Attached decks are a vulnerable component of a building in wildfire-prone areas. Once ignited, decks can expose a building’s cladding (siding), exterior wall components such as windows and doors, and the under-eave area to flames and radiant heat. This exposure can result in severe damage or destruction of the building.

In a wildfire, decks are typically exposed to embers (firebrands) on their top surface and flame impingement to their bottom side. In this research, the vulnerability of decks to ember attack on the top surface and flame impingement at the bottom was investigated. Moreover, the ability of the current deck standards (State Fire Marshal (SFM) Standard 12-7A-4A and ASTM E2632) to predict the performance of different decking assemblies during a wildfire was assessed from experimental and computational perspectives.

Deck research at IBHS began with the evaluation of the vulnerability of decks to wind-blown embers. This research provided evidence that the top surface of redwood (Sequoia sempervirens) decks was particularly vulnerable to ignition by embers. Ignition typically occurred from ember accumulation in gaps between deck boards at joist crossings, which is an area where wind-blown vegetative debris can accumulate. This finer fuel can facilitate ignition by embers. After ignition, fire propagated both parallel and perpendicular to the test building. Although the mechanism was different, propagation did not depend on the orientation of the deck boards (or support joists).

The research on decks was followed up by testing deck assemblies exposed to under deck flame impingement. Several combinations of substructure (i.e., the structural support system) and walking surfaces were evaluated. These results highlighted the vulnerability of joists in the deck assembly. In North America, a wood or plastic composite walking surfaces installed over a wood substructure is the most common decks are built. Our tests showed that once the joist ignites due to the initial flame impingement exposure, it can burn for an extended period and expose the bottom side of the deck boards to flames. It was observed that if a joist was not engulfed in flames, the boards above them do did burn. Hence, one major finding from this study was regarding the impact of the substructure overall deck performance and the importance of explicitly considering the structural support system in any standard test method used to evaluate performance. It also supported the benefit of advocating the use of a noncombustible structural support system in new construction.

While using metal substructures significantly reduced the vulnerability of decks to wildfires (considering both ember and flame exposures), it might not be an affordable option to retrofit existing decks. To address this issue, different types of walking surfaces were tested. It was concluded that continuous noncombustible walking surfaces such as no-gap metal boards or a concrete slab surface limited the availability of oxygen and stalled flame spread in the under-deck area.