What size ledger board do I need for my deck?

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.

Can I use 2×8 for a deck ledger?

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.

What's the difference between 2×10 and LVL ledger?

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.

Do all deck ledgers need flashing?

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.

Can I attach a ledger to engineered I-joists?

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.

What about ledger attachment to a log home?

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.

How long can a ledger board span be?

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.

Does snow load change my ledger size?

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.

Should I bolt the ledger or use Simpson hardware?

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.

Material · IRC R507.9 ledger sizing

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.

IRC R507.9AWC DCA-65 lumber options

5·Lumber options

IRC·R507.9 compliant

Z-flash·Always required

Auto·Engineer flag

Inputs

Joist spanft

Ledger lengthft

Load (psf)40+10 IRC

psf

Snow (psf)region snow

psf

House framing

Ledger · 2×10 PT pine

2×10390 lb/LF · $68 total

Total load

20 ft × 12 ft span

Load/LF

capacity-driving metric

Recommended

2×10

smallest compliant

Cost

@ $3.4/lf

IRC compliance + advisories

2×10 PT pine recommended

IRC R507.9 + AWC DCA-6

Load per LF = 390 lb. Lumber capacity meets demand with margin. Buy 20 ft @ $3.4/lf = $68.

Z-flashing + drip cap REQUIRED

IRC R703 + R507.9.1.4

Per 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 deck calculator in 3 steps.

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.

Enter ledger length

Full length of the ledger along the house (typically equals deck width).

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).

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.

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 the result — 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

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).

Total load on ledger

load = (joist_span / 2) × ledger_length × (live + dead + snow × 0.5)

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.

Load per linear foot

load_per_lf = total_load / ledger_length

6,000 lb / 20 ft = 300 lb/LF. Calc compares this to lumber capacity table to pick recommended size.

Lumber capacity sizing

smallest lumber where capacity >= load_per_lf

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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Deck calculator questions, answered.

Ledger Bolt Calculator

Ledger bolt count + spacing per IRC R507.9.1.3. Lag screw / through-bolt / SDS structural screw options vs deck tributary load. Edge + end distance + spacing compliance.

Joist Span Calculator

Maximum joist span for any size, species, and spacing — directly from IRC 2021 R507.6 tables.

Lateral Load Calculator

IRC R507.9.2 lateral connection design. Simpson DTT2Z hold-down count + spacing. Earthquake + wind lateral capacity check. Inspection-grade output.