Deck

What it's for

Takes off a rectangular deck from the boards down: decking, joists, screws, posts, footing tubes, concrete, beam, ledger, ledger fasteners, hangers and railing. You get the order, three drawings — a framing plan, the beam line with its footings, and the decking layout with the rip on the last board — and a material list you can work as a cut list. It checks the frame against the deck tables of the 2021 IRC, Section R507: the joist span and cantilever, the beam between its posts, the footing size for your soil, and the ledger fastening. A check within its limit reads green; one over it reads in red capitals.

Step by step

The example is the screen as it opens: PT Lumber, 5/4x6 boards 16 ft long with a 1/8" gap, a deck 16' 0" by 12' 0" and 3' 0" above grade, 2x10 joists at 16" o.c. in DF / hem-fir / SPF, a 6" cantilever, a 2-ply beam with posts no more than 8' 0" apart, clay or silt soil, ground snow 40 psf or less, attached to the house with 1/2" lag screws, 40' 0" of railing, 10% waste.

Deck inputs at their defaults: MATERIAL PT Lumber (Cedar and Composite beside it), BOARD WIDTH 5/4x6 (or 5/4x4), BOARD LENGTH 16' 0" (or 8' 0", 10' 0", 12' 0"), BOARD GAP 1/8", DECK LENGTH 16' 0", DECK WIDTH 12' 0", HEIGHT ABOVE GRADE 3' 0", JOIST SPACING 16" o.c., JOIST SIZE 2x10 and, at the foot, FRAMING SPECIES Not picked. The answer bar at the top reads 29 boards and, in amber, Joists 2x10 @ 16" o.c. span 11' 4-1/2" — within the 13' 7" max (IRC 2021 R507.6), then 464 lin ft decking (order) · 26 courses · 1 per row. — Steps 1 to 7. The answer bar at the top keeps the board count and the first check in view wherever you scroll. The check is amber until the framing species is picked.

Pick MATERIAL: PT Lumber, Cedar or Composite. The board widths and lengths on offer change with it. If the size you had picked is not stocked in the new material, the screen moves you to one that is.
Pick BOARD WIDTH and BOARD LENGTH, the stock length you will buy.
Enter BOARD GAP, the space you will leave between boards.
Enter DECK LENGTH, along the house, and DECK WIDTH, out from the house: from the house wall behind the ledger (free-standing: the outside of the far rim) to the outside face of the rim. The decking runs the length. The joists run the width, less the ledger and the rim — on the example, 12' 0" wide gives a joist span of 11' 4-1/2" from the face of the ledger to the center of the beam.
Enter HEIGHT ABOVE GRADE, from the ground to the top of the deck. It sets the post length in the material list.
Pick JOIST SPACING (16", 19.2" or 24" o.c.) and JOIST SIZE (2x6 to 2x12). The size names the joist, beam and ledger rows, and the joist span is checked against it. The code's least ledger is a 2x8 (IRC R507.9.1.1), so 2x6 joists buy a 2x8 ledger. The deck table prints 12″, 16″ and 24″ spacing only, so at 19.2″ the joist span says not checked.
Pick FRAMING SPECIES, the lumber the joists and beam are cut from, No. 2 grade: Southern pine; DF / hem-fir / SPF (Douglas fir-larch, hem-fir, spruce-pine-fir); or Redwood / cedar (redwood, western cedars, ponderosa pine, red pine). Every deck table is printed by these three groups. It starts at Not picked. Until you pick it, the checks read DF / hem-fir / SPF and their passing lines are amber, not green. The same goes for GROUND SNOW below. A line under the results names the species and loads the checks were read for. The drawing and the cut sheet carry it too.

Deck scrolled to the rest of the inputs: JOIST SIZE 2x10, FRAMING SPECIES Not picked, JOIST CANTILEVER 6", BEAM PLIES 2-ply (3-ply beside it), MAX POST SPACING 8' 0", SOIL Clay or silt · 1,500 psf, GROUND SNOW Not picked, ATTACHED TO HOUSE (LEDGER) on, LEDGER FASTENERS 1/2" lag screws, RAILING on, RAILING LENGTH 40' 0" and the top of WASTE FACTOR. — Steps 8 to 15, the inputs the deck code checks need and the rest of the form.

Enter JOIST CANTILEVER, how far the joists run past the center of the beam. The beam stands that far in from the outer edge. 0 puts the beam at the edge.
Pick BEAM PLIES, 2 or 3 of the joist size nailed together.
Enter MAX POST SPACING, the most you want between posts along a beam. The posts stand 6" in from each end of the beam and evenly between. On the example 8' 0" gives 3 posts 7' 6" apart. Close the posts up if the beam fails.
Pick SOIL, the bearing the IRC lets you assume without a soils report (Table R401.4.1): clay or silt 1,500 psf, sand 2,000 psf, gravel or rock 3,000 psf. It sizes the footings. If you don't know, leave clay or silt — it is the lowest value the table lets you assume. On fill, organic or very soft ground the IRC calls for a soils investigation, so get a soils check.
Pick GROUND SNOW, the ground snow load your county gives. A deck is built for 40 psf of people or for the ground snow, whichever is more (IRC R507.1). At 40 psf or less the checks read the 40 psf rows of the deck tables; at 50, 60 or 70 psf they read that row. Over 70 psf is past the tables: the checks say not checked and that the deck has to be designed (IRC R301.1.3), the beam-line drawing says HAVE IT DESIGNED, and what the drawings and list show is the 70 psf row's sizes, a starting point only.
Leave ATTACHED TO HOUSE (LEDGER) on for a deck hung off the house. You get a ledger, its fasteners, joist hangers and one beam. Turn it off for a freestanding deck. The ledger and hangers drop out and you get two beams, with posts, footings and concrete for both.
Pick LEDGER FASTENERS: 1/2" lag screws or 1/2" through-bolts through no more than 1/2" of sheathing, or 1/2" bolts through up to 1" of sheathing. The row shows only on an attached deck. It sets the fastener spacing and count.
Leave RAILING on to buy railing. RAILING LENGTH opens on the run this deck implies — the three open sides of a deck attached to the house (two widths plus the length; the house side takes no rail), or the whole perimeter of a free-standing one — and follows the deck as you change its size. Type over it and your figure is used instead. The row is hidden when railing is off. A deck more than 30" above grade needs a guard on its open sides (IRC R312.1.1): with railing off, the card says so in amber — Guard required … RAILING is off, so none is ordered — in case the guard is coming from another supplier. The railing row is a length; the guard's 36" height and its 4" openings are the railing maker's.
Set WASTE FACTOR. It is added to the decking, joists, screws, concrete, beam and railing. Posts, footing tubes, ledger, ledger fasteners and hangers are exact counts.

Reading the results

Deck scrolled to the results. The end of the inputs shows RAILING on, RAILING LENGTH 40' 0" and WASTE FACTOR 10% with its stepper. RESULTS reads 464 lin ft decking (order) · 26 courses · 1 per row, 13 joists · 15 ordered, a grey line IRC 2021 R507 read for — DF / hem-fir / SPF No.2 · 40 psf live · 10 psf dead, then seven amber rows: Joists 2x10 @ 16" o.c. span 11' 4-1/2" — within the 13' 7" max (IRC 2021 R507.6); Cantilever 6" — within the 2' 10-1/8" max (IRC 2021 R507.6); Beam (2) 2x10 spans 7' 6" — within the ≈ 8' 6-1/16" max (IRC 2021 R507.5); Footings 20" round — ≈ 19" needed for ≈ 58 sq ft a post (middle post × 1.25, the beam runs over it — DCA 6) on 1,500 psf soil (IRC 2021 R507.3.1); Posts 6x6 1' 4-1/2" beam to footing — within the 14' 0" max (IRC 2021 R507.4); Ledger: 1/2" lags @ 15" OC, staggered (IRC 2021 R507.9.1.3); and Decking 5/4 @ 16" o.c. — within the 16" o.c. max, each board on 3+ joists (IRC 2021 R507.7). The BASIS & WHAT WAS ASSUMED row follows. The answer bar keeps 29 boards. — Step 16, the waste factor, then steps 17 to 19: the answer, the decking and joist lines and the seven deck code checks.

The headline is decking boards to buy at your stock length, waste included. On the defaults: 29 boards. The count is for the pieces the decking layout cuts, packed onto your stock with the offcuts reused (a 1/8" saw kerf), so a short deck can cut two courses from one board and a long deck's short end pieces come out of other boards' offcuts.
Decking line. On the defaults: 464 lin ft decking (order) · 26 courses · 1 per row. Courses is the number of boards across the deck width, at board width plus gap. Per row is how many stock lengths it takes to cross the deck length. More than one means butt joints. Joists: 13 joists · 15 ordered — the first number is what you lay out and hang, the second the order, with waste.
The deck code checks, one row each, the section named at the end. On the defaults all seven are within their limits, in amber until you pick the species and ground snow: Joists 2x10 @ 16" o.c. span 11' 4-1/2" — within the 13' 7" max (IRC 2021 R507.6) — the joist span runs from the face of the ledger (on a freestanding deck, from the other beam) to the center of the beam; Cantilever 6" — within the 2' 10-1/8" max (IRC 2021 R507.6); Beam (2) 2x10 spans 7' 6" — within the ≈ 8' 6-1/16" max (IRC 2021 R507.5) — post to post; Footings 20" round — ≈ 19" needed for ≈ 58 sq ft a post (middle post × 1.25, the beam runs over it — DCA 6) on 1,500 psf soil (IRC 2021 R507.3.1) — the deck area the busiest post carries, and the tube the list buys. A middle post under a beam that runs on over it carries a quarter more than its share (AWC DCA 6), and the list buys each beam ply the full length of the deck, so the app sizes a middle post that way; Posts 6x6 1' 4-1/2" beam to footing — within the 14' 0" max (IRC 2021 R507.4) — the post from the underside of the beam to the top of the footing: the height above grade less 1" of decking, the joist and the beam (the beam is the joist's size, under the joists), with the footing top at grade; Ledger: 1/2" lags @ 15" OC, staggered (IRC 2021 R507.9.1.3); Decking 5/4 @ 16" o.c. — within the 16" o.c. max, each board on 3+ joists (IRC 2021 R507.7) — wood decking at 19.2" or 24" is over it. Composite decking spans by its maker's rules, so for composite the line reads not checked. A failed check reads in red capitals and says what it is over, for example JOISTS 2X8 @ 16" O.C. SPAN 11' 4-1/2" — OVER THE 11' 1" MAX (IRC 2021 R507.6). Every line names the edition it was checked against, the 2021 IRC. The 2024 edition re-tabulated deck beams by joist span and cantilever, and for some decks it allows a shorter beam span: where it does, an amber line says so — for example the default deck in Redwood / cedar, whose 7' 6" beam span passes 2021 but is over 2024's 7' 5" — so you can confirm which edition your jurisdiction adopted. The app does not switch editions. The strip at the top shows the first failure while you scroll. A case the tables do not reach reads as a plain not checked line instead of a guess.
BASIS & WHAT WAS ASSUMED opens to the load (40 psf live plus 10 psf dead, or the ground snow you picked plus 10 psf dead), the species and grade, where each width and span is measured, how the footing is sized and rounded up to a stock tube (12" least), how the post height is measured, and what is not checked: the lateral ties. It ends with IRC 2021 — verify local code and a sentence that states and municipalities amend the code: check the adopted edition and local amendments with your building department. The cut sheet prints the checks on its first page, a failed one at the top, with the load basis under them; the soil is with your inputs.
BASIS also says what is not checked: the loads are gravity only, so where IRC R301.2.1.1 requires wind design, or in Seismic Design Category D0 to E (R301.2.2; townhouses from C), the added rules are not applied.

Deck scrolled to the drawings. The last code rows (Posts, Ledger, Decking) end RESULTS, then the BASIS & WHAT WAS ASSUMED row, an outlined BUILD THE DECK STAIR button and the link Check before you cut · Measure twice. The DECK FRAMING plan is dimensioned 16' 0" by 12' 0" with 3 POSTS @ ≤8' 0" O.C. and BEAM, JOIST and LEDGER labelled, a footing detail 4' 0" deep marked POST and VERIFY FROST, the notes JOISTS @ 16" O.C. and DF / HEM-FIR / SPF NO.2 · 40 PSF LIVE · 10 PSF DEAD — VERIFY, and the title strip DECK FRAMING, 13 joists (15 ordered), NOT TO SCALE. Below it the next drawing starts: 3 POSTS PER BEAM @ 7' 6" O.C. and SECTION AT POST. — 20 the basis row, with BUILD THE DECK STAIR under it · 22 the framing plan · 23 where the beam line begins. The decking layout is below them.

DECK FRAMING is the plan: ledger, joists at their spacing (the end joists flush with each end), the beam where JOIST CANTILEVER puts it, the rim across the joist ends, and posts, with the deck dimensioned and a footing detail marked VERIFY FROST. Pinch to zoom, drag to pan, double-tap to reset, and use the expand button for full screen. See Drawings.
DECK BEAM LINE is an elevation along the beam and a section at a post. It dimensions the post centers along the beam, the height above grade and the footing depth, and draws the beam where the cantilever puts it. Its notes give where the end footings sit, the tube size, and SPANS: SEE THE IRC 2021 R507 CHECKS.
DECKING LAYOUT shows every course over the joists, with running dimensions from the house side. The corner detail gives the last board. On the defaults the course module is 5-5/8" and the last board is a 3-3/8" RIP. The notes spell out the module, 5-1/2" + 1/8" GAP, and whether there are butt joints. Joints fall on joist centres, drawn course by course: the courses alternate two patterns, so no two courses side by side joint on the same joist, and every board bears on at least three joists (two are allowed only at 12" o.c. — the code's single span, IRC R507.7). The cuts for the first course run along the bottom; the other pattern's are in the notes as ALT. COURSE. The joists are where the framing plan puts them.

MATERIALS. Each row carries its own note after the dash. On the defaults: joists 12' each; screws 350/box — 2 per joist crossing (+ splices); 6x6 posts 4' each — 3 per beam @ 7' 6" o.c.; footing tubes 20" — 48" — verify local frost depth; concrete 60-lb (0.45 cu ft) — ≈ 26.2 cu ft in 3 tubes 20" x 48" + waste, the whole pour rounded up once, the way Concrete counts the same footings; beam outer beam, 2 plies. An attached deck adds the ledger, marked flashed (IRC R703.4), its lag screws or bolts at the table's spacing, one hanger per joist, the ledger flashing (IRC R507.2.4) and two 1,500 lb lateral hold-downs within 24" of each end (or four 750 lb; IRC R507.9.2). Every deck also buys the hardware the deck code asks for, each row naming its section: the rim across the joist ends and blocking between the joists over the beam (IRC R507.6.2), a post base on each footing (R507.4.1) and a post cap on each post (R507.5.2), 10d nails for the rim and the beam plies (R507.6.2, R507.5), and 8d toe nails at the beam (R507.6.1). Connectors and fasteners outdoors are hot-dip galvanized or G185 (R507.2.3).
The buttons are at the very bottom. CUT SHEET shares a PDF of the drawing, inputs, the code checks and materials, and the list icon at its right end shares the material list alone. SAVE keeps the result in History. CUT LIST puts the material list on the Lock Screen as a cut list to work through. SEND LIST shares the list as a page someone else can open and tick off on their own phone, with no app.

Deck is a Pro calculator. CUT SHEET, SAVE, CUT LIST and SEND LIST are Pro as well, a one-time purchase. CUT SHEET, the list icon in it, CUT LIST and SEND LIST stay dim until you have entered your own measurements. SAVE stays lit, but until then a tap only answers Enter your job's numbers first and saves nothing.

Common mistakes

Swapping length and width. DECK LENGTH is along the house and sets the joist count, the beam and the ledger. DECK WIDTH is out from the house, measured to the outside of the rim — not the joist length, which is shorter by the ledger and the rim. Check the framing plan: the joists should run the way you mean to frame them.
Leaving FRAMING SPECIES on the wrong group. Ask the yard what the treated framing is. Southern pine spans a little farther than DF / hem-fir / SPF in most sizes, so a deck that passes in one group can fail in the other.
Leaving GROUND SNOW at 40 psf or less in snow country. Every green row is judged at the load you pick; look up your county's ground snow before you trust them.
Reading a pass as a permit. The checks cover the joists, beam, footings and ledger at the load you pick. They do not check the lateral ties, guards or stairs, or a ground snow load over 70 psf. Your building department has the last word.
Finding a thin rip at the last board on the day. Read the corner detail on the decking layout before you start. If the rip is too narrow to fasten, change BOARD GAP a little and watch it move.
Ordering 48" footing tubes where frost is deeper. The footing tube and concrete rows are figured on a 48" deep tube, and the tube row says to verify local frost depth. The post row is height above grade plus a foot, rounded up to even-foot stock. It does not change with frost depth, because the posts stand on the footings.
Entering the deck perimeter as RAILING LENGTH on an attached deck. The house side has no rail, and the row already opens on the three open sides. Type over it only when the job differs — a returned rail, a stair opening, a gate.

The stair

BUILD THE DECK STAIR, under the results, opens Stairs with the deck's height above grade as the stair's total rise and EXTERIOR (DECK) STAIR on, so the deck-guide rules apply there: 2x12 cut stringers at 18" apart at most, the 12' 0" rise limit and hangers at the top. The deck has no stair width, so Stairs keeps its own. Back returns to the deck.