In a recent safety assessment of residential decks in and around Charlottesville, Virginia, an alarming trend was noted. 7 out of 10 platforms had at least one security issue and 4 of the platforms had significant security issues that prompted immediate action. Most homeowners hire a contractor to build the deck, or the deck was attached to the house when the house was purchased. Some of the most serious problems found were:
- Insufficient methods of attachment to the house.
- Improper joist attachment
- Insufficient lateral bracing
- Improper or missing post attachment
- Undersized Structural Members
- Improper bras used
The most serious problem, insufficient attachment to the house, was found in almost half, 4 out of 10, of the covers inspected. When a stringer board or band board (the deck structural member that is attached to the home frame, to which the deck joists are attached) is attached to the home incorrectly, the deck exhibits a critical risk of collapse. . Ledger boards MUST be attached to the house using lag bolts that are securely attached to the rim joist or house framing, or carriage bolts that fully penetrate the house framing and are secured with a washer and nut on the other side. Problems we saw included the use of nothing more than 16 penny nails, lag screws that failed in the house framing members and fastened only to the exterior siding (plywood), and in one case, standard wood screws. Improper fastening techniques and materials are the leading cause of deck collapse, resulting in injury and death. When a platform is used for entertainment, the change in load as people move on the platform also changes the stresses in the platform structure. The more people, the more weight, and the greater the forces involved. One of the covers we inspected was so poorly constructed that the weight of one person was enough force to cause the cover to sway slightly. Put 20 people on that platform and everyone is in danger.
16-cent nails are simply not thick enough to support heavy loads and have too little strength to withstand lateral forces to resist removal. Any adult can drive a nail with the claw of a hammer using as much force as 30 pounds of weight. Put 20 people, with more than 2,000 pounds of weight force, on a platform nailed down, and you’ll see the problem. Nailing, screwing, or even screwing lag bolts to the exterior siding of the house is essentially useless. Most siding products are less than 1/2 inch thick. They are designed to provide lateral (sideways) force to a structure, not to resist pullout forces. The best method of attachment is to use carriage bolts that go all the way through the deck band board, through the house board or other structural member, and are secured with a washer and nut on the other side. When this is not possible, lag bolts should be used and secured to the structural members of the house.
The next serious issue noted was encountered with the attachment of the deck joists to the deck band board. Again, on nearly half of the decks inspected, the joists were simply nailed with toe nails into the band board, and/or nailed to the toe and sat on a larger 2″ X 2″ block placed under the joist. . The same problems of poor pull-out strength and load-bearing capacity apply here. The only correct way to support deck joists is to use a metal joist hanger. They are designed to carry load and withstand lateral forces when used correctly. Check below your deck. If you don’t see joist supports, nailed at 90 degrees to the deck band board and joist, your deck is not secure!
Most of the decks inspected used 4X4 treated wood posts that were buried in the ground below the frost line and set on a suitable concrete pad. Some of them were not. Two of the decks had 4X4 posts literally resting on the concrete, with no method of attaching to the concrete. If you are not burying the post at least 2 feet below ground level, you must use a metal support for the base of the post. The base of the post is attached to the concrete using the appropriate concrete fasteners, then the post is attached to the support using joist hanger nails or lag screws.
Next, a severe lack of lateral bracing was noted on most of the decks. Side braces prevent the deck structure from moving sideways. If you were to start taking a few steps in one direction on your platform and suddenly come to a hard stop, your platform SHOULD NOT MOVE. If it does, it’s not properly braced. Your deck needs to be at least as strong as the flooring inside your house, and often stronger. The exterior siding of your home frame provides the lateral reinforcement. On a deck, it should be made from dimensional lumber that is designed to withstand exposure to the elements. When planned properly, the lateral bracing of a deck can be incorporated into the design so that it adds to the appearance of the deck while also providing a structural purpose. If your deck is built above grade, or less than 24 inches above grade, it will be necessary to apply side bracing prior to laying the deck surface. On decks more than 24 inches above grade, one piece of dimensional lumber must be fastened at an angle to each side of each post, from the top of the post to some point below the top, with a minimum angle of 30 degrees. If you have a deck, have it inspected by a professional right away. An inspection should take about an hour and cost less than a hundred dollars if you call a contractor. A small price to pay for peace of mind.