Deck Framing Calculator
Joists, beams, footings and board counts for a deck — and the load each ledger fastener carries, which is where decks actually fail.
Wider or deeper?
What each ledger fastener carries, holding everything else fixed. Only one of these two dimensions is on the list.
Materials and framing
Where decks actually fail
Not the joists. The connection back to the house. A 12 ft deep deck puts 300 lb on every foot of ledger, so fasteners spaced like ordinary framing — 16″ apart — are each holding 400 lb in shear. That is a much bigger number than the spacing suggests anyone had in mind, and it is why ledger fastener spacing comes from a code table rather than from habit, and why that table is far tighter than 16″ and tightens further as the joists get longer.
Making a deck wider changes nothing per fastener. Making it deeper changes everything. The ledger carries half the joist span, so the load per foot depends on how far the deck reaches out and not at all on how far it runs along. Double the width and you get twice the fasteners carrying twice the total — each one unchanged. Extend the same deck from 12 ft to 20 ft deep and every fastener already in that wall goes from 400 lb to 667 lb without one being added. Extending a deck outward is a different job from extending it sideways.
And the decking gap rule runs backwards from intuition. Treated lumber is usually delivered wet and shrinks as it dries — an eighth to a quarter of an inch across a 5½″ board. So wet boards go down tight and open their own gap. Dry and kiln-dried boards have already shrunk, so they go down with a gap. Get it backwards and you end up either with half-inch gaps within a season, or with boards that have nowhere to go when they take up moisture and cup instead of closing up.
- A ledger must bear on solid framing, not through siding. A lag screw whose threads stop in sheathing carries almost nothing and looks exactly like one that is carrying its share.
- Flashing is half the ledger detail. Water trapped behind a ledger rots the band joist it is bolted to, and that connection fails years later with nothing visible from outside.
- Deck framing spans less than the same timber indoors. The wet-service factors take 15% off bending and 10% off stiffness before anything else is considered.
- Better soil buys less than you'd think. Tripling the bearing pressure cuts footing area to a third but the diameter only by 42%, because area goes with the square.
- Two screws per joist crossing, not one. A single fastener lets the board pivot and cup; the second is what keeps it flat.
- Diagonal decking spans 41% further between joists than square-laid decking, so it needs the joists closer — which is usually decided after the framing is already up.
How to use
- Enter the deck size — the depth out from the house is what loads the ledger.
- Set the joist size and spacing, and check the span warning before anything else.
- Read the load per ledger fastener, then get the actual spacing from your code table.
- Pick how the boards feel now, since wet and dry decking are laid differently.
Frequently asked questions
Where do decks usually fail?
At the ledger — the connection back to the house — rather than in the joists or the boards. The arithmetic makes it obvious why it deserves attention: a 12 foot deep deck at 50 psf puts 300 lb on every foot of ledger, so fasteners spaced like ordinary framing at 16 inches are each holding 400 lb in shear. That is a far larger number than the spacing suggests anyone had in mind.
Does a wider deck put more load on each ledger fastener?
No — not one pound. The ledger carries half the joist span, so the load per foot depends on how far the deck reaches out from the house and not at all on how far it runs along it. Doubling the width doubles the total on the ledger and doubles the number of fasteners sharing it, leaving each one exactly where it was.
What happens if I extend a deck outward?
Every fastener already in that wall picks up more load, and none are added. Taking a deck from 12 feet deep to 20 puts each one from 400 lb to 667 lb — a 67 per cent increase with the same fasteners in the same holes. That is why extending a deck outward is a genuinely different job from extending it sideways, and why the ledger has to be reconsidered rather than simply lengthened.
How far apart do ledger fasteners go?
Get it from the code table your jurisdiction has adopted, not from a calculation. The spacing is much tighter than framing spacing — often half of it or less — and tightens further as the joist span grows. Lag screws and through-bolts are also not interchangeable at the same diameter, with lags needing noticeably closer spacing. The load figure here exists so the table makes sense, not so you can substitute for it.
Can I attach a ledger through siding?
No. Fasteners have to reach solid framing with nothing compressible in between — no siding, no foam sheathing, no gap. A lag screw whose threads stop in sheathing carries almost nothing and looks exactly like one that is carrying its share, which is what makes it dangerous. The siding is cut away and the ledger sits against the sheathing or the band joist itself.
Why does flashing matter so much on a deck?
Because water trapped behind a ledger rots the band joist the whole deck is bolted to, and it does so out of sight. The connection then fails years later with nothing visible from outside and nothing obviously wrong on the deck. Flashing over the ledger is not a finishing detail — it is half of the structural connection, and it is the half that decides whether the other half is still there in ten years.
Should I leave a gap between deck boards?
It depends entirely on how wet the boards are now, and the rule runs backwards from most people’s instinct. Treated lumber is usually delivered wet and shrinks as it dries — an eighth to a quarter of an inch across a five and a half inch board. So wet boards go down tight and open their own gap. Dry and kiln-dried boards have already shrunk, so they need a gap left for them.
What happens if I gap wet deck boards?
You end up with half-inch gaps within a season, wide enough to lose things through and wide enough to look like a mistake, because the gap you left is added to the gap the shrinkage creates. The opposite error is just as bad in a different way: dry boards laid tight have nowhere to go when they take up moisture, so they cup and buckle rather than simply closing up.
Do deck joists span as far as floor joists?
No. The wet-service factors take 15 per cent off the bending value and 10 per cent off stiffness before anything else is considered, so identical timber spans noticeably less outdoors than it does inside. It is a common source of surprise when someone sizes deck joists from an interior floor span table.
How big does a deck footing need to be?
Divide the load on that post by the soil’s bearing value. On clay at 1,500 psf a post carrying 2,400 lb needs 1.6 square feet of bearing, which is about a 17 inch diameter footing. Better soil helps less than expected: tripling the bearing pressure cuts the required area to a third but the diameter only by 42 per cent, because area goes with the square of the diameter.
How deep do deck footings go?
Below the frost line for your area, which is a local figure and can be four feet or more. That is entirely separate from how wide they need to be — the width comes from the soil bearing calculation and the depth comes from frost. A footing of exactly the right area at the wrong depth will heave and lift the deck, usually unevenly and usually at one corner.
How many screws does a deck take?
Two per joist crossing per board, which adds up faster than people expect — a 16 by 12 foot deck runs to around 700. Two rather than one is not decoration: a single fastener lets the board pivot around it and cup, and the second one is what holds it flat. Diagonal decking needs more again, since it spans 41 per cent further between joists and usually calls for closer joist spacing.
Does this cover everything a deck needs?
No. It covers layout and quantity, and it does not size the beam, check the posts for buckling, or handle the lateral load the code requires a ledger-attached deck to resist — which is a separate connection back into the house framing and is very commonly omitted. Decks are inspected closely because they fail suddenly and with people standing on them. Get the ledger detail, the flashing and the footing depth from your local code.
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