Every year many injuries, and even deaths, are reported as the result of failing decks. In most cases, these tragedies occur with higher decks that are elevated quite a ways off the ground and from excessive weight (load) being placed on the deck at one time. Higher decks obviously require more care in the design and building phases than decks that sit close to ground level. In this post, I will review some of the main safety and structural requirements for elevated decks so you can be sure your deck is safe and sound for your next family gathering.
First of all, your deck should be made of either treated lumber of composite decking. If you have hired a professional to construct your deck this point should not be an issue at all, but homeowners attempting to build their own deck as a DIY project may use dimensional lumber without knowing this information. Untreated dimensional lumber is not rated for exposure to the elements and will fail before long if used.
If the deck is 6 feet or higher off the ground, 6×6 posts should be used for support rather than 4x4s, and cross bracing (angled bracing) should be used to increase lateral stability and prevent sway. The posts should rest on or be imbedded in concrete piers that extend below the frost line, and this depth varies depending on your region and climate zone. This will ensure that the deck is not affected by frost heave as the ground freezes and thaws with seasonal weather changes. Ideally, the concrete piers will extend several inches above ground so the wood posts are not in direct contact with soil.
A deck can either be attached to the house as part of the structure, or unattached, in which case it is referred to as a “floating” deck because it stands on its own. In my personal opinion, floating decks are the better option because they prevent several common issues associated with attached decks; but since attached decks are still more common I will focus on them here.
Decks attached to the house are attached by what’s called a ledger board. Unfortunately, ledger boards are hardly ever installed properly, and defects with ledger boards are a main cause of deck failures. First, the ledger board should be at least a 2×8 and should be attached with appropriate weather-rated fasteners (such as 1/2” or larger thru bolts) that extend fully through the board and rim joist. Proper location and spacing of bolts is essential to provide adequate support of the ledger board and prevent it from pulling away from the structure. Z flashing should be installed at the ledger board to direct water away from the structure and prevent rot of the wood. In my experience, it is unfortunately not too common to see all of these requirements in place and done properly.
Deck joists, like traditional floor joists, should be hung on the ends with joist hangers, and they should typically be spaced 16 inches on-center (O.C.) just like regular joists. The actual deck boards should span far distances as solid pieces, and short pieces of board should not be used. Installing deck boards perpendicular to joists is the easiest method, but installing them at a diagonal increases stability and adds visual appeal. In addition, a small gap should exist between the deck boards to allow for expansion and contraction and adequate drainage of water from the deck’s surface.
If the deck is intended to support large groups of people or heavy objects like a hot tub, extra support will likely be needed to support the additional load. Joists may need to be doubled and/or additional posts may be needed under the area where a hot tub will be, for example. A structural engineer should be consulted to ensure the deck design is suitable for supporting the intended load requirements.
Deck stairs often have many problems as well, but two of the most common are open risers and improper handrails. Risers that are greater than 4 inches high (and they almost always are) should not be open. They should have a board in place, just like an interior stairway. Likewise, a 2×6 handrail is not safe as it is too wide to be easily grasped if someone were to fall and needed to grab the railing quickly. The spindles, or balusters, of the railings should be no more than about 4 inches apart, which is yet another safety measure. Railings should be at least 36 inches high as a general rule, and if the deck is especially high, 42 inches (6” higher than the normal minimum) is preferred as a means of extra safety.
As you can see, deck construction comes with many stringent requirements, but they exist for good reason and help ensure safety. Given the many injuries and deaths that result from insecure, poorly built decks each year, it is a good idea to look over your deck (or better yet, have it professionally inspected) so you can rest at ease knowing it is stable and your family and friends will be safe.
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