How to Frame a Deck: Joists, Beams, and Spans

Start by picturing where the weight goes, because that load path is what every table is really protecting. A person standing on the deck pushes down on the boards, the boards hand that weight to the joists, the joists hand it to a beam, the beam hands it to the posts, and the posts carry it down to footings in the ground. Every table below is protecting one link in that chain. You frame it in the reverse order, from the ground up:

1. 1Get your two numbers first: which code edition your building department enforces, and your ground snow load. Everything downstream is read off those.
2. 2Set the footings and posts, checking the post height against your tributary area rather than assuming a 4x4 will do.
3. 3Set the beam line, fasten the plies the way the code specifies, and keep the cantilever inside its limit.
4. 4Run the joists across the beam and the house-side support, hanging any that frame into the side of a beam or ledger.
5. 5Restrain the joist ends so they cannot roll, and add blocking wherever a load concentrates or your decking asks for it.
6. 6Check the frame for square while it can still move, then lay the boards at the spacing your decking manufacturer publishes.

Which code your town is actually on, because it probably is not the newest one

You'll read everywhere that decks are built to the 2021 International Residential Code, section R507. Don't take that on faith, because the country is nowhere near that tidy. On the ICC's own adoption chart, dated January 2024, fifteen states had a statewide 2021 IRC, sixteen were still on the 2018 edition, seven were on 2015, and Texas was on the 2012 code. That is twenty-four states behind the 2021 edition against fifteen on it. Ten more states carry an X, which the chart's legend defines as one or more jurisdictions having adopted an edition while the code "is not used as a standard for all buildings," and Wisconsin's cell is blank, meaning no jurisdiction in the state has adopted the IRC at all. So which edition governs your yard has a real answer, and it comes from your building department rather than from a search result.

This matters because the editions disagree. The 2018 code and the 2021 code word the cantilever rule differently, and the 2021 code split the beam table into four. So the first call you make is to your building department, and the question is two sentences long: which code edition are you enforcing, and what is my ground snow load? From there it is a matter of reading the correct row, once you have those two answers. The American Wood Council also publishes a free prescriptive guide, DCA-6, that many departments hand out. It is genuinely useful, but know that the current edition is keyed to the 2015 IRC, so it sits two editions behind the code most people quote.

How far apart deck joists go, and how far they can span

Joists are the repeating boards that carry the decking, and two measurements govern them: how far apart they sit, called spacing, and how far they can reach between supports, called span. The 2021 IRC states the rule for both in section R507.6, and the whole section is two sentences:

Read that second sentence twice, because it kills the most common myth in deck building: that 16 inches on center is simply the right answer. Spacing is decided by the boards you plan to lay. Span is decided by the lumber. Table R507.6 gives the reach for No. 2 grade joists at 12, 16, and 24 inches on center. Here are the common ones at 16 inches, in the 40 psf live-load block, which is the no-snow case:

Now watch what snow does to those. That southern pine 2x10 reading 14 feet drops to 13 feet 9 inches at 50 psf of ground snow and to 11 feet 11 inches at 70 psf. The 2x8 falls from 11 feet 10 inches to 9 feet 10 inches. A builder in Buffalo who frames off the numbers a builder in Houston is using has overspanned his joists by two feet, and the deck will feel fine on the day it is finished. Species matters as much as size, so a cedar 2x10 will not go as far as a southern pine one. When your joist run is longer than your row allows, you do not reach for a wider board and hope. You drop in a second beam to shorten the span, or you step up to a deeper joist.

Beams: what sets the span, and why the nailing pattern is not optional

The beam is the horizontal member the joists land on, built up from boards nailed face to face. How far it can run between posts depends on the beam size and on how much deck it carries, which the tables express as the length of the joists resting on it. Here is section R507.5 in full:

Notice the plural. There are four beam tables in the 2021 edition, R507.5(1) through R507.5(4), and they are the same four load cases as the joist tables: 40 psf live, then 50, 60, and 70 psf of ground snow. Older editions print a single beam table instead. A beam is also doing two jobs at once: its own size and ply count set what it could carry, and the length of joist landing on it sets what it has to carry. Add a ply and the beam reaches further; hand it longer joists and it reaches less far.

Read that middle sentence of the code again. Two rows of 10d nails at 16 inches on center are what make loose boards act as one beam. Leave them out, or space them how you feel like it, and the plies slide against each other and the beam is weaker than the table promises. And while we are here, drop the line that structural screws always beat nails. The code prescribes the fastener by where it goes, and a 10d nail in a beam ply is doing the job it was tested for.

Cantilevers: one-fourth of the actual span, not the allowable one

A cantilever is the bit of frame that reaches past its last support, like joists running a foot or two beyond the beam so the deck edge floats without a post under it. It is a clean look and a fine technique, but it has a hard ceiling, and the exact wording of that ceiling is worth a minute because it is where a lot of decks get built wrong.

The 2021 code says a beam may cantilever up to one-fourth of the actual beam span. Actual, not allowable. The difference is real money and real safety. If your table says a beam of that size could span 9 feet, but you only ran it 6 feet between posts, your cantilever limit is a quarter of 6 feet, which is 18 inches. It is not a quarter of 9 feet. Read it the other way and you build a 27-inch overhang that the code never permitted. Push a cantilever past its limit and the deck bounces at the edge and pries up on the connection at the far end, which is exactly where springy, unnerving decks come from. When your design wants a longer overhang than the rule gives you, move the beam outward. Do not let the member reach further.

For joists, the 2021 code handles cantilevers in the table rather than in the section text: Table R507.6 carries its own maximum cantilever block, indexed by the joist's back span. Look it up rather than reciting a ratio. And if your town is on an older edition, check the wording yourself before you use the rule above, because it is not the same sentence. The 2018 edition's R507.5 reads that beams "shall be permitted to cantilever at each end up to one-fourth of the allowable beam span". Allowable, where the 2021 edition says actual. One word, and it moves the limit. On an older code a beam that could have spanned 9 feet but was run 6 gets its quarter measured against the 9, not the 6. That is the whole reason you called the building department first.

Posts: the table sets a maximum height, not a minimum size

Here is a section that is almost always described backwards. Section R507.4 reads, in full: "For single-level decks, wood post size shall be in accordance with Table R507.4." And Table R507.4 is titled DECK POST HEIGHT. It isn't a list of minimum post sizes. It's a table of maximum heights, and you enter it with four things: your load case, your species, your post size, and your tributary area, meaning how much deck that one post is carrying. Worth knowing before you read any of it: DCA-6, the free guide this page keeps pointing you at, does not accept a 4x4 deck post at all. It requires that "all deck post sizes shall be 6x6 (nominal) or larger," and its commentary explains why, saying the code's minimum 4x4 "would often be overstressed" in the situations the guide covers. So the 4x4 conversation below is the code's position, not the whole trade's, and if you are building to DCA-6 or your department hands it to you, the answer is a 6x6 before you open any table.

Work an example, and note this is the 2021 table. Southern pine, no snow. A 4x4 post can stand 14 feet tall if it is only carrying 20 square feet of deck. Give that same 4x4 a tributary area of 160 square feet and its maximum height collapses to 6 feet 2 inches. A 6x6 holds 14 feet all the way across the row. And in some rows the table simply reads NP, for Not Permitted: a redwood 4x4 carrying 120 square feet is not allowed at any height. So a 4x4 isn't automatically fine just because your deck is low. It's fine or it isn't, depending on how much deck is leaning on it. Note too that R507.4 covers single-level decks only.

Your town can be stricter, and plenty are. San Diego County's residential deck handout, PDS 044, writes a flat ceiling into its standard plan: "Deck shall be strictly limited to a maximum post height of 10 feet." That same handout also says "Deck shall not be supported by an overhang or cantilever," which is a good illustration of why you check locally instead of trusting a national article. Cantilever your joists in that county and you are outside their standard plan no matter what the IRC allows.

Bearing and hangers: two rules that hold the frame together

Where a beam or joist lands on something, the code cares how much of it is actually sitting on solid support. Section R507.5.1 requires that "the ends of beams shall have not less than 1 1/2 inches of bearing on wood or metal and not less than 3 inches of bearing on concrete or masonry for the entire width of the beam." Section R507.6.1 says the same thing for joist ends. A beam clinging to the very edge of a post does not meet that.

Where a joist lands on top of a beam, gravity and a couple of fasteners hold it. Where a joist meets the side of a beam or the rim, gravity is working against you, and R507.6.1 is blunt: "Joist framing into the side of a beam or ledger board shall be supported by approved joist hangers." Toenailing a side-bearing joist is one of the failures an inspector catches on sight, because a hanger is a listed connector rated for a real load and a few angled nails are not. The American Wood Council's DCA-6 adds a spec worth knowing: hangers should have a depth of at least 60 percent of the ledger or beam depth, and clip angles or brackets are not an acceptable substitute for a hanger.

Where you actually need blocking

There is a stubborn belief that you owe a row of blocking every 4 feet down the joists, and it is usually quoted as though it came from the code. No part of R507 sets a uniform intermediate blocking interval. What that means is narrower than "skip the blocking," though. It means the code is not the thing asking for it. Your decking manufacturer may well be. TimberTech's AZEK guide tells you to "install solid wood blocking between each joist, placed in rows at maximum every 4' - 6' within the structure," to stop the joists moving and twisting under the boards. So the every-4-feet habit is not a code rule and not folklore either. It is a decking rule that got repeated until people thought it came from the IRC. Read your board's installation guide before you decide the mid-span rows are optional. The code sets a floor here, not a ceiling.

What is not optional is restraint at the ends. Section R507.6.2 requires that "joist ends and bearing locations shall be provided with lateral resistance to prevent rotation," and it puts a number on it: "where lateral restraint is provided by joist hangers or blocking between joists, their depth shall equal not less than 60 percent of the joist depth." If a rim joist is doing that job instead, it has to be fastened to the end of each joist with at least three 10d nails or three No. 10 screws three inches long. So blocking isn't discretionary. It's discretionary in the middle and mandatory at the supports, and that 60 percent figure is exactly the sort of thing an inspector puts a tape on.

Beyond that, put blocking where a load concentrates: under a guard post, so the post has something solid to bolt to instead of racking a lone joist, and where stair stringers land on the frame. Guard posts are worth knowing the numbers for, because a guard is what stops someone going over the edge. DCA-6 requires that a deck guard post for a required guard be a minimum 4x4 nominal "with an adjusted bending design value not less than 1,100 psi," and, the part people miss, that the joists and rim joists those posts attach to be a minimum of 2x8 nominal. Bolt a guard post to a 2x6 rim and the post may be fine while the thing holding it is not.

Hardware that survives treated lumber

Modern pressure-treated lumber is soaked in copper-based preservatives that chew through ordinary steel, so bright or lightly coated hardware corrodes from the inside of the joint where you will never see it. Section R317.3.1 names the acceptable metals: "Fasteners, including nuts and washers, for preservative-treated wood shall be of hot-dipped, zinc-coated galvanized steel, stainless steel, silicon bronze or copper. Staples shall be of stainless steel." Note the nuts and washers, which people forget, and note that staples get their own harder rule.

Two things that section adds. First, there is an exception for steel bolts a half inch in diameter or larger, which is exactly the fastener you are likely to use at a post-to-beam connection. Second, if your deck is near the ocean, this is not a preference. The American Wood Council's DCA-6 puts it plainly: "Fasteners and connectors exposed to salt water or located within 300 feet of a salt water shoreline shall be stainless steel grade 304 or 316." Note that it names a grade, not just a metal, and that it covers connectors as well as fasteners. Inside that line, stainless is the rule, not the upgrade.

Framing for composite and PVC boards

This is where joist spacing comes back around, and where the code does something people never expect: it declines to give you a number. Table R507.7 is titled MAXIMUM JOIST SPACING FOR WOOD DECKING, and every row in it is wood. It has two of them, for wood an inch and a quarter thick and wood two inches thick, and each one splits again depending on whether the board runs over a single span or several. The inch-and-a-quarter board gets 12 inches over a single span and 16 over multiple, dropping to 8 and 12 on a diagonal. The two-inch board gets 24 either way, and 18 or 24 on a diagonal. For plastic composite the table prints no figure at all. What it does instead is point you to section R507.2, which requires the board to comply with ASTM D7032 and requires the board or its packaging to be labelled with the allowable load and the maximum allowable span determined under that standard. So the code does set a limit for composite. It just makes the manufacturer print it on the product rather than printing it in a table, which means your number is on the label in your hand and in the install guide, not in the IRC.

What helps you instead is knowing what the guides actually say, and the only honest way to write this is one brand at a time, because the numbers are not interchangeable and the guides get revised. Trex's installation guide prints a span chart: 16 inches on center for its 1-inch boards on a residential deck, and 24 inches for the 2x6 profile. The same chart then adjusts for angle, which is where most people come unstuck. At 60 degrees the maximum spanning drops 2 inches, at 45 degrees it drops 4 inches, and at 30 degrees it is halved. So a 1-inch Trex board laid straight at 16 inches goes to 12 inches on a 45-degree diagonal and 8 inches at 30. Trex also prints a separate, lower set of numbers for commercial decks and marinas, and a different chart again for rooftop and sleeper systems, so the figure you want is the one from the chart for the deck you are actually building. TimberTech's AZEK guide sets a different rule again: joist spacing there "should never exceed 16" on center," with 12 inches preferred for a more rigid feel. Two brands, two documents, and no number that carries across.

Squaring the frame: the 3-4-5 rule

Before the boards go on, the frame has to be square, and you do not need a fancy tool for it. Measure 3 feet along one side from a corner and mark it. Measure 4 feet along the other side from the same corner and mark that. If the corner is a true 90 degrees, the straight-line distance between your two marks is exactly 5 feet. If it reads more, the corner is open; if it reads less, it is closed. Nudge the frame until the diagonal hits 5 feet on the nose.

On a bigger deck, scale it up and keep the same ratio, so 6, 8, and 10 feet, or 9, 12, and 15. The longer the legs, the more precisely the check reads. The other version of the same test is to measure both diagonals of the whole rectangle and adjust until they match. Do this while the frame is still loose enough to move, because once the joists are hung and the boards are down, an out-of-square deck stays out of square forever, and every board you cut to fit it will tell the story.

Freestanding versus ledger-attached frames

There are two ways a deck gets its house-side support. A ledger-attached deck bolts a board, the ledger, flat to the house, and the joists hang off that while a beam and posts carry the outer edge. It uses less lumber and it's the default for a plain rectangle. The catch is that bolting to the house is its own exacting job, with a fastener schedule, flashing, and a hard look at what the wall is made of. It's the connection the trade has studied hardest, which is why the code prints a whole fastener table for it and for almost nothing else, and why a thin bid loves to skip it. We pull that apart in our guide to why one deck quote comes in so far under another.

A freestanding deck skips the house connection entirely. It stands on its own posts and beams on all sides, carrying every bit of its load straight down to footings, and it only leans against the house for looks. Sometimes that is a preference and sometimes the code makes the decision for you. Section R507.9.1.1 says flatly that "deck ledgers shall not be supported on stone or masonry veneer," so if your wall is brick or stone veneer, a ledger is off the table and a freestanding frame is the answer. Be careful how far you carry that, though, because the code names veneer and only veneer. Stucco and concrete block are not in that sentence, and builders often go freestanding on those walls for practical reasons rather than because a section says to. The trade is more posts, more footings, and often a second beam line near the house so the joists span cleanly between two beams. It costs more frame, and in return you sidestep the connection that gives a ledger deck the most trouble.

The mistakes that actually show up

Five, in rough order of how often an inspector or a bouncy deck finds them.

1. 1Reading a span number without knowing your ground snow load, so the row you used was never yours.
2. 2Measuring a cantilever against the span the table would have allowed instead of the span you actually built, and on a pre-2021 code the reverse.
3. 3Treating Table R507.4 as a minimum post size when it is a maximum post height that shrinks fast as tributary area grows.
4. 4Skipping lateral restraint at the joist ends, which R507.6.2 requires, while dutifully adding mid-span blocking the code never asked for.
5. 5Using bright or lightly coated hardware in treated lumber, which corrodes inside the joint where nobody sees it until it lets go.

Where this stops being a DIY job

Most of a rectangular deck is table work, and a careful homeowner can do it. Four things are not. A ledger connection is its own exacting job with a fastener schedule and flashing, and it is the connection that fails most often, so a first-timer is better off building freestanding. Anything carrying a concentrated load, whether a hot tub, a planter wall or a stair landing, sits outside the prescriptive tables entirely, and DCA-6 says so in its own limitations. A deck more than one storey up, or one where the posts are tall enough that the guard becomes the thing between someone and a fall. And any site the tables do not cover: a ground snow load above the highest block, a slope, poor soil, or a wall the ledger cannot attach to. In those cases the answer is not a better article. It is a design stamped by someone who carries the liability for it.

Whichever you build, the frame isn't a place to eyeball anything. Get your ground snow load, pull the right block of the right table for your species, nail the beam plies the way R507.5 reads, restrain the joist ends, hang the side-bearing joists, and use hardware rated for treated wood. Do that and the skeleton under your boards is one you can jump on without a second thought, which is the entire point of getting the part nobody sees exactly right.