Plan the decking before you buy it. Enter the deck size, the board you are using and the lengths your supplier stocks, and this works out how many rows you get, where every board starts and stops, and how to place the butt joints so each one lands on a joist and is staggered clear of the row next to it. You get a top-down plan, a row-by-row cut list, a buy list by board length and an estimate of offcut and screws. Everything runs in your browser — nothing is uploaded, no sign-up.
Deck & boards
longer side, plan view
shorter side
joists sit at 90° to this
actual face width
Stock, border & jointing
comma separated, in the unit above
min offset between rows
shortest offcut you will lay
lost per cross cut
Notes
Why does the planner refuse to put a joint anywhere except on a joist?
Because a butt joint in mid-air is a trip hazard within two seasons. Both board ends need bearing, and each end needs its own fixings, which is why a joint doubles the screw count at that joist. If your run needs joints and the numbers here look awkward, the usual fixes are to double up the joist under the joint line, or to add a row of blocking so the two ends each get solid timber under them. Some builders prefer a full-width “breaker board” across the deck instead of scattered joints — this tool does not lay that out, but it will tell you how long each run is so you can decide.
How is the stagger actually decided?
The first piece of each row is shortened by a whole number of joist bays, cycling through as many distinct starting lengths as will fit before the piece gets shorter than your minimum. That pushes the joint line sideways by at least the stagger you asked for, row after row, and then the pattern repeats. It is the same thing you would do on site with a story pole, just counted for you. The result panel reports the smallest gap between joints in adjacent rows so you can check it against what you actually want.
Why is the last row a different width, and can I avoid it?
A deck almost never divides into a whole number of boards plus gaps. The planner fills with full-width boards and then rips the last one to suit, because that is what happens in practice. If the rip comes out as a sliver, the honest answer is not to rip it — nudge the gap by a fraction of a millimetre across all the rows instead, and the result panel gives you the exact gap that makes the boards land whole. Splitting the difference between the first and last row is the other classic dodge, and it looks better against a wall.
What this does not do
It does not design the frame — joist sizing, spans, beams, posts, footings and ledger fixing are structural work and are outside this tool. It assumes a rectangular deck on evenly spaced parallel joists, with the last bay shortened if the run does not divide evenly. It ignores expansion gaps at walls, board bow, and the extra blocking a picture frame needs under its mitres and under every field board end. The buy list is an exact-material figure with saw kerf allowed for on each cut — add your own contingency, five to ten per cent is normal, more if you are picking through the stack for straight boards. Offcuts are counted as waste even when they are long enough to be worth keeping.