Wildland-Urban Interface Demonstration Building - Near-home
| Wildland-Urban Interface Demonstration Building | |||||
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| Roofing & Gutters | Vents | Eaves & Soffits | Windows | Siding | Near-home |
This is a close up view of the noncombustible (rock) mulch in the area immediately adjacent to an exterior wall (stucco clad). Vegetative debris can still accumulate, and should be removed on a regular schedule.
The demonstration building doesn’t have an attached deck, but since many homes have them, information regarding decks is included here.
In a wildfire, decks are vulnerable from an ember attack from above, and surface fire from below. If ignited, the deck can cause longer term flame impingement exposure, potentially igniting the siding, and/or breaking the glass of a window or door. Fire can then enter the stud cavity, or occupied space of the building, increasing the chance that the home or building will be lost.
The performance of the deck will depend on three things: 1) the relative siting of the building and deck on the parcel (i.e., mid-slope, top of slope, on a flat lot, etc.), 2) any combustible material that is stored under (or on top of) the deck, and 3) the deck material.
See a video demonstration of some issues to keep in mind about decks and wildfire.
This photograph shows a narrow deck that wraps around a home located at the top of a steep slope. A large window is located next to the deck. The siding on this home is brick (non-combustible). The deck is a fiber-plastic composite product. The vegetation on the slope isn’t well maintained, so it is likely that a surface fire on the slope would reach the deck, imposing a flaming exposure to the underside of the deck. A photograph taken from the deck to the vegetation is shown in the next slide.
If the vegetation ignites, a flaming exposure on the underside of the deck is likely. If the deck also ignites, the glass in the window next to the deck may break, depending on the extent and duration of the flaming exposure from the decking (and vegetation).
This is a photograph looking down slope from the deck.
In this case, a wildfire burned up the hill, in the draw, and ignited the lower deck on this home.
Depending on the amount and type, storage of combustibles under your deck will greatly impact the vulnerability of the deck. This material could be ignited by either surface fire or embers. The burning materials would provide a longer under-deck flaming exposure to the deck, igniting the solid wood deck boards.
Combustible materials on top of your deck (such as patio furniture, brooms, and mats) are also vulnerable to ember attack. These materials should be pulled away from the home or building during wildfire.
Most deck boards are combustible, although noncombustible (metal) deck boards are available. Three deck products are shown in this photograph. From right to left, they include redwood, a ‘Chapter 7A’ code compliant wood-plastic composite product, and a wood-plastic composite product that isn’t compliant with the provisions of Chapter 7A.
As you can see, there can be significant differences in the performance of combustible decking products. In this demonstration, two of the three decks self-extinguished. This photo was taken about 30 minutes after initially igniting the decks with standard ‘B’ brands (one per deck, as shown in the inset photograph). Combustible materials either under the deck, or on top of the deck, would have resulted in a larger fire. Also, in this case, gypsum wallboard (largely a noncombustible product) was used as the ‘siding’ product.
Some decks that have a solid surface (such as a light-weight concrete deck) are also noncombustible (see the next slide). Solid surface decks aren’t as common as the decks with gapped deck boards.
This is a photograph of the top surface of a solid surface (in this case, concrete) deck. It is a ‘membrane’ deck because there is a water proofing membrane underlying the concrete.
Solid surface decks are more expensive than gapped-board decks. Use of a solid surface deck can be an aesthetic choice, or be selected because of a desired function. In this case, it was a second story deck. Use of a solid surface deck provided some protection from rain for the first floor deck. The roof overhang provided the same protection for this second story deck.
These are examples of various types of wood plastic composite (WPCs) materials. Performance of these products will depend on the type of plastic used in the product, the board geometry (solid, channeled, hollow), and the additives used in the formulation.
Tests conducted at the University of California Forest Products Fire Research Laboratory in 2000-2002 indicated that boards with channeled construction didn’t perform as well when subjected to an under deck flame impingement exposure. Boards with a hollow construction didn’t perform as well when exposed to the (‘A’) brand (on top of deck) test. These tests resulted in the development of a standard fire test procedure that is now used by testing labs to qualify decking for use in wildfire prone areas in California.
Most WPCs today have a solid cross-section, and are usually a little more than one-inch thick. Channeled and hollow configurations aren’t as common.
Photographs of decking during the under-deck fire test (upper left) and top-of-deck brand test (lower right). These tests were conducted at the UC Fire Research Laboratory.
Oil-based penetrating stains are often recommended as one of a number of maintenance procedures to extend the useful life of a wood deck. Penetrating stains aren’t effective for more than a year or two, so reapplication should happen on a regular basis. Those concerned about wildfires worry that these oil-based stains would have a negative impact on the fire performance of the deck.
To investigate this, stained and unstained redwood decks were incorporated into fire demonstrations held during wildfire workshops in Ukiah, and Woodland, California during May, 2009. An oil-based stain was applied to a small redwood deck which had been stored outside for about a year. In each case, the stain was applied within 24 hours of the demonstration. The volatile components of the stain will dissipate relatively quickly, so conducting the test on freshly stained material was thought to be the most severe condition.
As seen in this slide, at least for the conditions tested here, the oil-based stain had minimal impact on the performance of the deck.
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 where the siding is attached to a framing system integral to the vertical support columns, or 2) enclose horizontally, again with a panelized siding product. The siding product is then attached to the bottom of the horizontal support joists.
If 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).
Additional information about building protection issues from wildfire exposures can be found by visiting the Homeowner's Wildfire Mitigation Guide or by completing the Homeowner Wildfire Assessment online.
| Wildland-Urban Interface Demonstration Building | |||||
|---|---|---|---|---|---|
| Roofing & Gutters | Vents | Eaves & Soffits | Windows | Siding | Near-home |