Read the deck upside down before you dig
A deck is a stack of handoffs. The board takes the foot, the joist takes the board, the beam takes the joist, the post takes the beam, and the footing hands everything to the ground. Every dispute about member size on a job site traces back to somebody sizing one of those members without knowing the number arriving from above. So the first hour on a new deck belongs to arithmetic, not to a shovel — walk the stack downward on paper, write the tributary width against each member, and only then decide where the holes go.
Tributary area is the whole trick. A joist spanning between a ledger and a beam gives half its load to each; a beam under joists landing from both sides carries half the span on either hand. Get in the habit of writing that number on the plan next to each member — beam carries six feet of tributary over a fourteen foot span — because it is the single figure a building official will ask for and the single figure most home-built decks never established. Everything downstream, including how deep and how wide the concrete goes, is that number multiplied out.
Loads themselves are set by your local adopted code, not by a national default, and the residential deck live load in one jurisdiction is not the live load two states over. Snow country adds a ground snow load that can dwarf occupancy. Elevated decks near assembly-style use get treated differently again. Name the adopted code edition on your drawing and size to it; publishing a single number here would be wrong somewhere, and wrong somewhere on a deck means a collapse during a party.
The stack of handoffs, drawn
- Deck boards — the only layer chosen partly for how it looks, and the one whose maximum span dictates how tightly the joists below it have to sit Deck Board Calculator
- Joists and their hangers — half their load goes to the beam and half to the ledger, which is why the two are never sized off the same reaction
- Beam over the posts — collects joists arriving from both hands, so tributary width and clear span multiply here rather than add Timber Beam Bending Stress Calculator
- Posts and their bases — seldom crushed and often leaning, so slenderness rather than crushing is what governs a tall post on an open deck Timber Column Euler Buckling Calculator
- Footings in undisturbed ground — how many the perimeter needs falls out of post spacing; how wide and how deep each one is comes from the beam reaction and the local frost line, which this site does not size Deck Footing Calculator
The footing: where the number from above lands
Concrete does not care how pretty the framing is. A footing sized for a modest joist layout under a deck that later got a hot tub, a pergola and an outdoor kitchen is a footing that will settle out of level and drag the whole frame with it. Size the pad for the beam reaction it actually receives, and size the depth for frost — frost depth is set by the local authority having jurisdiction and varies enormously, from essentially nil in warm coastal ground to well past four feet inland at latitude. If you are unsure, the building department will tell you over the phone in thirty seconds. Do not guess.
Bearing capacity of the soil is the other half. Presumptive values in code tables cover common soils, but fill, expansive clay and anything within the zone of an old excavation want a geotechnical opinion rather than a table lookup. A hole augured through eight inches of topsoil into undisturbed subgrade behaves; the same hole stopped short in backfill will hand you a post that has dropped a full inch by the second spring. Dig until the auger is fighting you, not until the tape reads the number you hoped for.
Water is what actually kills footings. Bell the base if your code permits it, keep the top of the pier above finished grade so the post base is never sitting in a puddle, and use a standoff post base that lifts end grain clear of the concrete. A post cut flush to the pier top wicks water into its end grain and rots from the inside where nobody can see it; the visible face still looks fine the year it fails. Where the pier top will be seen, form it — a tube cut square and struck flush reads as intentional, a hand-dug flare reads as a repair.
Post spacing sets how many holes the auger has to drill, and that count is the number worth fixing before it arrives.
The length of the deck.
The width of the deck.
The maximum spacing between footings along the beam line.
Minimum footings needed
8 footings (minimum)
Footing count and placement are structural and code-critical — this is a rough planning estimate only. Always verify final footing count, size, and depth (including frost depth) against your local building code or a structural engineer before digging.
- Perimeter beam length
- 49 linear ft
- Footings along each long side
- 3 footings
- Footings along each short side
- 3 footings
They open the calculator with your figures already in it
Deck Footing Calculator: 8 footings (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- COUNTS FOOTINGS ALONG A PERIMETER, and says nothing about how big each one has to be. Footing SIZE comes from the load each post carries — its tributary area of deck, plus live load — divided by the soil's bearing capacity, and a deck on soft ground needs the same number of much larger pads.
- Spacing is driven by the BEAM, not by the perimeter: how far a beam can span between posts depends on its size, species and grade and on how much deck it carries, and that span is what should set the spacing entered above rather than a habit.
- Frost depth governs how deep every one of them goes, and it is a local figure. A footing at the right size and the wrong depth lifts the deck every winter and drops it every spring.
- Assumes a rectangular deck supported around its perimeter. A deck ledgered to the house carries much of its load on that ledger and needs fewer footings on that side but a correctly flashed and bolted ledger instead — which is the connection that fails most often on a deck.
- Stairs, cantilevers, hot tubs and any point load need their own footings sized for what they carry, and none of them are in this count.
Post and beam: the compression and bending pair
Posts carry axial load, and for typical residential heights the timber is rarely governed by crushing — it is governed by slenderness and by the connections at both ends. A tall post on an exposed deck is a column with a lateral job as well: it resists the sway that people walking, and wind, put into an elevated frame. Undersized posts do not crush, they lean, and the lean shows up as a squeak long before it shows up as a crack.
Bolting the beam to the post is where inspectors focus, and rightly. A beam merely toe-nailed to the top of a post relies on fasteners loaded in withdrawal, the weakest way to use them. Notch the post and through-bolt the beam into the notch, or use a rated post cap, so the load path is bearing rather than fastener shear. Bolt holes want the fastener manufacturer's specified clearance, snug but not oversized, with washers under both head and nut so the hardware bears on wood rather than sinking into it.
Beams themselves are sized by span and tributary width together, and the two trade off. Doubling the beam span roughly quadruples the bending demand, so shortening the span by adding one more post-and-footing pair is almost always cheaper in labour and material than jumping two beam sizes. Where you build up a beam from plies, fasten to the connector schedule and orient the plies crown up as a set; a built-up beam assembled with mismatched crowns has a permanent kink you will chase all the way to the deck surface.
Cantilever earns its own paragraph because it is the most frequently abused detail on residential decks. A beam or joist may overhang its support by a limited proportion of its backspan — the proportion is set by the code you are building to, and it is not a number to eyeball. Overhang beyond it and the backspan lifts: the far end of the joist tries to rise out of its hanger, and the deck develops a springy edge that owners describe as bouncy and engineers describe as the beginning of the end.
Once the tributary width and the clear span give you a moment, this is what tells you whether the section on the drawing carries it or whether an extra post pair is the cheaper answer.
The maximum bending moment the beam must resist.
The beam's cross-sectional width (the narrower dimension).
The beam's cross-sectional depth (the dimension parallel to the bending load).
Actual bending stress
1,442 psi
They open the calculator with your figures already in it
Timber Beam Bending Stress Calculator: 1,442 psi — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1,442 psi — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The figure returned is the stress demand on the section, not a verdict on the beam. Nothing here compares it against the bending strength of your species and grade, and none of the modification factors are applied — load duration and wet service under the NDS, kmod and the material partial factor under Eurocode 5. Under Eurocode 5 those two alone typically cut the characteristic bending strength to around 60 percent of its value, and the factors themselves are jurisdictional.
- Bending is only one of the checks a timber beam has to pass. This page does not calculate shear stress, which for a rectangular section is 3V/(2bd) and commonly governs on short heavily loaded spans, nor deflection, which usually governs on long ones and is limited by span ratios such as span/360 for imposed load depending on the jurisdiction. Bearing stress at the supports is also outside its scope.
- The flexure formula assumes the compression edge is held against sideways movement. A deep, narrow beam loaded on edge with an unrestrained compression edge can fail by lateral torsional buckling at a moment well below the one this stress figure implies is safe, which is why codes apply a beam stability factor (CL in the NDS, kcrit in Eurocode 5). No such reduction is made here.
- The calculation uses the full uncut rectangle you type in. Notches, birdsmouths, bolt holes and drilled service holes reduce the section modulus, and a notch on the tension face adds a stress concentration that simply reducing the depth does not represent, which is why code rules restrict notch depth and position. Enter actual dressed dimensions as well: a nominal 2x10 measures 1.5 by 9.25 inches (38 by 235 mm), and using the nominal figures understates the bending stress by about a third.
- Depth must be the dimension in the plane of the load, not just the larger one. Entering a 100 by 300 mm joist the wrong way round — 300 wide by 100 deep, as if laid flat — triples the reported stress, and the calculator cannot tell which orientation you meant. Every box follows the unit switch — the moment in kN·m or kip·ft, the section in millimetres or inches — and the stress comes back in MPa or psi.
Joists: span, spacing, and the ledger that holds half of them
Joist size follows from span and spacing, and spacing follows from what the decking above needs — which makes joist layout the hinge in the whole stack. Standard spacing suits most solid timber boards run square to the joists. Diagonal decking increases the effective span of each board and tightens the required spacing accordingly. Composite and plastic-composite boards carry their own manufacturer-published maximum joist spacing, and that spacing is a condition of the product warranty; ASTM D7032 is the specification such products are rated under, and the manufacturer's installation instructions govern in the field.
Ledger attachment deserves more care than any other single connection on the deck, because roughly half the deck's load goes through it into the house. The failures that make the news are ledger failures. Fasten to the rim board or to solid blocking, never to sheathing or to brick veneer, and use a fastener schedule from the connector manufacturer's published tables rather than a pattern copied off a previous job. Lag screws and structural screws have different withdrawal behaviour, edge distances and required penetration; the schedule accounts for all three.
Flashing behind the ledger is not weatherproofing trim, it is structural preservation. Water that gets behind an unflashed ledger rots the rim board, and a rotted rim board holds nothing regardless of how good the fasteners were. Cut the siding back, install continuous flashing lapped correctly with the water-resistive barrier above and turned out over the ledger below, and do not rely on sealant to do the lapping's job. Order flashing to a measured length rather than piecing offcuts — a lapped joint mid-ledger is a funnel.
Lateral load connection is the companion requirement in most current codes: hardware tying deck framing back into the house floor framing, resisting the deck's tendency to pull away rather than fall down. It is separate from the ledger fasteners and is not satisfied by them. Confirm which detail your jurisdiction requires — hold-down devices at specified locations, or a full continuous load path — before the framing is closed in, because retrofitting it means pulling boards.
Half the deck arrives at the house through this one line of fasteners, so divide the shear it has to carry by the published capacity for the screw and rim board you actually have before a single hole is bored.
The total shear load the ledger connection must carry.
The rated shear capacity of a single lag screw, from the applicable NDS/IRC table or manufacturer data.
Lag screws needed
9 screws
Use the actual IRC/NDS-table or manufacturer rated capacity for your specific screw diameter, species, and ledger/rim board thickness combination — this calculator only divides your total load by whatever per-fastener value you enter.
They open the calculator with your figures already in it
Ledger Lag Screw Shear Count Calculator: 9 screws — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The total shear load is the input this page needs most and explains least. It is the deck area landing on the ledger — half the joist span times the ledger length — times the design floor load, which on a residential deck is 40 psf (1.9 kPa) live over 10 psf (0.5 kPa) dead before any snow, planter or hot tub is added. Enter it low and the screw count comes out proportionally low, and nothing downstream catches the error.
- Screws in shear hold the deck UP; they do nothing to hold it against the house. Code now requires a separate lateral load connection — tension devices anchored back into the floor framing, rated in the region of 1,500 lb (680 kg) at two locations — because a ledger fastening resists a deck pulling away from the wall very poorly, and that separation is what actually drops decks off houses. No number of lag screws substitutes for those devices.
- A tabulated per-screw capacity assumes the screw lands in sound solid rim of the thickness the table names. Through brick veneer, over a cantilevered floor, into hollow block or stacked masonry, or across any sheathing gap the ledger does not bear tight on, the tabulated figure does not apply at any count — and several of those attachments are prohibited outright rather than merely derated. A rim that has been wet behind unflashed ledger for a few seasons holds a fraction of the table value at every screw at once.
Boards: the layer everybody sees
Decking is the only member sized by comfort and appearance rather than by structure alone, but it still carries the point load of a foot between two joists. Run the boards, count them honestly with the gap you actually intend to hold, and order a surplus for the cuts — a board layout planned as about right always ends with a sliver rip at the house wall, which is the one row a client sees every day from the kitchen window. Where a run exceeds stock length, plan the butt joints as a deliberate pattern over doubled blocking rather than letting them fall where the stock runs out.
Gapping depends on the material and its moisture state at installation. Kiln-dried timber installed dry will shrink less than green treated stock, which arrives wet and closes its gaps as it dries — install green boards tight or near-tight and they will open on their own, while installing them pre-gapped gives you a slatted deck by August. Composite boards move with temperature rather than moisture, and their end gaps are specified by the manufacturer against install-day temperature; ignoring that spec is how a summer-installed composite deck buckles in its first heatwave.
Fastener metallurgy interacts with the board. Face screws want stainless or a coating rated for contact with the preservative in your treated timber — AWPA U1 covers the use category and hence the preservative retention, and the wrong metallurgy in ACQ-type treatment corrodes visibly within a season. Hidden clip systems shift the load path into the board edge and demand a groove profile and a spacing the clip maker specifies. Pre-drill hardwood ends without exception; the split you cause at a board end is permanent and telegraphs straight along the grain.
Crown, cup and the direction of the growth rings all get argued about on site. What matters more is consistency: pick a convention, hold it across the whole deck, and keep the boards' factory-sealed ends where you can. Cut ends of treated timber lose their protection and need field-applied preservative before they go down — a bundle of untreated cut ends in the middle of a deck is a hundred small rot sites nobody will find until the boards feel soft underfoot.
Board count, run layout and the surplus for cuts — settled before the delivery, not on the pile.
The length of the deck in the direction the boards run.
The width of the deck, perpendicular to the board direction.
The length of deck board stock you plan to buy.
Estimated deck board needed
56 boards
- Deck area
- 253.5 sq ft
- Rows of boards (across width)
- 28 rows
- Boards per row (along length)
- 2 boards
They open the calculator with your figures already in it
Deck Board Calculator: 56 boards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Board width and the gap between boards are fixed together as a single 5.625 in pitch — a 5.5 in board plus a 1/8 in drainage gap — and neither is offered as an input, so the row count only describes nominal 5/4x6 stock. Narrower composite or hardwood boards change it quickly: at 5.25 in wide the same deck needs about 5 per cent more rows, and the wider gap that grooved hidden-fastener clips set gives you fewer.
- Boards per row is worked out once and then multiplied by the number of rows, so an offcut is never carried across to start the next row. On the default 6 m deck that is two full 12 ft boards bought for every one of the 28 rows and a 4.3 ft tail discarded 28 times, when in practice many of those tails would be reused — whether they can be depends on the seams landing on a joist and on how you stagger them, and neither is modelled here.
- No waste, breakage or cutting allowance is added anywhere in the figure. Boards culled at the yard for crown, cup or splits, the ripping the outermost row usually needs to bring the deck to its finished width, and cuts around posts, notches and openings all come out of the same count, so ordering exactly this number leaves nothing to reject.
- The footprint is treated as one plain rectangle with every board running the same way: area is length times width, and rows come straight off the width figure. An L-shaped or wrapped deck has to be entered as separate rectangles and the counts added, and any surface where the boards turn to run the other way has to be worked out on its own, because the width input is what defines the direction rows are counted in.
- The board-length choices are held in feet and the metric labels are rounded: choosing 2.4 m (~8 ft) still puts a full 8 ft into the arithmetic, which is 2.4384 m and about 1.6 per cent longer than the board you would actually be handed. On a deck length that only just divides into the nominal figure, that difference is enough to leave each row one board short.
- The square-foot figure in the breakdown is the gross rectangle, not the timber it takes to cover it — roughly one part in 45 of that surface is drainage gap once the boards are down. Pricing decking from the area and pricing it from the board count will not agree, and the board count is the one to order from.
Where the stack gets checked
Sequence your inspections to the hierarchy. Holes open and depths measured before concrete. Post bases and beam connections visible before joists go on. Ledger flashing, fasteners and lateral connectors visible before decking. Once the boards are down, every one of those becomes a demolition job to verify, and an inspector who cannot see a connection will assume the worst about it.
Paperwork belongs with the frame. Connector part numbers, fastener schedules, the manufacturer's installation instructions for composite boards, the treatment tag off the lumber bundle — photograph them against the work. Warranty claims on decking and hardware turn on demonstrating the product was installed to its published instructions, and a photo of the ledger schedule before the boards went on has settled more disputes than any amount of recollection.
Finish work sits on top of the same hierarchy. Guard posts are not decorative; they transmit a specified horizontal load into the frame, and ICC-ES AC273 is the acceptance criteria such assemblies are evaluated against. A guard post bolted only through the rim, without blocking and a tension tie into a joist, will pass a shove and fail a crowd. Balusters, stair rise consistency and the finish schedule all follow once the load path underneath them is closed.
Spindle count between posts is fixed by the sphere that must not pass and by the width of the stock you bought, and getting it wrong shows up on every bay of the guard at once.
The length of railing between two posts.
The width of a single baluster (spindle), from the product spec.
Balusters needed
17 balusters
The count is the fewest spindles that keeps every clear gap at or under the largest gap shown: 100 mm (3.94 in) when you work in metric, the sphere England's Part K and Canada's code use, and 4 in (101.6 mm) in imperial, the IRC's. The figure your own building rule sets is the one that governs, and some jurisdictions set stricter guard requirements.
- Rail section length
- 96 in
- Actual gap between balusters
- 3.92 in
- Largest gap allowed
- 4 in
They open the calculator with your figures already in it
Deck Baluster Calculator: 17 balusters — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The count covers one post-to-post bay and includes no spares, so a railing made of several sections has to be worked out and ordered bay by bay, with your own allowance on top for a spindle that splits at the fixing hole.
- Spacing is solved as one repeating baluster-plus-gap unit, which assumes every spindle is exactly the width entered and that the two end gaps against the posts finish the same as the gaps in the middle; setting out from a centred spindle, or leaving a wider gap at one post, breaks the figure shown for 'Actual gap between balusters'.
- Rail section length is taken as the clear opening between posts exactly as typed, with nothing deducted for post faces, newel trim or rail-end brackets, so a centre-to-centre measurement overstates the run and buys more spindles than the real opening needs — squeezed into the true span, the finished gaps close up tighter than the breakdown reports.
- The largest gap follows the unit system the page is worked in, not the place the guard is built: 100 mm (3.94 in) in metric, the sphere of England's Approved Document K and Canada's National Building Code, and 4 in (101.6 mm) in imperial, the IRC's. A Canadian guard set out in inches is still held to 100 mm, so work it in metric; where a code allows a larger sphere, such as Australia's 125 mm (4.92 in), this count can hold more spindles than that code asks for, never fewer.
- Nothing in the result speaks to strength or fixing: baluster thickness, whether each spindle is screwed, pocketed or clipped to the rails, and the load the infill has to resist are all absent from the arithmetic, which counts spacing and nothing else.
- Only the horizontal gaps between spindles are sized here — the gap beneath the bottom rail, the clearance where the guard meets the deck surface and the height of the guard itself are never computed, so a bay that passes on spindle spacing can still leave an opening somewhere the count never looked.
Deck take-off, from the ground up
Work the two calculators in load-path order — concrete first, surface last — carrying the tributary numbers between them.
- Footings and piers — Count set by beam spans; depth set by local frost depth from the authority having jurisdiction.
- Beam and post stock — Size against tributary width, not span alone; shortening a span usually beats upsizing the beam.
- Joist hangers and ledger fasteners — Use the connector manufacturer's published schedule; lag and structural screw values differ.
- Ledger flashing — Order continuous to measured length — a mid-run lap funnels water into the rim board.
- Decking with cut surplus — Confirm the gap convention against material and moisture state before ordering the run.
- Preservative for cut ends — Every field cut in treated timber loses its protection and needs sealing before it goes down.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.