How the Deck Designer sizes your deck
Every size on an AerialQuote deck plan comes from the deck tables of the national residential code. This page shows which table each number comes from, the assumptions behind them, and where the Designer deliberately stays on the safe side — so you, your building department or your contractor can check the work.
The code it follows
The Designer uses the 2021 International Residential Code (IRC), section R507 — Exterior decks, and the American Wood Council’s DCA 6 guide, which explains the same tables for builders. Section R507 is “prescriptive”: if a deck follows its tables, it meets the code without an engineer’s calculations. That is exactly what the Designer does — it looks up the tables, it does not engineer anything new.
Your town may be on a different edition. States and towns adopt the IRC on their own schedule — many are on the 2015 or 2018 edition — and most add local amendments (frost depth, snow load, guard height, footing rules). The deck section changed between editions. Your building department’s rules always win; if they ask for a change, you can make it on the plan and print a new revision.
Loads
- Standard — 40 psf live load + 10 psf dead load (IRC Table R301.5). This is what the R507 tables are built on.
- Snow country — 60 psf snow + 10 psf dead. Spans are scaled down from the 40 psf tables by the square root of the load ratio (about 15% shorter). The 2021 IRC also publishes separate snow tables; until they are entered, this scaling is the Designer’s starting point.
- Heavy duty (hot tubs, outdoor kitchens) — 100 psf + 15 psf. The code tables don’t cover these loads, so the Designer only shows a beefed-up starting point and does not offer a permit set — a hot tub needs an engineer.
- Soil — 1,500 psf bearing capacity, the code’s presumptive value for clay and silt (Table R401.4.1). Better soil only makes the footings safer.
Lumber
Pressure-treated #2 grade. The species is set from your address: Southern Pine in the East, South and Midwest, and Douglas fir-larch / hem-fir / spruce-pine-fir in the West (WA, OR, CA, NV, ID, MT, WY, UT, CO, AZ, NM, AK, HI), because that is what lumberyards there stock.
Joists
The Designer picks the smallest joist that spans the distance at 16" on center, and tightens to 12" only if it has to. Maximum spans, IRC 2021 Table R507.6 (40 psf live, 10 psf dead):
| Species | Size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|---|
| Southern Pine | 2×6 | 9'-11" | 9'-0" | 7'-7" |
| 2×8 | 13'-1" | 11'-10" | 9'-8" | |
| 2×10 | 16'-2" | 14'-0" | 11'-5" | |
| 2×12 | 18'-0" | 16'-6" | 13'-6" | |
| Douglas fir-larch / hem-fir / SPF | 2×6 | 9'-6" | 8'-8" | 7'-2" |
| 2×8 | 12'-6" | 11'-1" | 9'-1" | |
| 2×10 | 15'-8" | 13'-7" | 11'-1" | |
| 2×12 | 18'-0" | 15'-9" | 12'-10" |
The joist span is measured from the ledger to the beam. Composite decking and 5/4 boards are laid on joists no more than 16" apart.
Deck board patterns. Boards laid straight, square to the joists, need joists no more than 16" apart. A diagonal field (boards at 45°) needs them 12" apart (IRC Table R507.7; composite makers say the same), so the Designer drops to 12" on center whenever a diagonal pattern is picked. A picture frame border adds a nailer joist inside each end joist for every border row, so the frame boards on the two sides that run with the joists are fully supported, and a diagonal field adds blocking between the joists along the house and outer edges, where the angled board ends land. The shopping list adds 7% extra boards for a framed straight field, 10% for the mitred frame boards and 15% for a diagonal field.
Side-by-side frames. On a long deck, the field can be split into two or more sections along the house, each in its own double picture frame, so the boards inside each run straight across on a shorter stock length — a 29'-6" deck becomes two 14'-9" sections on 14 ft boards. Each section gets nailer joists under its border rows, and each joint between sections sits on a doubled joist.
Composite edges. Grooved composite boards are held by hidden clips in the grooves, so any board whose edge shows — the outer row of a picture frame on the three open sides, the last row of a straight layout, and the front of each stair tread — has to be a square-edge (no-groove) board, fastened with color-matched face screws. They come in 12, 16 and 20 ft lengths, and the list counts them separately.
Overhang past the beam
Joists can run past the beam (a cantilever), usually 1 to 1½ ft. The Designer keeps every overhang within a quarter of the back span and within the cantilever limits in Table R507.6, which get shorter as the back span gets longer and are not allowed at all near a joist’s full span. On 24" spacing and western species it takes those limits a further 10–20% shorter until the full table is entered.
Beam and posts
The beam runs along the outside, with posts set about 1 ft in from each end. Beam spans come from IRC 2021 Table R507.5. At a 12 ft joist span, for Southern Pine:
| Beam | Max post spacing |
|---|---|
| 2-ply 2×8 | 6'-2" |
| 2-ply 2×10 | 7'-4" |
| 2-ply 2×12 | 8'-7" |
| 3-ply 2×8 | 7'-9" |
| 3-ply 2×10 | 9'-2" |
| 3-ply 2×12 | 10'-9" |
For other joist spans the Designer scales from this column, and it measures the joist span including the overhang — a longer figure than the code’s own method, so its post spacings come out shorter than the table allows. Among the beams that work, it picks the combination of beam size and number of footings that costs least to build, and never uses a beam shallower than the joists.
Posts are 6×6 by default (good to 14 ft tall, Table R507.4). If you switch to 4×4, the allowed height depends on how much deck each post carries, and the Designer caps it at 8 ft as DCA 6 does. A 3-ply beam needs a 6×6 to sit on fully.
Footings and concrete
Each inside post carries half the joist span plus the overhang, times the post spacing:
footing area = load ÷ 1,500 psf → diameter, +10% for the footing’s own weight
→ rounded up to a stock tube: 12, 14, 16, 18, 20 or 24 in
The +10% brings the sizes in line with IRC Table R507.3.1. Depth is your frost depth plus 3 inches, rounded up to the next 6 inches (42" frost → 48" deep), and never less than 12 inches into undisturbed soil (R507.3.2). The default frost depth is 42"; you change it to your town’s number. Concrete is counted at 0.6 cu ft per 80 lb bag, including the couple of inches the pier stands above grade.
Ledger and lateral load
The ledger is bolted to the house’s rim board, at least a 2×8 (R507.9.1.2), flashed above. Fastener spacing, staggered in two rows, IRC 2021 Table R507.9.1.3(1):
| Joist span | ½" lag screws | ½" through-bolts |
|---|---|---|
| up to 6 ft | 30" | 36" |
| up to 8 ft | 23" | 36" |
| up to 10 ft | 18" | 34" |
| up to 12 ft | 15" | 29" |
| up to 14 ft | 13" | 24" |
| up to 16 ft | 11" | 21" |
| up to 18 ft | 10" | 19" |
For snow loads the spacing is tightened in proportion. Every attached deck also gets the lateral load connection of R507.9.2: two hold-downs rated at least 1,500 lb each (or four at 750 lb), tying the deck joists into the house floor.
Stairs and guards
- Stairs (R311.7): risers no taller than 7¾", treads at least 10" (the Designer uses two 5/4×6 boards, 11¼"), at least 36" wide, a graspable handrail 34"–38" high on runs of four or more risers, and a concrete landing pad at the bottom.
- Guards (R312): required on every open side of a deck more than 30" above grade, at least 36" high, with openings small enough that a 4" ball can’t pass.
The shopping list and the price
Quantities come straight from the plan: joists counted at the spacing plus the two ends, boards in real stock lengths (8 to 20 ft), deck boards at 5½" plus a 3/16" gap with 5% extra, two screws per board at every joist (or one hidden clip), ten nails per hanger and tie, concrete by the bag. The price range uses national mid-range material prices and labor per square foot; it is a planning guide, not a bid.
Where it stays on the safe side
- Beam spans are measured with the overhang included, which shortens post spacing.
- Footings get 10% extra diameter and round up to the next tube size.
- Ledger spacing reads the table column for the next longer joist span when yours falls between columns.
- Snow-country spans and western-species beams are scaled down rather than read from tables not yet entered.
- Every change you make on the plan is checked against these limits, with a warning when it goes past them.
What it won’t plan
Hot tubs and other heavy loads, decks more than 12 ft high, decks too deep for a single joist span, freestanding decks, multi-level decks, odd shapes, landings, roofs and pergolas. For any of these, use the drawings as a starting point with an engineer or a deck designer.
Not an engineered or architectural drawing. AerialQuote deck plans are starting points built from the code’s prescriptive tables, using assumptions that may not match your site. They are not reviewed by a licensed engineer or architect. Have your plan reviewed by your building department, and by a licensed professional where your town requires one, before you build. AerialQuote is not responsible for construction or for compliance with local codes. See our terms.