Sketch your deck from above, drawn to scale. Set the size and shape, joists, beam and post spacing, board width and direction, and optional stairs. The designer draws the framing and decking plan, checks joist spans and builds a material list.
Last updated:
Ledger / rimJoistsBeamPostsFootingsBlocking
Material list
Item
Qty
Size / length
Notes
Counts are planning estimates without a waste allowance unless noted. Confirm sizes, spans and fastening with your local building code.
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How to use the Deck Designer
Choose the shape and enter the width along the house and the depth out from it. For an L shape, set the size and side of the cut-out corner.
Set the framing: joist size and spacing, how far the joists cantilever past the beam, the beam and the maximum post spacing.
Set the decking: board width, gap, stock length and direction. Tick Picture-frame border for a finished edge.
Add stairs if you need them and enter the deck height above the ground.
Switch views between framing, decking and both, and read any span or height warnings.
Download the PNG plan or copy the material list for the lumber yard.
Typical maximum joist spans (reference)
These values are from the 2021 International Residential Code, Table R507.6, for Southern Pine No. 2 joists carrying a 40 psf live load and 10 psf dead load. Other species and grades, such as Hem-fir or Spruce-pine-fir, allow shorter spans, and snow loads or local amendments can change them. The designer uses this table only for its span warning.
Joist
12 in on center
16 in on center
24 in on center
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"
How the plan is laid out
Joists run straight out from the house. They are placed every 12, 16 or 24 in measured from the left end, with an end joist at each side and, on an L-shaped deck, one at the inside corner.
Joist span = depth of that part of the deck − cantilever (for a freestanding deck, minus the cantilever at the house side as well).
Beams sit under the joists, set back from the front edge by the cantilever. A freestanding deck gets a second beam near the house instead of a ledger.
Posts: each beam gets posts no more than your maximum spacing apart, starting up to 1 ft in from each end: posts = ⌈(beam length − 2 × inset) ÷ max spacing⌉ + 1.
Boards parallel to the house: rows = depth ÷ (board width + gap). Rows longer than a stock board are joined over a joist, with joints on alternate rows moved two joist bays to stagger them. Short leftovers are reused where they fit.
Screws: 2 per board at every joist it crosses.
Stairs: risers = ⌈height ÷ 7.75 in⌉, treads = risers − 1, each 10.5 in deep.
Design tips
Match the deck depth to your joist and board lengths (for example 12 or 16 ft) to cut waste.
A deck that steps down from the door by one riser keeps snow and rain away from the threshold; allow for it in the height.
Plan the stairs where people actually walk, usually toward the yard, path or patio.
It depends on lumber species, grade, size, spacing and the design load. As a reference, the 2021 International Residential Code (Table R507.6) lists, for Southern Pine No. 2 joists under a 40 psf live load, about 11 ft 10 in for 2×8 joists at 16 in on center, 14 ft for 2×10 and 16 ft 6 in for 2×12. Other species and grades are often shorter, so check your local code before you build.
How far can joists cantilever past the beam?
A common rule, reflected in the IRC deck provisions, is that a joist may overhang the beam by up to one quarter of its back span, and the IRC sets further limits by size and spacing. The designer warns you when the cantilever is more than a quarter of the joist span. A modest overhang of 1 to 2 ft hides the beam and posts and looks lighter.
Which way should deck boards run?
Most decks run boards parallel to the house, at right angles to the joists, which is the simplest and uses the least material. Diagonal boards at 45° look striking but need more cuts and waste, and most decking makers require joists no more than 12 in on center for diagonal boards, so check the instructions for your boards.
What is a picture-frame border?
A picture-frame border is a row of boards that runs around the outside edge of the deck and covers the cut ends of the field boards. It gives a finished, furniture-like edge. The border boards along the sides sit on the end joists, and extra blocking is usually added so the mitered corners and ends are fully supported.
How are the stairs calculated?
The number of risers is the deck height divided by the maximum riser height of 7 3/4 in allowed by the IRC, rounded up. The riser height is the deck height divided by that number, there is one fewer tread than risers, and each tread is drawn 10 1/2 in deep, above the 10 in code minimum. Stringers are counted at about 16 in apart.
Do I need a railing?
Under the IRC, walking surfaces more than 30 in above the grade below need a guard at least 36 in high. The designer flags decks higher than 30 in and estimates the railing length along the open sides, excluding the stair opening. Use the Deck Railing Calculator to count posts and balusters.
Is this a substitute for engineered or permit drawings?
No. The designer is a planning tool that helps you visualize a layout and estimate materials. Beam sizes, post sizes, footings, ledger attachment and fasteners must meet your local building code, and many areas require a permit and inspection for decks.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.