Why this page exists at all
Ledger failure is the reason deck collapses make the news. A deck that fails structurally in the field bends and sags and gives people a warning. A deck that loses its ledger goes all at once, hinging down off the outer posts, and it usually happens when the deck is loaded, which is to say when it is full of people. Investigations of these failures keep returning the same two findings: the ledger was attached with something that was never rated for the job, and there was no flashing, so the rim board behind it had been quietly rotting for a decade.
What this calculator does is narrow. You supply a spacing from a detail somebody qualified gave you, and it tells you how many fasteners that spacing produces along your ledger and what the even layout actually works out to. It also converts your deck size and design load into the demand each fastener would see if the load shared out evenly. It does not check capacity and it will never tell you an attachment is adequate.
Round the intervals up, not the count down
This looks like a triviality and it is where the arithmetic usually goes wrong. If you have 188 inches to divide and a 16 inch maximum spacing, dividing gives 11.75. Round that to eleven spaces and you get twelve fasteners at 17.1 inches apart, which is over your limit everywhere. Round up to twelve spaces and you get thirteen fasteners at 15.67 inches, which is under it everywhere.
The count of fasteners is always one more than the count of spaces, and the number you round is the spaces. The calculator does this and reports the actual spacing so you can see it landed under the figure you entered rather than over it. If it ever reports a spacing larger than what you typed, something is wrong and you should not proceed.
End distance is deducted from both ends before the division. Those distances are not cosmetic. A bolt too close to the end of a ledger board splits it out under load, and a bolt too close to the end of the rim board behind the wall has nothing to bear against.
The load figure and what it is not
| Quantity | Where it comes from | What it tells you |
|---|---|---|
| Joist span | Projection minus cantilever | The clear distance the joists actually bridge |
| Tributary depth | Half the joist span | The ledger takes one end, the beam takes the other |
| Ledger load | Tributary area times design load | Total force trying to pull the deck off the wall |
| Demand per fastener | Ledger load divided by count | An input to a capacity check, not the check itself |
Note what the cantilever does. Joists that run past the beam load the beam more and the ledger less, because the tributary split happens between the two supports and the overhang hangs off one of them. A deck with a long cantilever puts less on the ledger and more on the posts, which changes the footing sizing on the other side of the deck. That is the same arithmetic as the deck footing load calculator, seen from the other end.
The demand figure assumes the load spreads evenly across every fastener, which real connections do not do perfectly. It also ignores lateral load entirely, which is a separate problem with a separate connector, and it ignores what happens when a section of rim board has softened. Treat it as a sanity check on the order of magnitude, and take the actual sizing from the code tables or from an engineer.
Flashing is not a detail, it is the reason the ledger lasts
Water arriving at a wall runs down the drainage plane behind the siding. Bolt a wide board flat against that plane and you have built a shelf across the path. Without flashing, water sits in the joint between the ledger and the wall, wicks into the end grain of both, and works on the rim board inside the wall where nobody looks. By the time there is a visible symptom the fasteners are bearing in soft wood.
Flashing has to shed water over the top of the ledger and out, and the wall drainage plane has to lap over the top of the flashing, not under it. Getting that lap order backwards funnels water into the wall more efficiently than no flashing at all. The ends have to be closed and returned rather than cut off square, which is why the calculator adds a run past each end and lists two end pieces. The same lapping logic applies to any wall penetration, and the window and door flashing calculator covers the general case if the wall detail is unfamiliar.
Some assemblies also want the ledger held off the wall on spacers so water can drain and the back of the board can dry. Whether that is appropriate depends on the wall, the cladding and the adopted code. It is a decision for the person doing the wall detail, not a default.
This is inspected work
Decks generally require a permit, and the ledger connection is normally inspected before the framing is covered. That inspection exists because the connection is invisible afterwards and because the failure mode is sudden. If a deck already exists and you cannot see how the ledger is attached or whether it was flashed, that is a question for a professional inspection rather than a calculation. An existing deck with lag screws into sheathing and no flashing is not a maintenance item.
Once the ledger detail is settled by somebody qualified, the rest of the frame is arithmetic: the deck framing calculator for joists, rim, beam and hardware, the header and beam load calculator for the tributary figures a span table needs, and the deck board calculator for the surface once the frame is right.
Questions people ask
What fastener spacing should I use on a deck ledger?
This page will not tell you, and you should be suspicious of any page that does. Ledger fastener spacing depends on the fastener type and diameter, the ledger thickness, what is behind the sheathing, the joist span, the design load and the code edition your jurisdiction has adopted. The published tables are specific to a whole assembly, and pulling one number out of one of them and applying it to a different assembly is exactly how ledgers fail. Get the detail from the building department, the fastener manufacturer table that matches your assembly, or an engineer, then put that spacing into the field here.
Can I attach a ledger with lag screws through the siding?
Structural questions about existing walls are not answerable from a web page, but the general shape of the problem is worth knowing. A fastener has to reach solid framing and bear against it, and siding, sheathing and any gap between the ledger and the framing all reduce what the fastener is actually gripping. A ledger standing off the wall on a compressible layer is a different connection from one bearing tight against a rim board, and the published capacities assume the latter. This is one of the recurring findings in collapse investigations. If you cannot see and verify what the fasteners land in, do not guess.
Why does the calculator ask for the cantilever?
Because it changes what the ledger carries. Joists supported at the ledger and at a beam split their load between the two, half each. If the joists run past the beam, that overhang loads the beam and slightly relieves the ledger, so the tributary depth at the ledger is half the clear span between supports rather than half the whole projection. On a 12 foot deck with a 2 foot cantilever the ledger carries a 5 foot strip, not a 6 foot one. The difference is around 17 percent, which matters when you are looking at the demand per fastener.
What is the flashing developed width?
The total width of the metal measured across the flat before it is bent, so a piece that goes six inches up the wall, across the two inch ledger top and down an inch over the face has a developed width of nine inches plus whatever hem you fold. Enter the flat dimension because that is what the metal is sold and cut by. The calculator gives the lineal feet including the run past each end and the area if you supplied a width, which is what you need for ordering coil stock or for a bending charge.
Is a freestanding deck simpler than a ledger?
It removes the ledger question and replaces it with more footings and a lateral stability question. A deck not attached to the building needs its own resistance to racking and its own posts and footings along what would have been the ledger line, roughly doubling the footing count on that side. Whether it is simpler depends on your ground, your frost depth and how far the deck is off the ground. It is a real option and it is worth asking about, particularly on a wall where the ledger connection is awkward, but it is a design decision for the building department conversation rather than a shortcut around one.