Deck Building: Best Practices

In This Article
Ledger Attachment
Ledger Flashing
Lateral Bracing to House
Posts & Piers                      View all DECKS & PORCHES Articles

Deck building looks pretty simple at a glance, but constructing 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 key 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.

Deck collapse failure

The ledger connection failed due to undersized fasteners and flashing errors that lead to wood decay in the band joist.

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.

 

Ledger Attachement

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.

Where a deck ledger is bolted to a band joist, it's important that the band joist be securely tied to the house structure.

Where a deck ledger is bolted to a band joist, it’s important that the band joist be securely tied to the house structure. CLICK TO ENLARGE

 

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:

Fastener Spacing Deck Ledger to Band Joist

CLICK TO ENLARGE

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.

Ledger Fastener Placement with Gap

CLICK TO ENLARGE

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:

Fastener Placement Ledgers

To prevent splitting and provide adequate support, stagger bolts and maintain proper clearances from edges. CLICK TO ENLARGE

Flashing the Ledger

Proper deck flashing is critical since leakage around the ledger could lead to decay in the band joist and failure 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.

CLICK TO ENLARGE

Though deck collapses are rare, deck ledgers are often at fault. It’s critical to properly flash the ledger area with materials that are compatible with pressure-treated wood.                           CLICK TO ENLARGE

Spaced Deck Ledger Detail

Spacing the ledger away from the house helps prevent rot in the band joist. A membrane flashing across the band joist area offers an extra layer of protection, useful in wet climates.                                               CLICK TO ENLARGE

 

 

 

 

 

 

 

 

 

 

 

 

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.

Poor flashing let to water intrusion and rot in the plywood sheathing and band joist beneath. The deck was only a few years old and looked terrific until it collapsed under the weight of a heavy snowfall. CLICK TO ENLARGE

Faulty flashing led to water intrusion and rot in the plywood sheathing and band joist. The deck was only a few years old and looked terrific until it collapsed under the weight of a heavy snowfall. CLICK TO ENLARGE

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.

Lateral Bracing of Deck to House

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 this issue are fuzzy and 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.

Where deck joists run parallel to house joists, the IRC shows one way to provide lateral bracing. Use two hold-down braces per deck, each brace rated at 1500 lbs. minimum. Drawing adapted from 2012 IRC Section R507.2.3

Where deck joists run parallel to house joists, the IRC shows one way to provide lateral bracing. Use two hold-down braces per deck, each brace rated at 1500 lbs. minimum. Drawing adapted from 2012 IRC Section R507.2.3 CLICK TO ENLARGE

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 several joists in the house, lining up the solid blocking with the deck joist. Then attach inside  hold-down bracket to the solid blocking.

This approach is difficult to implement, especially  in retrofits where the interior  joist bays may not be accessible. Alternative solutions that provide equal support are allowed, but it is up to you and the building inspector to work this out.

Fortunately, the new 2015 IRC has provided 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.

The 2015 IRC has provided a simpler way to provide lateral bracing using four steel brackets on the exterior. Details are shown in this IRC illustration. CLICK TO ENLARGE

The 2015 IRC has provided a simpler way to provide lateral bracing using four steel brackets on the exterior. Details are shown in this  2015 IRC illustration. CLICK TO ENLARGE

Four of Simpson's new DTT1Z bracket spaced along the deck ledger provide the lateral support required by the IRC. Courtesy of Simpson Strong Tie. CLICK TO ENLARGE

Four of Simpson’s new DTT1Z brackets spaced along the deck ledger provide the lateral support required by the IRC. Courtesy of Simpson Strong Tie. CLICK TO ENLARGE


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 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.

Posts and Piers

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.

Deck Post and Pier

Galvanized or stainless-steel post bases keep the wood out of contact with soil or concrete. Where wind uplift is an issue, use anchor bolts and post bases rated for the required uplift loads. CLICK TO ENLARGE

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.

Big_Foot Piers

Bigfoot footing forms with cardboard construction tubes ready for concrete. Courtesy of Bigfoot Systems

Footing_Tube

The Footing Tube provides a single pier and footing form. The slippery plastic form resists frost adhesion. Courtesy of The Footing Tube

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.

 

 

 

 

Diagonal sway bracing helps stiffen tall posts and protects against racking of the deck structure. CLICK TO ENLARGE

Diagonal sway bracing helps stiffen tall posts and protects against racking of the deck structure. CLICK TO ENLARGE

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.

See also Deck Fasteners & Connectors

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Comments

  1. Hanging Deck off Cantilevered Joists?

    I am a drafter with a client who wants to stabilize a second-floor deck attached to an addition. The deck cannot attach to the rim joist because it is cantilevered about a foot. He wants the joists to insert into the 2nd-floor house joists (sistered to the joists with a 3 ft. overlap). The deck extends 9ft-6in off the house and is supported by 6×6 posts 7ft-6in on center with a 18-inch cantilever at the front. The deck joists are 2×6 lumber 16 oc. The info I find says that this can cause thermal bridging and I cannot find a flashing detail for this, so I am recommending he just add two more posts for a four-post deck instead . Any thoughts? thanks

    • buildingadvisor says:

      Your suggestion to create a freestanding deck by adding two posts is probably the best solution as it is very difficult to insulate and flash cantilevered decks (where the house joists extend beyond the exterior wall to form a deck). You would end up with the same sort of detail if you sistered the deck joists to the house joists as your client requested.

      I’ve seen cantilevered joists used to create narrow decks on condos and they do not perform well over time. Because of the flashing difficulties, water leakage and wood decay are common with this detail.

      One other possibility is to use hold-down hardware, such as the Simpson DTT2Z, to tie the new ledger back to the house floor joists. This detail for lateral bracing of decks attached to a standard band joist is incorporated into the latest version of the IRC. However, it should work for a cantilevered floor as well and be easier to flash.

      The IRC calls for two lateral braces for each deck ledger. Because you are attaching to a cantilevered floor, however, you may need more than the two lateral braces specified by the code. A quick review by a structural engineer is always a good idea when getting creative with structural details or building outside of standard construction.

      The cantilever at the outboard side of the new deck should be fine. The latest version of the IRC allows for exterior decks to cantilever up to one-fourth the main span of the joists, up to a maximum of 3 ft.

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