There are two basic kinds of decks – those that have a solid surface and those that utilize (usually gapped) deck boards over the structural framing. The upper surface of a solid surface deck and the underside of gapped-board deck are shown in this slide.

During a wildfire, decks can be ignited from a surface fire from below and/or ember attack from above. If ignited, the burning deck will present a long term flame impingement exposure to the side of the house, potentially igniting or otherwise resulting in failure of the siding, and/or breaking the glass of a window or sliding glass door. If the decking and/or siding is very combustible, flames could spread to the eave.

Issues:

This is an example of a solid surface deck, with an occupied space below (in this case, a garage). The surface is usually noncombustible (light weight concrete or stone). In this case, the wildfire threat would be from an ember exposure to the top of the deck. More critical than the noncombustible surface would be debris that accumulates on the deck, and any combustibles stored or used on the deck (such as firewood and furniture).

This is an example of a membrane deck with a wood deck. The wood deck boards are attached to 2 x 4 sleepers that rest on a waterproof membrane that is attached to the sheathing. In this example, the wildfire threat would be from embers igniting accumulated debris on deck, accumulated embers igniting the deck boards, and embers igniting deck furniture. Embers and debris would tend to accumulate at a deck-to-wall area, particularly at an interior corner. If the siding is combustible, it could also ignite.

Depending on how far off the ground the deck is, a surface fire burning up the deck could result in a flame impingement exposure to the structural framing members, and the sheathing.

This is an example of a solid surface deck that is enclosed horizontally on the underside. Sometimes enclosed solid surface decks incorporate vents on the underside to allow for some drying should the top surface develop a leak. Depending on the size and location of the leak, venting may or may not be adequate to dry out the space.

Although some wildfire guides suggest that decks be enclosed, it doesn’t always make sense to do this. If your defensible space requirements have been met, and you aren’t storing combustible materials under your deck, the benefits of enclosing your deck are minimal, and moisture-related degradation issues become more of a problem.

There are two ways that decks can be enclosed: 1) enclose ‘vertically’ using a siding product. The siding would be attached to a framing system integral to the vertical support columns, and 2) enclose horizontally , again with a panelized siding product. The siding product would be attached to the bottom of the horizontal support joists.

In a deck consists of gapped deck boards, either of these enclosure methods would result in water moving into the enclosed areas. Some drainage or ventilation system would have to be incorporated into the design, or fungal decay would soon be a problem. The ‘vertically’ enclosed deck, shown in this slide, has incorporated vents into the design (shown with arrows).

The lattice fencing used as a vertical enclosure on this deck, plus the vegetation, would be readily ignitable during a wildfire and would not be part a good fire safe design.

This chain link fence enclosure was probably installed to keep rodents, and maybe kids, from getting under the deck and into the crawl space. This fence still allows vegetative debris to accumulate and makes it more inconvenient to remove it. The deck height is high enough to allow for storage of materials, some combustible.

Decks that are located close to the ground increase the difficulty of removing debris.

Proximity to the ground: decks set to low to the ground increase difficulty of removing debris (a minus). These decks are also harder to store things under (a plus). The noncombustible rock mulch under the deck on the lower right photo is a good feature.

Debris in and storage of construction debris in under deck area makes this deck vulnerable to ignition. If storage space is needed, then enclosing the deck vertically would be advisable. Including a solid surface membrane-deck would provide an added benefit in this case.

As seen in this slide, tightly installed deck boards will limit the ability of flames to move from the lower to upper surface of a gapped-board deck. However, this would overall be a bad detail given the increased potential to trap water in the joints, thereby increasing the potential for fungal decay.