Lag Screw vs. Bolt

A lag screw relies on its threads in the band joist, so the code allows less capacity per fastener than a through-bolt. A 1/2 in. lag screw is not interchangeable with a 1/2 in. through-bolt.

Pilot Holes

The IRC calls for predrilling with two bits: a 1/2 in. hole through the ledger and any sheathing, and a 5/16 in. to 3/8 in. hole through the band joist. Lag screws are turned in with a wrench or driver, not hammered.

Thread Penetration

The tip of the lag screw must fully extend beyond the inside face of the band joist, and lag screws must be full-body diameter screws. The AWC notes that a visible tip confirms the screw is the right length.

Spacing Schedule

The first column of the main chart gives the lag-screw schedule. A lag screw at a 12 ft span needs to be 15 in. on center; a through-bolt in the same condition can be 29 in. Lag sizes are in the Lag Bolt Sizes Chart.

The spacing table assumes a solid-sawn band joist. Engineered rim joists change the numbers, as AWC’s Table 5 shows.

Solid-Sawn Lumber

A 1-1/2 in. solid-sawn band joist gives the widest spacing: 30 in. to 10 in. for lag screws and 36 in. to 19 in. for bolts.

Source: AWC DCA 6 (2015), Table 5. The through-bolt rows for LVL rim joists are not shown because they are not verified here.

LVL Rim Joists

A 1 in. LVL rim joist brings the 12 ft lag spacing down to 12 in. and a 1-1/8 in. LVL to 14 in., against 15 in. in solid lumber. Thinner rim members hold lag screws less well.

Engineered Rim Boards

The IRC says engineered rim joists follow the manufacturer’s recommendations, and it lets rim boards of at least 1 in. nominal serve as the band joist. Use the evaluated connection for the exact product.

The ledger is a minimum nominal 2×8 of No. 2 grade or better, pressure-preservative-treated Southern pine, incised pressure-treated hem-fir, or decay-resistant wood.

2×8 Ledger

A 2×8 is 1-1/2 in. by 7-1/4 in. The staggered layout needs at least 2 in. at the top, 1-5/8 in. between rows and 3/4 in. at the bottom, a total of 4-3/8 in., so a 2×8 has room.

2×10 and 2×12 Ledgers

Deeper ledgers give more room for the rows but do not change the table spacing. Actual sizes come from the Lumber Size Chart.

Minimum Prescriptive Requirements

A ledger may not carry concentrated loads from beams or girders, and it may not be supported on stone or masonry veneer.

Two-Row Staggered Layout

The IRC requires lag screws or bolts to be staggered from the top to the bottom along the horizontal run of the ledger, following its placement figure. Alternating between an upper and a lower row spreads the load and avoids splitting the ledger along a single grain line.

Alternating Fasteners

Consecutive fasteners alternate rows, so no two fasteners sit one above the other.

Horizontal Spacing

The on-center spacing in the chart is the maximum along the ledger. It does not change with the row you choose.

Vertical Row Spacing

Rows must be at least 1-5/8 in. apart. The placement figures, not the simplified table, set the upper and lower row locations. Do not invent fastener locations.

Top and Bottom Edge Clearances

The ledger and the band joist have different edge minimums because they sit at different heights.

Source: IRC Table R507.9.1.3(2) and Figures R507.9.1.3(1) and (2), 2024 IRC; same values in the 2018 and 2021 editions.

Ledger End Distances

Fasteners must sit at least 2 in. from the ends, and the table footnote caps that distance at 5 in.

House Band-Joist Clearances

The band joist needs 3/4 in. at the top and 2 in. at the bottom. For engineered rim joists, the manufacturer’s recommendations govern.

1/2-Inch Sheathing

The lag screw column and the first bolt column assume sheathing up to 1/2 in. thick. The sheathing must be wood structural panel or solid-sawn lumber.

1-Inch Sheathing

The third column covers up to 1 in. of sheathing, which can be wood structural panel, gypsum board, fiberboard, lumber or foam sheathing. It carries closer spacing.

Stacked Washer Conditions

Up to 1/2 in. of stacked washers can replace up to 1/2 in. of allowable sheathing thickness where combined with wood structural panel or lumber sheathing. The AWC notes stacked washers are not permitted with gypsum board or foam sheathing.

Prohibited Substitutions

The maximum distance between the ledger face and the band joist face is 1 in. Do not treat thick siding or cladding as structural sheathing.

1/2-Inch Fasteners

The prescriptive tables are written for 1/2 in. bolts and lag screws. Fastener dimensions are in the Lag Bolt Sizes Chart.

Hot-Dip Galvanized Hardware

Fasteners and connectors in contact with preservative-treated wood must be hot-dipped zinc-coated galvanized steel or another corrosion-resistant material under the code’s fastener provisions (Section R507.2.3 for deck fasteners).

Stainless Steel

Stainless steel is used for higher corrosion exposure. Do not mix galvanized and stainless components in contact.

Treated-Wood Compatibility

Use the approved ledger hardware. A general Bolt Grade Chart describes bolt material but does not make a bolt an approved ledger fastener.

Ledger Length and Maximum Spacing

Start with the ledger length and the maximum spacing from the chart for your span and fastener.

End Offsets

Keep each end offset between 2 in. and 5 in. The examples use 3 in.

Worked Example

Counts for the Same Ledger

Deck Area Loads

Design loads come from the code and the deck use. See the Deck Load Chart for live, dead and snow load values.

Joist Tributary Load

Ledger Reaction

Why Longer Joists Need Closer Spacing

The line load rises in direct proportion to the joist span, which is why the spacing tightens.

Do not convert these loads into a fastener spacing with an invented bolt capacity. The IRC table already does that for the stated details.

Identifying the Actual Joist Span

Use the span of the deck joists, from the ledger to the outer beam. Deck depth and ledger length are not the same thing, and neither is the joist’s overall length if it cantilevers.

Selecting the Correct Table Column

Choose the span row, then the fastener and sheathing column that match your connection. For 12 ft joists use the ‘over 10 to 12 ft’ row: 15, 29 or 24 in. For 16 ft joists use the ‘over 14 to 16 ft’ row: 11, 21 or 18 in.

Comparing Bolt and Lag Spacing

Through-bolts with 1/2 in. sheathing allow about twice the lag spacing at 12 ft (29 in. versus 15 in.) and about 1.9 times at 16 ft (21 in. versus 11 in.).

The wood-band-joist table does not apply to concrete or masonry.

Concrete Foundation Walls

A ledger on a poured concrete foundation needs approved anchors and an anchorage design. Spacing and edge distance come from the anchor’s evaluation, as in the Concrete Anchor Spacing Chart.

Solid Masonry

Solid masonry also needs engineered anchorage. Hollow masonry requires a specifically approved solution.

Approved Expansion and Adhesive Anchors

Use anchors approved for the base material. General anchor sizes are in the Anchor Bolt Size Chart.

Anchor Embedment and Edge Distance

Embedment, edge distance and spacing depend on the exact product. The IRC allows alternate ledger details that meet its load requirements (R507.9.1.4).

Brick and Stone Veneer

The IRC says a deck ledger may not be supported on stone or masonry veneer. Veneer is not a structural support.

Hollow Masonry

Hollow masonry needs an approved engineered connection.

Cantilevered Floor Overhangs

The band joist must bear fully on the primary structure. A floor that cantilevers past the wall, or a bay window, does not qualify, and AWC DCA 6 prohibits the prescriptive ledger connection there.

Freestanding Deck Alternative

Where the house cannot support a ledger, a freestanding deck with its own posts and beams avoids the problem.

Vertical vs. Lateral Resistance

Ledger bolts resist vertical load. A deck also needs a lateral load path so it does not pull away from or slide along the house. The ledger spacing chart does not cover that.

1,500-Pound Hold-Down Option

Where the IRC’s first lateral detail is used, hold-down tension devices go in at least two locations per deck, within 24 in. of each end of the deck, and each device must have an allowable stress design capacity of at least 1,500 lb.

750-Pound Hold-Down Option

Where the second detail is used, hold-downs go in at least four locations per deck and each must be rated for at least 750 lb.

Manufacturer-Approved Details

These alternatives have placement and installation requirements of their own, and engineered floor systems such as I-joists need the floor manufacturer’s lateral-connection detail. Use hold-downs approved for the purpose.

Continuous Flashing

Where a ledger is attached to wood-frame construction, flashing goes above the ledger. It must extend at least 2 in. above the ledger vertically and at least 4 in. horizontally beyond the ledger face, or reach the ledger face and turn down at least 1/4 in. A window or door within 2 in. above the ledger means the flashing runs to the bottom of that opening.

Water Intrusion

The water-resistive barrier is lapped over the vertical flashing leg at least 2 in. Existing walls without a barrier need one installed behind the ledger and flashing.

Ledger Deterioration

Flashing is not required where the ledger stands off the wall covering at least 1/4 in. for drainage and ventilation. Water that gets behind a ledger can rot the band joist and defeat a correctly spaced connection.

Inspection

Check the flashing and the wood behind the ledger whenever you inspect a deck.

Confirm House Framing

1. Confirm the house framing and the band joist type.

Mark Bolt Locations and Drill

2. Mark bolt locations on the ledger in a staggered layout.
3. Drill the correct holes: clearance holes for bolts, or two-bit predrilling for lag screws.

Install Bolts or Lag Screws and Washers

4. Install the bolts or lag screws with washers.
5. Tighten the hardware.

Inspect and Tighten

6. Inspect the connection and the flashing.

Turn lag screws in; do not hammer them. Washers go under heads and nuts. Do not use a universal tightening torque; follow the governing details. Joist-to-ledger connections use approved hangers; see the Joist Hanger Size Chart.

Proprietary Structural Screws

Structural ledger screws are tested products with their own geometry, hole requirements and spacing tables. One manufacturer’s ledger screw guide, for instance, specifies a 3-3/4 in. minimum end distance and a 1-3/4 in. minimum edge distance in a V-shaped stagger, which differ from the IRC bolt values.

Manufacturer Spacing Tables

Each product publishes spacing by joist span and rim material. Use only that table, not the bolt chart.

Approved Substitution

Substitution is allowed only where the product’s evaluation report or the code approves it for the exact ledger and band joist.

Evaluation Reports

Snow Loads

The main chart uses a 40 psf live load. The IRC table also has rows for 50, 60 and 70 psf ground snow loads, which tighten the spacing as the table below shows.

Source: IRC Table R507.9.1.3(1), 2024 IRC. See the table for the other joist spans.

Hot Tubs and Concentrated Loads

The table does not cover hot tubs, planters, heavy furniture or other concentrated loads. They need engineering. Load values are in the Deck Load Chart.

Roofed Decks

A roof over a deck adds roof and snow loads that the table does not include.

Engineered Connection Requirements

Ordinary 40 psf live load schedules can be insufficient in these conditions. Use an engineered connection.

Missing or Undersized Fasteners

Count the fasteners and compare them with the chart, but also check the type, diameter, washers and penetration. For lag screws, the tip should show beyond the inside face of the band joist.

Wood Rot

Probe the ledger and band joist, especially at the top edge behind flashing. A correctly spaced connection in rotted wood is not safe.

Corrosion

Look for corroded or missing hardware, and for galvanized or stainless fasteners that do not match their connectors.

Reinforcement Options

Adding bolts does not repair a deteriorated ledger connection. Replace damaged wood, restore flashing and add lateral connections, or build a freestanding deck.