By decks, we are including all types of horizontal walkways, including landings, porches, and patios that are directly connected or very close to a house. Decks are described by the surface that you walk on (called the deck covering). There are two basic kinds of decks – those that use deck boards as the deck covering, and those that have a solid surface deck covering. The deck boards are almost always made from combustible materials (wood or one of the wood fiber – plastic composite or 100% plastic deck board products). Solid surface deck coverings are usually made from noncombustible materials, and include light-weight concrete or stone. They are built over occupied (living) space. Occasionally an open frame deck will be installed over a water-proof membrane, again built over occupied space. As with normal decks, this open-frame deck will also be vulnerable to accumulation of debris, and ignition by burning embers.
The figure above shows wood deck, built on wood 2x6 ‘sleepers’ on a solid surface deck (above a garage).
The most important features of decks are deck boards, ledgers, access to the underside, under-deck drainage systems (for raised decks to shelter the lower level), and adjacent doors and windows. This drawing shows some of the important deck elements:
Decks: Potential Problems
There are two major problems that decks present. First, they are a great source of fuel and an ignited deck will also certainly endanger many portions of a structure. Second, nearly all decks are adjacent to large windows or glass sliders. The heat from the deck fire can cause the glass to fail and permit the fire to enter the house, where entry means certain destruction.
In general, the thicker the deck boards (about 1.5 inches thick), the better. Thin boards (about 1 inch or less thick) release heat much faster and are a higher hazard. You may have noticed how much easier it is for thin materials to burn in a fireplace.
One of the greatest risks to structures is a "thin-board" wooden deck (about 1 inch thick). In general, the thicker the deck boards, the better. You may have noticed how much easier it is for thin materials to burn in a fireplace. Thicker materials tend to release heat much more slowly and are a lower hazard.
Deck board gaps (which are there for drainage and ventilation) can permit embers to lodge and cause ignition. In this ground-level deck (or patio), you can see literally tens of char marks from embers. Although this deck survived, a very similar one next door did not and the townhouse was lost:
The deck above also suffered from having debris in the deck board gaps, and possibly decay at some of the joints, such as where the stairs met the deck.
Raised decks offer another problem - storage of combustibles underneath. This photo may be an extreme case, but consider what would happen if even a single ember got in the stack of wood!
Also, some raised decks have drain systems to permit rainwater to drain away from the deck area. While this can offer a nice dry area, it also encourages storage of combustibles. The drain system can accumulate debris, such as tree needles or leaves that can go through the deck board openings:
Looking at the last two photos, consider the consequences of the next deck that ignited from below, permitting fire entry through the windows (even though the siding was non-combustible).
Quite often, decks that are raised, including those not being used for storage are open to flames or embers, especially those on slopes:
The deck above illustrates another problem — the growth of vegetation under the deck, that when dry, can be a fire hazard.
Ledger boards, used to attach the deck to the house, are often recommended to be installed with a gap for drainage of rain from the siding so that it doesn’t pour onto the deck. However, this is a very good trap for burning embers. The ledger board attachment detail is an example of a conflict between moisture and fire regarding ‘good’ design. In order to maintain adequate performance, this joint must be inspected and maintained. Debris must be cleared. Durable materials should be used to minimize the potential for fungal decay.
Fascia boards are often used as decorative edges on decks, but often cause decay to develop between the fascia and deck. This deck corner ignited in a decayed area at the deck corner:
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Deck surface materials must be carefully considered. In tests conducted at the UC Forest Products Laboratory starting in the late 1990s, many wood-plastic and plastic decks were found deficient in behavior. This information is posted on the following web page: http://nature.berkeley.edu/~fbeall/WDDeckIntro.htm
As a rule of thumb, the thicker the wooden decking material, the better the fire performance. That does not mean that such decks will survive a wildfire, but they will not contribute enough heat release to endanger the house.
Decks that are just above ground level should be screened to prevent combustible debris from entering. This also keeps out animals, such as skunks! Also, note the stones being used to minimize growth of vegetation.
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The gaps between deck boards (about 3/16 in.) are there for drainage and ventilation. Keeping the gaps clean also protects the deck boards and joists from decay. The best advice is to keep these clean of debris, especially before and during the fire season.
The problem with gaps between the deck and the house (such as offset ledger board construction) are probably best handled by covering the gaps with screening having about 1/8 in. openings to prevent lodging of debris and embers. Ledger boards that are attached without gaps should be flashed (see below)--this not only provides protection against water penetration, but also acts as a fire barrier to embers.
Replacement of deck boards is obviously expensive, but could be one of the best investments you can make. For replacements, consider any material (plastic, plastic lumber, fire-retardant treated lumber for exterior use, or lumber) that passes the standard posted on the web page of the Office of the State Fire Marshal:
http://www.fire.ca.gov/fire_prevention/fire_prevention_wildland_codes.php#testingstandards
Then click on 'Testing Standards CA SFM12.7A-4 Decking'
This and other testing procedures listed on this site have been approved for use by the Office of the State Fire Marshal, and have been incorporated into the new California Building Code for new construction in designated wildland urban interface areas. You should check with your local building and fire officials to find out what products are acceptable in your area. In the tests done by the UC Forest Products Laboratory, all-heart 1 1/2 inch thick redwood (2 x 6s), a widely used decking, and some (but not all) plastic-fiber composite decking products, performed acceptably. Note the information in bold; redwood sapwood was not tested in that study, so its performance was not determined. Also, because any sapwood will readily decay, it wouldn't be the most appropriate choice. Thinner material could be risky (for mechanical failure as well as fire). They found that certain types of wood-plastic board profiles (hollow and channeled) did not perform well:
Decks: Possible Solutions