The most common cause of deck collapse is when a ledger pulls away from the band joists of homes and buildings.
The two most common ways to correctly attach a ledger to a structure are with lag screws or through-bolts. The installation of through-bolts requires access to the back-side of the rim joist which, in some cases, is not possible without significant removal of drywall within the structure. Most building codes state that, where positive connections to the primary building structure cannot be verified during inspection, decks shall be self-supporting (free-standing). Determining the exact required spacing for the ledger fasteners is based on many factors, including:
joist length;
type of fastener;
diameter of fastener;
sheathing thickness;
use of stacked washers;
type of wood species;
moisture content;
band joist integrity; and
deck loads...
...and so is beyond the scope of a visual inspection.
Joist Connection to Beam or Ledger
Deck joist framing into the side of a beam or ledger board (where the ends of the deck joists connect to the beam or ledger board) must be supported by approved joist hangers. (2022 IRC 507.6.1).
Code for On-Center Spacing
Fasteners used in deck ledger connections should be installed in accordance with a building standard that describes the deck ledger connection to the band joist. Refer to 2021 IRC Table R507.9.1.3(1) below.
For example, assume a deck is built with a 40 psf live load design, the joists are spanning 10 feet, and 1/2-inch diameter lag screws are used. The on-center of spacing of fasteners is 18 inches. The tip of the lag screw should fully extend through and beyond the inside surface of the band joist.
The on-center spacing of ledger fasteners required is primarily determined by the span of the joists.
InterNACHI's ledger fastener spacing formula provides inspectors with this general rule of thumb:
On-center ledger spacing of 1/2-inch lag screws in inches = 190 / joist span in feet.
On-center ledger spacing 1/2-inch bolts in inches = 330 / joist span in feet.
But this is a very general rule of thumb that may differ considerably from modern building standards, best practices, and residential codes.
For example using 2021 IRC Table R507.9.1.3(1), if the deck is built with a 40 psf live load design, the joist span is 12 feet, and the fasteners are 1/2-inch diameter bolt with 1/2-inch maximum sheathing, the on-center spacing of fasteners at the deck ledger board connection to the house band joist is 29 inches (864 mm). If the fasteners are 1/2-inch lag screws with 1/2-inch maximum sheathing, the on-center spacing is 15 inches (381 mm).
Placement of Fasteners at Ledgers
Lag screws or bolts must be installed through the ledger board in a staggered pattern, staggered from the top to the bottom along the horizontal run of the deck ledger.
The fasteners at the deck ledger board should be staggered in two rows spaced 1 5/8 inches to 5 inches apart from each other.
The maximum distance from the bottom row of lag screws or bolts at the deck ledger board to the top edge of the ledger is at least 5.5" for 2x8s, at least 6.5" for 2x10s, and at least 7.5" for 2x12s.
The placement of the lag screws or bolts at the deck ledger board should be 2 inches from the top edge, 3/4 inches from the bottom edge, and 2 to 5 inches from the ends.
Through-bolts should be a minimum of 1/2-inch in diameter and have washers at the bolt head and nut. Lag screws should also be a minimum of 1/2-inch in diameter and have washers. Expansion and adhesive anchors should also have washers.
Ledger Boards
Deck ledgers should be made of at least 2x8 pressure-treated wood. Joist framing into the side of a beam or ledger board must be supported by approved joist hangers.
Band Joists
Band joists supporting a ledger must be a minimum 2-inch-nominal (51 mm), solid-sawn, spruce-pine-fir or better lumber, or a minimum 1-inch (25 mm) nominal engineered wood rim boards. Refer to 2021 IRC R502.1.7.
The tip of the lag screw must fully extend beyond the inside face of the band joist.
There are several ways a joist may be observed attached to a ledger.
The first is by resting the joist on a ledger strip. The illustration above depicts a floor joist properly resting on a 2x2-inch ledger strip. (Refer to 2021 IRC R502.6.2). In 2021 IRC R507.6.1, it says that deck joists that connect to the side of a deck beam or ledger board must be supported by approved joist hangers.
The second is by notching over a ledger strip. The illustration above depicts a notched joist resting a 2x2-inch ledger strip.
The third is by hanging the joists with joist hangers. The illustration above depicts joists properly attached to a ledger using metal joist hangers.
The illustration above depicts a joist cut too short. Joists may rest on 2x2-inch ledgers like the one above (or in joist hangers), but joists must be cut long enough to reach the ledger or band joist that is supporting them.
The illustration above depicts joists that are not fully resting in their joist hangers. Joists should be fully resting in their joist hangers. Ledger Board and Band Joist Contact
The image above depicts a ledger board and band joist sandwiching the structural sheathing.
Sheathing is permitted to be wood structural panel, gypsum board, fiberboard, lumber or foam sheathing.
Open Space
2021 IRC Table 507.9.1.3(1), for ledger connections, allows up to 1 inch of open space between the house band and the ledger. This provides for wall sheathing and/or drainage and ventilation. It is not intended for cladding.
Up to 1/2-inch thickness of stacked washers are permitted to substitute for up to 1/2 inch of allowable sheathing thickness when combined with wood structural panel or lumber sheathing.
Removing the cladding and reinstalling it properly around the ledger depends on what type of exterior wall covering is installed. Vinyl siding and adhered stone veneer are different from each other.
Code specifies a clearance below all these products or references the manufacturer's installation instructions. However, none of the clearances reference “decks” specifically. Throughout the IRC and manufacturer instructions, minimum clearances are required between exterior wall covering (cladding) and the ground, exposed earth, paved surfaces, and finished ground level, “grade,” but rarely does anything ever refer directly to “decks.”
Bolts All through-bolts should have washers at the bolt head and nut.
The image above depicts a hold-down tension device. Code requires hold-down tension devices at no fewer than two locations per deck.
Codes in some areas outright forbid attaching a ledger board to an open-web floor truss.
The image above depicts a ledger board attached to a concrete wall. Caulking rather than flashing is used.
The image above depicts a ledger board attached to hollow masonry. When the ledger is attached to a hollow masonry wall, the cell should be grouted.
The image above depicts a ledger board improperly supported by brick veneer. Ledger boards should not be supported by manufactured stone or brick veneer. Deck ledgers are not permitted to be supported on stone or masonry veneer.
Ledger boards should not be attached directly (surface-mounted) to stucco or EIFS. Stucco and EIFS have to be cut back so that ledger boards can be attached directly to band joists; however, cut-back stucco and EIFS are difficult to flash and weatherproof.
Ledger Board Flashing
Ledger flashing is intended to divert moisture away from the wooden components of the house. A deck ledger connection is only as strong as the wood it is attached to, and rotted wood is not very strong at all.
Ledger flashing is only required for decks attached to wood-frame construction. Since the ledger board is required to be decay resistant, the flashing is primarily to protect the house.
Ledger boards attached to concrete foundations do not require this type of flashing, and a durable sealant is recommended in these applications.
The illustration above depicts flashing installed over and under the ledger board. The ledger board should be flashed even if the home has a protective roof overhang.
Aluminum flashing is commonly available but should not be used. Contact with pressure-treated wood or galvanized fasteners can lead to rapid corrosion of the aluminum.
Information used with permission of nachi.org