce-pine-fir10‑1113‑916‑1019‑69‑912‑415‑117‑6
24Douglas fir-larch9‑1012‑615‑317‑98‑1011‑213‑815‑10
24Hem-fir9‑712‑214‑1017‑38‑710‑1013‑315‑5
24Southern pine9‑211‑713‑916‑28‑210‑412‑314‑6
24Spruce-pine-fir9‑912‑415‑117‑68‑811‑013‑615‑7

Sources: 2021 IRC Chapter 5 — Tables R502.3.1(2) and R507.6 and 2021 IRC Chapter 8 — Tables R802.4.1(1) and R802.4.1(3), International Code Council, read free at codes.iccsafe.org · AWC Span Options Calculator for Wood Joists and Rafters and its DCA 6 Prescriptive Residential Wood Deck Construction Guide (Table 2), the American Wood Council tables the IRC adopts. 1 in = 25.4 mm exactly.

How It Works

This is a lookup, not a formula. Every result is a cell read from a 2021 International Residential Code span table: Table R507.6 for deck joists (40 psf live load or 50, 60 and 70 psf ground snow, 10 psf dead, No. 2 grade with the wet service factor, L/360), Table R502.3.1(2) for floor joists in living areas (40 psf live, 10 or 20 psf dead, L/360) and Tables R802.4.1(1) and R802.4.1(3) for rafters with the ceiling not attached (20 psf roof live load or 30 psf ground snow, 10 or 20 psf dead, L/180). The full rows the tool can return are printed below the calculator with the table number.

Spacing is on-centre in inches. The deck table lists 12, 16 and 24 in only; the floor and rafter tables add 19.2 in. Species are the four No. 2 groups common at the lumber yard; the deck table treats Douglas fir-larch, hem-fir and spruce-pine-fir as one row group and adds a redwood, western cedars, ponderosa pine and red pine group that the floor and rafter tables do not carry, so that option only works for decks.

Spans are clear spans between the faces of supports. The tables cover only the grades, loads and deflection limits named in their headings; other grades, heavier snow, attached ceilings, cantilevers and heel-joint connections are separate tables. Where a rafter cell reads “Note b” the span exceeds 26 ft and the code gives no figure.

Worked Example

A Southern pine No. 2 2x8 deck joist at 16 in on centre under the 40 psf live load case: Table R507.6 gives 11’-10” (142 in, 3.61 m). At 12 in on centre the same joist reads 13’-1”. Move the joist indoors to a living-area floor with 10 psf dead load and Table R502.3.1(2) allows 11’-10” for the same lumber at 16 in, because the deck table’s wet service factor and the floor table’s dry conditions happen to meet at that cell.

FAQ

Why does my local code give a different number?

Jurisdictions adopt different IRC editions and amend them. The figures here are from the 2021 IRC as published by the ICC; your building department’s adopted edition and amendments govern.

What does “clear span” mean?

The distance between the inside faces of the two supports, not the length of the board. Add bearing length at each end when you cut.

Can I interpolate between spacings?

Table R507.6 (note g) allows interpolation and forbids extrapolation. The floor and rafter tables list 12, 16, 19.2 and 24 in; use the next wider spacing if yours falls between.

Where are the cantilever and beam figures?

Table R507.6 also lists maximum cantilevers by back span, and beams are in Tables R507.5(1) to (4). This tool reproduces the joist span columns only; open the cited chapter for the rest.

Which grades does this cover?

No. 2 only. Select Structural and No. 1 span further and No. 3 shorter; those rows are in the same tables at the source.

Sources

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