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The total shear load the ledger connection must carry.
The ledger carries HALF the deck, not all of it — the outer beam takes the rest. So this is the tributary load: half the joist span, across the ledger's length, at the design load for the deck. Getting that halving wrong in the safe direction wastes fasteners; getting it wrong the other way is the connection that fails, and deck ledger failures are the ones that injure people.
The rated shear capacity of a single lag screw, from the applicable NDS/IRC table or manufacturer data.
From the row that matches what is actually being built: the species of the band joist the screw bites into, whether sheathing sits between ledger and rim, and whether there is a drainage spacer behind the ledger. The spacer matters most — it is increasingly common because it lets the wall drain, and it changes the fastener's capacity substantially rather than marginally.
Lag screws needed
9 screws
Use the actual IRC/NDS-table or manufacturer rated capacity for your specific screw diameter, species, and ledger/rim board thickness combination — this calculator only divides your total load by whatever per-fastener value you enter.
They open the calculator with your figures already in it
Ledger Lag Screw Shear Count Calculator: 9 screws — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Deck ledger lag screw shear capacity is governed by NDS/IRC fastener tables specific to screw diameter, species, and ledger/rim board thickness — this calculator divides your total required shear load by your own fastener's rated capacity from that table
Inputs used
- Total Shear Load
- 3.37 kip
- Per-Fastener Rated Capacity
- 0.4 kip
Confidence note: Use the actual IRC/NDS-table or manufacturer rated capacity for your specific screw diameter, species, and ledger/rim board thickness combination — this calculator only divides your total load by whatever per-fastener value you enter.
What this calculation does not cover
- The total shear load is the input this page needs most and explains least. It is the deck area landing on the ledger — half the joist span times the ledger length — times the design floor load, which on a residential deck is 40 psf (1.9 kPa) live over 10 psf (0.5 kPa) dead before any snow, planter or hot tub is added. Enter it low and the screw count comes out proportionally low, and nothing downstream catches the error.
- Screws in shear hold the deck UP; they do nothing to hold it against the house. Code now requires a separate lateral load connection — tension devices anchored back into the floor framing, rated in the region of 1,500 lb (680 kg) at two locations — because a ledger fastening resists a deck pulling away from the wall very poorly, and that separation is what actually drops decks off houses. No number of lag screws substitutes for those devices.
- A tabulated per-screw capacity assumes the screw lands in sound solid rim of the thickness the table names. Through brick veneer, over a cantilevered floor, into hollow block or stacked masonry, or across any sheathing gap the ledger does not bear tight on, the tabulated figure does not apply at any count — and several of those attachments are prohibited outright rather than merely derated. A rim that has been wet behind unflashed ledger for a few seasons holds a fraction of the table value at every screw at once.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Deck ledger lag screw shear capacity is governed by NDS/IRC fastener tables specific to screw diameter, species, and ledger/rim board thickness — this calculator divides your total required shear load by your own fastener's rated capacity from that table
Which documents these citations point at
- International Residential Code (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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