Deck Ledger Board Calculator
Size the ledger lumber given joist span + tributary load + ledger length + house framing + snow load. Output: smallest IRC-compliant ledger (2×8 PT through LVL 2×12) + total load + load per LF + Z-flashing reminder. Auto-flags engineered I-joist + log home + unknown framing as 'needs engineering'. IRC R507.9 + AWC DCA-6 compliant. Pair with Ledger Bolt + Lateral Load Calculators for complete attachment spec.
Inputs
IRC compliance + advisories
2×10 PT pine recommended
IRC R507.9 + AWC DCA-6Load per LF = 390 lb. Lumber capacity meets demand with margin. Buy 20 ft @ $3.4/lf = $68.
Z-flashing + drip cap REQUIRED
IRC R703 + R507.9.1.4Per IRC R703 + R507.9.1.4 — water trapped between ledger and siding causes the #1 cause of deck collapses. Flashing kit $25-50 — non-negotiable.
IRC R507.9 + AWC DCA-6. Pair with Ledger Bolt + Lateral Load + Joist Span Calculators for complete attachment spec. Z-flashing always required.
How to use
How to use the ledger board calculator in 5 steps.
- 1
Enter joist span + tributary load
Joist span = clear distance joist will cover (typically 8-16 ft). Tributary load = IRC default 50 PSF (40 live + 10 dead). Add 50% of snow PSF for cold zones.
- 2
Enter ledger length
Full length of the ledger along the house (typically equals deck width).
- 3
Pick house framing type
2×10 rim (most homes 2000+). 2×12 rim (older or larger homes). Engineered I-joist (modern truss systems — NEEDS ENGINEERING). Log home (special connector). Stucco-veneer (needs depth check). Unknown (default to engineering).
- 4
Enter snow load (region-specific)
0-15 PSF Gulf/South. 25-40 PSF Mid-Atlantic. 50-70 PSF Upper Midwest/Northeast. Snow Load Calculator gives state-specific values.
- 5
Read recommended lumber + flags
Calculator picks the smallest IRC-compliant ledger. Engineered I-joist or unknown framing always triggers 'needs engineering' — book a $400-800 SE letter before installing.
How we calculate
How DeckMath calculates this — IRC 2021 sources.
The Ledger Board Calculator sizes the ledger lumber given joist span + tributary load + ledger length + house framing type + snow zone. Output: smallest ledger lumber that handles the load (2×8 PT through LVL 2×12), total load + load per LF, flashing strategy reminder, and house-framing compatibility check. Auto-flags engineered I-joist + log home + unknown framing types as 'needs engineering' (not prescriptive). IRC R507.9 compliant. Pair with the Ledger Bolt Calculator to spec the fasteners.
IRC references
- IRC R507.9 — Ledger attachment + flashing requirements
- AWC DCA-6 deck guide — ledger sizing tables
- IRC R703 + R507.9.1.4 — Z-flashing + drip cap required
Verify against the published source: 2021 International Residential Code (ICC).
IRC 2021 R507.9 + AWC DCA-6 deck guide. Lumber capacity tables from manufacturer span charts (PT pine NDS 2018, LVL via Boise Cascade + Weyerhaeuser specs).
Half the joist span is tributary to the ledger (other half goes to the beam). 12 ft span × 20 ft ledger × 50 PSF = 6,000 lb total ledger load. Snow contributes 0.5× as conservative reduction.
6,000 lb / 20 ft = 300 lb/LF. Calc compares this to lumber capacity table to pick recommended size.
2×8 PT = 250 lb/LF. 2×10 PT = 400 lb/LF. 2×12 PT = 600 lb/LF. LVL 2×10 = 800. LVL 2×12 = 1100. 300 lb/LF → 2×10 PT recommended.
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Ledger board questions, answered.
Depends on joist span + tributary load + snow. For a 12 ft joist span with 50 PSF load + 30 PSF snow on a 20 ft ledger: load per LF = 360 lb → 2×10 PT recommended. Always step up one size if framing is older or uncertain. The calculator picks the smallest IRC-compliant size automatically.
Only for short spans (joist span ≤ 8 ft) + low loads (no snow + small deck). 2×8 PT capacity is ~250 lb/LF — handles short residential decks but flags as undersized for typical decks. Most builders default to 2×10 PT minimum for any house-attached deck for safety margin.
2×10 PT = $3.40/LF, handles 400 lb/LF. LVL 2×10 (laminated veneer lumber) = $12.50/LF, handles 800 lb/LF — 2× the capacity. Use LVL for: (1) very long spans, (2) heavy point loads (hot tub above ledger), (3) any deck where the calculator flags 'needs engineering'. The premium pays off in safety margin + warranty.
Yes — IRC R703 + R507.9.1.4 mandate Z-flashing and drip cap on ALL house-attached deck ledgers. No exceptions. Water trapped between ledger and house siding is the #1 cause of catastrophic deck failures (ledger rot → bolt pull-out → deck collapses). Flashing kit $25-50 — cheap insurance.
NOT without engineering. IRC explicitly excludes I-joists from prescriptive ledger attachment — they're not solid lumber, so lag bolts can split the web. Solutions: (1) hire a SE for stamped connection detail ($400-800), (2) build freestanding (no ledger), (3) use Simpson DTT2Z hold-downs through-bolted to specific I-joist hardware. The calculator flags this combo automatically.
Special connector required (Simpson LSC or similar) — logs shrink and settle over decades, and a rigid ledger can't accommodate this movement. Always engineering-stamped. Log home builders typically prefer freestanding decks for this exact reason. Calculator flags log-home framing.
There's no hard IRC limit — you size up the lumber until capacity meets demand. A 30 ft ledger × 16 ft joist span × 50 PSF + 40 PSF snow = 720 lb/LF → LVL 2×10 territory ($300+ in lumber for the ledger alone). Most contractors break very long decks into multiple shorter ledgers with separate engineering details + freestanding column supports.
Yes — heavy snow zones (50+ PSF) can push you from 2×10 to 2×12 or LVL. The calculator adds 0.5× snow PSF to the live load (conservative — snow + occupants don't typically peak together). 30 PSF snow zone is the breakpoint — below that, snow rarely changes the recommendation. Use the Snow Load Calculator for your state-specific value.
Both required: lag bolts (typically 1/2″ × 6″ or longer) PER IRC R507.9.1.3 + Simpson hold-downs (DTT2Z minimum) PER IRC R507.9.2 for lateral. The bolts handle vertical load; the hold-downs prevent the deck from pulling AWAY from the house. The Ledger Bolt Calculator computes bolt spacing + the Lateral Load Calculator computes hold-down count.
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