A solid 4×8 deck beam can span between 4 ft 7 in and 8 ft 2 in between posts under the American Wood Council (AWC) DCA 6 deck guide, and only for the species group, grade and loads that table assumes. This guide gives you those published numbers with their conditions, shows what actually controls beam span, and says plainly when to stop and ask the building department or an engineer.

Measure the Right Things Before You Check Any 4×8 Beam Span

A "4×8" is a nominal size. The finished lumber you buy is smaller than the name suggests (a 4×8 measures roughly 3-1/2 in by 7-1/4 in), so always measure the real piece in your hand, and measure the real project. Two measurements decide which row of a span table applies to you:

• Joist span: DCA 6 measures it from the face of the support at one end of the joist to the face of the support at the other end, not counting any overhang. On a deck beam, this is the figure the table uses to find the beam span.
• Beam span: the distance between posts that carry the beam. This is the number you compare against the table result.

The calculator this page used to offer produced a deflection figure from a formula. A formula result is not the same as a code-based span, because allowable stress, grade, wet service conditions and load assumptions are baked into the published tables. That is why this page now sticks to the published table instead of generating its own answer.

4×8 Deck Beam Span Table From AWC DCA 6 (Table 3A)

The table below reproduces the 4×8 row, with the 4×6 and 4×10 rows for context, from Table 3A of the AWC Prescriptive Residential Wood Deck Construction Guide (DCA 6, 2015 edition). Read across by the joist span your beam supports. The result is the maximum beam span between posts.

Beam size	6 ft joists	8 ft joists	10 ft joists	12 ft joists	14 ft joists	16 ft joists	18 ft joists
4×6	6 ft 2 in	5 ft 3 in	4 ft 8 in	4 ft 3 in	3 ft 11 in	3 ft 8 in	3 ft 5 in
4×8	8 ft 2 in	7 ft 0 in	6 ft 3 in	5 ft 8 in	5 ft 3 in	4 ft 11 in	4 ft 7 in
4×10	9 ft 8 in	8 ft 4 in	7 ft 5 in	6 ft 9 in	6 ft 3 in	5 ft 10 in	5 ft 5 in

The conditions behind every number in that table, as printed in the guide:

• 40 psf live load and 10 psf dead load.
• L/360 simple-span beam deflection limit, and a cantilever-length/180 limit.
• No. 2 grade lumber and wet service conditions.
• Incising is assumed for Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir; Ponderosa Pine and Red Pine values assume northern species with no incising.
• The table covers a beam supporting a single span of joists, with or without overhangs.
• If joist hangers attach the joists to the side of the beam, the beam must be at least as deep as the joists.

Southern Pine has no solid 4×8 row in this table. In the edition read for this update, the Southern Pine entries are built-up beams made of two or three 2x members. If you plan to use Southern Pine, use those built-up rows or ask your building department what it accepts.

What the Table Shows That Most Beam Calculators Hide

The joists decide the beam span, not the beam alone. The same 4×8 spans 8 ft 2 in under 6 ft joists but only 5 ft 8 in under 12 ft joists, a drop of about 30 percent, and 4 ft 7 in under 18 ft joists, a drop of about 44 percent. Longer joists put a wider strip of deck load onto the beam, so a bigger deck needs posts closer together or a bigger beam.

A solid 4×8 is not the strongest 8-inch option. A common assumption is that one solid timber beats several boards nailed together. In the same table, a built-up beam of three 2x8s spans farther than a solid 4×8 at every joist span listed. At 8 ft joists, the Douglas Fir-Larch group reads like this:

Beam at 8 ft joists	Max span between posts
4×6	5 ft 3 in
2-2×8 (two 2x8s)	5 ft 8 in
4×8	7 ft 0 in
2-2×10 (two 2x10s)	7 ft 0 in
3-2×8 (three 2x8s)	8 ft 3 in
4×10	8 ft 4 in

So a solid 4×8 sits above a pair of 2x8s and equals a pair of 2x10s at that joist span, but three 2x8s beat it. The guide also notes that a 3-ply beam must connect to its post with a post cap. Cost, availability and how you plan to connect the post are the practical reasons to choose one over the other, not a belief that solid timber is automatically stronger.

AWC DCA 6 also limits where the table applies, which brings us to the part most people skip.

Where This 4×8 Beam Span Table Does Not Apply

DCA 6 is written for a specific job. Before you borrow a number from it, check that your project fits its stated limits:

• It applies to single-level residential wood decks attached to the house to resist lateral forces.
• It does not apply to decks that will see snow loads, snow drift loads or sliding snow loads above 40 psf.
• Decks supporting large concentrated loads such as hot tubs are outside its scope.
• Lumber must be a naturally durable species or preservative-treated, and fasteners and connectors must be corrosion resistant (hot-dipped galvanized or stainless steel).
• Where the guide and the local code differ, the code applies, and every deck needs the approval of the authority having jurisdiction.

This is a deck table. It is not a table for floor beams, headers over openings, or roof beams, which carry different loads and are sized from different tables. Because that source was not read for this update, this page gives no span numbers for those uses. If you need one, ask your building department or an engineer.

If your question is about loads on shelves, plywood or fasteners rather than a structural beam, our wood load capacity calculator is the better starting point.

Why Beam Span Is So Sensitive to Small Changes

Standard beam theory explains why the table numbers change so quickly. For a simply supported beam under a uniform load, deflection grows with the fourth power of the span, and the stiffness of a rectangular member grows with the cube of its depth. Two practical consequences follow:

• Doubling a span multiplies the sag under the same load by 16, which is why a few extra feet between posts is a big deal.
• Going from a 6-inch-deep member to an 8-inch-deep member stiffens it far more than the extra two inches suggest.

That sensitivity is the reason to keep your beam size on a published table instead of rounding up by feel. It is also why this page no longer offers a do-it-yourself calculator: the free AWC Span Options Calculator is aimed at joists and rafters, not beams, so for a beam the practical routes are the DCA 6 table for decks and your building department or an engineer for everything else.

Posts, Connections and Hardware That Go With a 4×8 Beam

A beam is only as good as what holds it up. These requirements come from the same guide:

• Posts must be 6×6 nominal or larger, with the maximum post height taken from the guide’s own table.
• The beam attaches to the post either by notching it as shown in the guide or with an approved post cap. Fixing a beam to the side of a post without notching is prohibited.
• The beam may extend past the post face by up to one quarter of the beam span.
• Joist hangers must be at least 60 percent of the beam depth, and hangers, clips and fasteners must be galvanized or stainless steel.
• Cut ends and notches in treated lumber must be field treated with an approved preservative.

Fastener choice matters as much as the beam size on treated lumber. Our guide to deck framing screws or nails covers what to use at each connection, and the best screws for pressure-treated wood explains how to match the coating to the exposure.

Before You Build: A Short Checklist

• Confirm your project is a single-level residential deck with no hot tub and no snow load over 40 psf.
• Measure the real joist span, face to face of supports, and the real post spacing with a tape.
• Find your species group and grade on the lumber stamp, then read the row in a published table.
• Check the post cap or notch details, hanger depth and corrosion-resistant hardware requirements.
• Ask your building department whether it requires a permit, drawings or inspection before you start.

Our broader American Wood Council span calculator guide covers general span questions, and the wide decking steps guide is the next read if your deck includes stairs.

4×8 Beam Span FAQs

Can a 4×8 beam span 10 feet?

Not as a deck beam under DCA 6. The longest published span for a solid 4×8 in the Douglas Fir-Larch group is 8 ft 2 in, and that is with 6 ft joists. For a longer span, the same table shows built-up and deeper beams that reach farther. For any other use, ask your building department or an engineer.

Is a 4×8 beam the same as two 2x8s?

No. In the DCA 6 table, two 2x8s span 5 ft 8 in with 8 ft joists, while a solid 4×8 spans 7 ft 0 in. Three 2x8s span 8 ft 3 in. They are different beams with different published spans and different connection rules, so use the row for the exact beam you build.

Can I use Southern Pine for a 4×8 deck beam?

The edition of the table read for this page lists no solid 4×8 row for Southern Pine, only built-up 2x beams. Use those rows or ask your building department what it accepts for your lumber. Do not borrow the Douglas Fir-Larch group numbers for a different species.

Does a longer joist span shorten the beam span?

Yes. Longer joists put more deck load on the beam, so the allowed beam span falls. For a 4×8 in the table, it drops from 8 ft 2 in with 6 ft joists to 5 ft 8 in with 12 ft joists, and to 4 ft 7 in with 18 ft joists.

Can I use this page to get a permit?

No. It reports published table values and their conditions for education and planning only. A permit needs drawings and sizes your local building department accepts, and any beam that carries people or a structure should be confirmed by that department or a licensed engineer.

Sources and Scope

The span figures and requirements on this page come from the American Wood Council’s Prescriptive Residential Wood Deck Construction Guide, DCA 6 (2015 edition), Table 3A and its stated conditions. Nominal-versus-actual lumber sizes, the fourth-power span effect and the depth-cubed stiffness effect are standard structural facts rather than figures from that guide. Always check the current edition and your local code before you build.