The structure that supports a deck looks pretty simple at a glance. But building a safe and durable deck requires good planning, careful material choices, and attention to detail —especially structural details that are sometimes overlooked by both owner-builders and contractors. Modern building codes pay a lot of attention to critical details such as ledgers and railings, but codes are not always consistently enforced. So it’s best to familiar yourself with the recommended materials and techniques.
Most residential decks are supported on one side by a ledger that is bolted or lagged to the home’s band joist. This connection is critical, since a failure here can cause a deck to collapse.
Failure of the ledger can be caused by too few or undersized fasteners, or by decay in the ledger or band joist. Lags or bolts provide little support when fastened to rotted wood. So proper flashing of the ledger and band joist area is critical. It is also important that the band joist be nailed adequately to the surrounding structure, since the ledger is only as strong as the structural member it is attached to.
A deck depends on a number of key structural connections, starting with the deck-t0-house connection at the ledger. The component you are fastening to must be strong enough to hold the ledger. A rotted band joist won’t work, and few existing ones are pressure-treated. In some cases the band joist was not properly nailed when installed and will need additional nailing. Also, take special precautions when fastening to engineered lumber. LVL at least 1 in. thick is OK, but I-joists need special connection details approved by the I-joist manufacturer.
Band Joist-to-House Connection. In new construction, if a deck is planned, make sure the band joist is pressure treated and adequately nailed to the sole plate above and the sill or top plate below, using stainless-steel or double-hot-dipped galvanized nails. Fastening with 16d common nails at 8 inches on-center is recommended. If the nailing cannot be confirmed in a retrofit, extra toenails driven through the exterior can help to reinforce this connection.
Ledger-to-Band Joist Connection. After a number of widely reported catastrophic deck collapses across the US, engineers at Virginia Tech conducted extensive studies to determine safe fastening details. Their results, originally reported in The Journal of Light Construction, are now incorporated into the IRC and shown in the diagrams and tables below.
Through-bolts are the most reliable connection, but lag bolts are adequate as long as they are long enough to fully penetrate the band joist. For through-bolts, drill holes 1⁄16 inch larger than the bolt. For lags, drill a full-diameter hole for the unthreaded portion and a smaller hole (65 to 75% of the lag’s diameter) for the threaded portion. Use washers under the head of the lag bolt or at both ends of through-bolts to keep the head from crushing the wood. Soak the holes with a preservative before inserting the bolts. Spacing for bolts and lags are shown in the Table below:
Spaced Ledgers. Some deck builders prefer to leave a ½” gap between the ledger and band joist to permit drainage and help with drying if the area gets wet. It’s a nice detail, approved by the IRC, but not used too often. I would strongly consider it in very wet or humid climates or coastal areas where wood decay is more common. I would also strongly consider it in retrofits where the band joist is not pressure-treated and is difficult to inspect.. To make this connection work, through-bolts should be used, and placed a little closer together, as shown in the Table below.
The IRC has established rules for where to place fasteners in the ledger. These are designed to maximize the strength of the connection and to prevent splitting. Lag screws or bolts should be staggered as shown, and held back two inches from top and side edges as shown in the illustration below:
Proper flashing of the deck ledger is critical since leakage around the ledger could lead to decay in the band joist and collapse of the deck. The siding should be removed over the band-joist area, and a wide band of peel-and-stick membrane or non-corrosive metal flashing should run over the band joist and up under the building felt or house wrap. A second cap flashing should direct water over the ledger and away from the house. Details for a standard and spaced ledger are shown below.
Flashing Leaks. Deck collapses are uncommon, but do occur. In one study by the Consumer Products Safety Commission (CPSC), nearly 700 people per year were injured in deck collapses. Ledger failures are often at fault in collapses.
To ensure a safe ledger connection, it is critical to properly flash the ledger area and to only use membrane and metal components that are compatible with today’s highly corrosive pressure-treated wood. The best lag screws and bolts in the world won’t help if the band joist they attach to has rotted due to water intrusion, as in the deck shown here. Periodic inspection of the ledger, band joist, and other critical deck connections is always a good idea.
Caution: Do not use aluminum flashings with pressure-treated lumber unless a durable barrier material, such as a bituminous membrane, separates the aluminum from contact with the wood. Preferably use membrane-type flashings, G185 galvanized steel, or copper. However, copper flashings should not contact galvanized hardware or fasteners.
Deck ledgers are engineered to resist “shear” forces, essentially keeping the deck from falling vertically. However, decks are also supposed to be braced to resist lateral forces trying to pull the deck away from the house. Code requirements on lateral bracing are not yet enforced in many areas. However, with large decks, high decks, or in areas with high winds or seismic activity, tying the deck to the house laterally is certainly a good idea.
The 2012 IRC shows one solution — where the deck joists run parallel to the house joists. The code requires two hold-down braces per deck, such as the Simpson Strong-Tie DTT2Z or USP DTB-TZ, each rated at the required 1,500 lbs.
Where the deck joists run perpendicular to the house joists, one solution is to nail solid blocking between the first two joists, lining up the solid blocking with the deck joist. Then attach the inside hold-down bracket to the solid blocking.
This approach provides optimal bracing, but is difficult to implement, especially in retrofits where the interior joist bays may not be accessible. Alternative solutions may be acceptable, but it is up to you and the building inspector to work this out.
The newer 2015 IRC provides a simpler alternative, using four simple angle brackets that can be installed from the exterior on new or existing decks. The four connectors must be rated at 750 lbs. each, and be spaced equally along the ledger. The outer two brackets should be within 2 ft. of each end.
Each connector screws or nails into the bottom of a deck joist and is fastened to the house framing with a single 3/8″ lag screw. The lag screws must penetrate 3 inches (all threaded) into the house framing — into a top plate, stud, header, or sill plate.
You can use either the Simpson DTT1Z, as shown, or USP LTS19-TZ. The code shows this connection for use where the deck joists run parallel to the floor joists, but your inspector may allow it where joists run perpendicular.
One caution, however: This connection still relies on the withdrawal strength of lag screws, which could fail due to decay in the framing. In a critical connection, such as an elevated deck in a wet climate, the DTT2Z hold-downs described above offer a safer solution.
In frost-susceptible soils, all posts should sit on concrete piers that extend below the frost depth. The tops of the concrete piers should extend slightly above grade to keep the post ends out of soil and standing water. It is also a good idea to use steel post bases to keep the wood out of direct contact with concrete.
Where uplift from wind is an issue, use structural post bases that are rated for the necessary uplift loads and connected to the concrete with anchor bolts. Many of these are adjustable laterally so the footing and anchor bolt do not need to be precisely placed. Use pressure-treated posts rated for ground contact. Treat any cut post ends and do not place cut ends in contact with soil or concrete.
Depending on the load and soil conditions, you may need a spread footing at the concrete post base. In freezing climates, a concrete pier with a widened base will help protect against frost lifting the post. Even with the base below the frost line, frozen soil can grab the sides of concrete posts and lift them. Backfilling with sand or gravel helps prevent this.
Also, there are a number of products, such as Big Foot and Square Foot that simplify the process of pouring concrete piers with widened bases. Some, like the Footing Tube, provide a single plastic form, which stays in place. The plastic resists adhesion to frost.
Helical Piers. Where access is difficult or space is limited for excavating post holes. no-dig posts are another option for supporting a deck. The most common approach in residential structures is the use of helical piers. These are are a form of heavy-gauge galvanized steel piles, with a large screw head welded to the bottom. The typical piling is 7 ft. long and screwed into the earth by heavy pneumatic equipment. A pressure gauge indicates when the piling is deep enough to support the required loads.
Since this is an engineered solution, it is widely accepted by building departments and and becoming more familiar to contractors and customers. When installed professionally, they are at least as sturdy as a concrete pier and are designed to last for many decades. With franchises and specialty contractors offering this service in some markets, it is often cost competitive with traditional concrete piers and a much faster and more convenient approach. Read more on Helical Piers.
Sway bracing. While 4×4 posts can be used up to about 10 feet in height, depending on loads, tall pressure-treated 4x4s are prone to warping and twisting when they dry. Some local codes limit the use of 4x4s to 8 feet. Upgrading to 6x6s for tall posts is a good idea whether required by code or not.
Diagonal sway bracing is an easy way to help stiffen tall posts and provide resistance against racking. The simplest approach is to run short 2×4 or 4×4 sway braces from posts to beams.
There are a number of situations where it is difficult or impossible to achieve a solid deck ledger connection. In this case, the best alternative is to eliminate the ledger altogether and to support the deck independently on its own posts and piers. This type of freestanding deck must have adequate diagonal bracing to prevent racking in the deck structure.
The deck can be braced to the house as well, to help resist racking, but all vertical loads are carried to the ground on posts. The deck should be designed to be structurally independent of the house.
To keep the piers a safe distance from the foundation, the deck will need to be cantilevered on the side facing the house. In general, you can safely cantilever the deck joists up to one quarter their total length. For example, a 12-foot deck joist can be cantilevered up to 3 feet. This assumes that the loading of the joists is uniform with no additional loads on the cantilevered portion of the joists.