FAQs about using framing tables for decks, porches & other wood construction: joist & beam spans & sizes.

This article series gives recommended or allowable joist & beam spans and explains typical spans for deck joists, deck beams, and deck flooring, giving both standard span tables and a quick "rule of thumb" that works pretty well.

Deck Joist & Beam Sizing Tables & Rule of Thumb Joist Sizing FAQs

These questions & answers about finding & using wood framing span tables for decks, porches, other common building floor structures were posted originally

at DECK FRAMING TABLES, SPANS - be sure to consult the tables and rules of thumb given there.

Is 12" O.C. spacing acceptable for a porch deck?

I’m using 3/4”White Oak for a porch deck. I’m planning on 12” OC for the joist spacing but can’t find anything in the code that states that is correct.

On 2019-03-01 by Mike

Answer by (mod) -

Mike

Standard deck framing for joists of various depths (you don't give the depth nor span of your porch floor joists) are in the article above.

Just specifying "12" OC joist spacing is not enough to even make a wild guess at spans allowed.

I agree that building codes are probably not going to specify joist spacing for 3/4" thick boards (of unspecified width) directly placed onto the joists.

Common porch flooring is not 3/4" thick but rather is 7/8" in these widths:
Tongue-and-groove profile – 3-1/8" x 7/8" x 10', 12' and 16'
Solid profile – 3-1/8" x 7/8" x 12'

Those are commonly installed on joists that are 16" o.c.

So your 12" o.c. with the thinner flooring may be ok.

Comparing a porch floor with 3/4" boards to a deck with heavier flooring:

Typically actual deck boards are 5/4 thick lumber and requires 16" o.c. joists. If you were building a deck (not a porch) using 2x lumber (like 2x6s) that flooring can be installed over joists that are up to 24" o.c.

Indoors where 3/4" thick flooring is installed in modern construction it's placed over at least a layer of 3/4" subfloor. So when you're going to a non-standard construction plan for an indoor floor you probably need to find suggestions either from your deck board supplier/manufacturer or from a local engineer or architect.

Keep in mind that the joist tables above are for the deck structure and don't address whether or not your flooring will be saggy or bouncy as it spans the joist themselves.

More tables that you might want to check out are at FRAMING AGE, SIZE, SPACING, TYPES

Concern for moisture trapped between members of built-up beams

Why does your beam span table consist of only built up 2" by packages instead of 4" by or 6" by actual beams.

I know that in the midwest and east headers in a 2x4 wall consist of two 2" bys with a 1/2" plywood spacer but in the west where the labor to do so costs more it's common to use 4x6 through 4x12 depending on span.

I guess either is OK for interior work but I would never package two or three structural elements because of moisture that is trapped between plies. Even pressure treated lumber can rot. Why take that chance? I'll continue to use 4" by beams under any deck. On 2019-05-21 by Suburbangeorge

- by (mod) - we recommended flashing atop the built-up beam

Thank you for your comment

You make a good point.

Where I have used built-up beams under decks, which is quite common in some parts of the country, I've recommended flashing atop the built-up beam to prevent runoff from going between the members. I agree with you that that's potential rot point

 

What's the size needed for a middle support beam for my Pergola Roof?

I would like to build a Pergola over my flagstone patio. The size of the Pergola would be 25' x 25' and would be attached to my brick house.

What size middle support beam would carry the weight of the 2' x 10' cedar joists ? Also, what size vertical cedar post's would be needed ? On 2019-08-14 by Anonymous

Answer by (mod) -

Anon you need to either meet standard flat roof framing specifications (which is over-building since there's no weight of the roof that's not going to be installed atop the joists) OR have an engineer do the calculations for you since the frame is almost a zero load except for the weight of the framing itself - not something in the framing tables.

Our own little pergola, a bit smaller than yours, that I built using 2x10s around the perimeter of a 20 x 15' pergola is supported on 6x6 posts, 2x10s through bolted to the posts; the joists are simple 2x4s 16" o.c.

It's been there without sagging, with added weight of an insane vine growth and occasional snow, for nearly 20 years.

I agree that it makes sense to have a single or doubled center beam so that your 25' span in one direction uses 2x10 cedar joists that are just 12 1/2' long. An engineer needs to calculate your load as it's not a standard deck or roof framing situation.

I'll do some looking to see what I can find.

As a general rule of thumb for beam sizing for a deck (figure that's more load than a pergola roof), if you're supporting joists that span 12 ft (almost your size), then a doubled 2x12 beam itself can span 12' (the number of ft of span = the doubled beam nominal depth in inches).

To go to a longer span for your center beam you'll expect to go to an engineered deck beam rated for outdoor use - such as a glulam beam.

But ticking with that 12' joist span example, there's STILL no engineered beam rated for a 25 ft. beam span. I think you need some mid-span posts for your pergola, else your'e going to be building an ugly huge beamed pergola that looks out of balance.

Where we spanned a 25' cathedral ceiling our glulam beam was a tripled 2x14 engineered wood beam

What size lumber should I use for a 10 foot span with no joists

What size of lumber should I use for decking that has no joists but will span 10 feet? On 2018-09-17 by Rob

Reply by (mod) -

Bob

Please check the joist span tables in the article DECK FRAMING TABLES, SPANS.

There you will see that the span distance that you're allowed depends on the wood species, the joist dimensions, and the joist spacing.

What is the maximum span for 2x8 PTs when used as decking boards?

What is the max span for 2x8 PTs when used as decking boards? On 2018-02-27 by Bob R -

Answer by (mod) - deck's supporting joists should be no more than 24" o.c. for nominal 2" thick SYP deck boards

Bob,

Bottom line: your deck's supporting joists should be no more than 24" o.c. for nominal 2" thick SYP deck boards and the joist depth needed depends on the joist spans and beam placements.

I would not frame a deck with joists at more than 24" o.c.

So your "floor board span" is never more than 24" joist - to - joist.

ANSWER:

Using

5/4" thick SPF (Spruce Pine Fir) PT (pressure treated lumber) deck flooring, the 2x deck joists should be 16" o.c. and the joist depth required depends on the joist span.

2" thick SPF, Hem-Fir, northern white cedar, redwood, or western red cedar, or southern pine deck flooring, the 2x deck joists may be spaced 24" o.c.

Incidentally, other readers, nominal 2x8 SPF pressure treated lumber is really only 1 1/2" thick

Can I use 32" on center joist spacing ?

If I have 1.25" thick deck boards can I use 32" on center joist spacing On 2016-07-26 by Tara

Answer by (mod) - no, not in normal framing with normnal 1 1/4" thick deck boards

No

What size deck beams do I need to include a 2 foot snow load?

I need to span 16 ft. With a beam that will carry 2"x6" rafters which will be sheeted with opaque corrugated fiberglass to serve as a deck cover.

Considering a snow load of up to two feet, what size beam do l need? Camino On 2016-06-20 by Doyle Terrell

Reply by (mod) -

In the article DECK FRAMING TABLES, SPANS see Table of Maximum Beam Spans for Exterior Decks.

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Continue reading at DECK FRAMING TABLES, SPANS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Wood Frame Construction Articles

• DECK CONSTRUCTION BEST PRACTICES - home
• DECK DESIGN & BUILD - home
• DECK & PORCH CONNECTIONS: LEDGER BOARDS
• DECK LEDGER BOARD INSTALL
• DECK NAILS, SCREWS, HIDDEN FASTENERS
• FRAMING AGE, SIZE, SPACING, TYPES
• FRAMING CONNECTORS & JOIST HANGERS
• FRAMING DAMAGE, INSPECTION, REPAIR - home
• FRAMING FASTENERS, NAILS, SCREWS
• FRAMING MATERIALS, AGE, TYPES
• FRAMING METHODS, Age, Types
• FRAMING SQUARE-UP 6-8-10 RULE
• DECK FRAMING TABLES, SPANS
• FRAMING TRIANGLES & CALCULATIONS
• HURRICANE TIE DOWN CONNECTORS & FASTENERS
• PORCH CONSTRUCTION & SCREENING
• PRESERVATIVE TREATED LUMBER
• RAILING CODES & SPECIFICATIONS - home
• ROT-RESISTANT DECK LUMBER & FLOORING
• STAIRS, RAILINGS, LANDINGS, RAMPS - home

Suggested citation for this web page

DECK FRAMING TABLES, SPANS FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING DECKS & PORCHES

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