Deck Building 101 – Types of Structural Supports
What type of supports should I provide for my deck?
There are many aspects of a deck that should be considered when discussing the structure. These include: joists, beams, post size, supports, spacing, decking thickness, guard/railing height, and spindle spacing. These aspects are especially important when a deck is high off the ground or supporting large loads.
The joists are used to support the deck’s surface. The size and spacing of joists are directly proportional, that is, as the size of the joist increases, the allowable spacing also increases. Larger joists can carry a larger load, therefore, fewer joists are required. The connection of the joists to the beam or ledger board is very important (please see the following discussion on beams for further information regarding these components). If the joist is not properly secured, then it does not matter how big it is! The joists can rest directly on the beam or ledger, as long as an acceptable length of joist is resting on the beam or ledger (usually 3-4 cm). Joist hangers are also used to connect the joist to the ledger board. The proper number of nails should be used with the joist hangers to ensure that the joists are properly secured. Joists should not just be “toenailed” into the ledger board since the nails can pull away from the wood over time and detach the joist.
The beams are provided to support the end and sometimes middle of the joists. This reduces excessive vertical flexing and side-to-side (lateral) movement. As with the joists, the size and spacing of the beams are directly proportional. In some cases, the joists are supported by a ledger board, a beam that is secured to the house at one end, and one or two additional beams at the mid-span and ends of the joists. The ledger should be secured to either the foundation or an interior structural ceiling joist, and should be attached using the proper type of bolts. This is critically important because if the bolts are not adequate or are not secured properly, the deck could fail. Although this is not related to the deck structure, it is important to make sure a proper flashing is installed around the ledger board as well. A small leak that goes undetected for a long period of time can cause significant damage to the interior wood structure and finishes.
The deck posts transfer the load from the beams to the ground. An undersized post or posts that are installed too far apart will be incapable of supporting the deck and can eventually crack, which would be a serious safety concern. Deck posts should be at least 15 ¼ cm x 15 ¼ cm (6”x6”), unless the deck is very close to the ground. Very high and very large decks, or decks that support large loads may require larger posts, or posts spaced closer together. The method of supporting the post in the soil should also be carefully considered. Ideally, a concrete footing that extends below the frost line should be installed and the post secured to the top of the footing above the soil level. This not only prevents wood to earth contact (which could lead to premature deterioration and rot of the wood), but also prevents the deck from heaving during the freeze-thaw cycle. Small concrete deck blocks that are placed directly on the ground surface and not buried may also be used, but only for smaller (i.e. lower) decks with very good drainage in the underlying and surrounding soil. A thick layer of well-drained sand and gravel is required under deck blocks to reduce the potential for water accumulation, freezing, and heaving, during the freeze-thaw cycle.
Guards and Railings
The railing or guard is what keeps guests from falling off of the deck. Therefore, the guard/railing should be very sturdy and tall. The requirement for guard and railing height is mandated by local building codes and varies from municipality to municipality. Typically, guards/railings are required to be installed if a deck is more than 0.45 (1.5 m) off of the ground. Guard/railing heights are typically 0.9 meters (36 inches) for decks that are no more than 1.8 m (6 feet) above finished ground level. For decks that are more than 1.8 m (6 feet) above ground, guards are typically required to be a minimum of 1.06 m (42 inches) high. To prevent small children from crawling through the spindles (or becoming stuck), the spindles should be at the most 10 cm (4 inches) apart. The spindles should be installed vertically, since horizontal spindles could be tempting for children to climb. The above requirements are typically required for stairs associated with decks, as well. In order to confirm local requirements, your local building department should always be consulted.
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