WestCon Tribune

January Review

Protecting Structures From Wildfire:

Materials and Design Issues

Presented by Stephen Quarles, University of California Forest Products Laboratory

At January’s meeting, Steve Quarles, a Westcon member and UCCE Wood Building Durability Advisor from the University of California Forest Products Laboratory, reviewed testing protocols and fire tests in support of the performance-based codes. Mr. Quarles brought in and explained the different materials which had been tested for fire and imposed loads, and the difference between design features that would be used to protect the exterior envelope from moisture and rain and those that would be used for wildfire.

The following information is taken from an excellent paper written by Mr. Quarles “Conflicting Design Issues in Wood Frame Construction” which covers a good deal of the Mr. Quarles presentation... minus some great test photos:

In order for structures to provide adequate long-term performance, they must be designed and built to resist the imposed loads. These loads can be both structural and environmental in nature. Of the environmental elements (e.g., moisture, fire, uv exposure), it is the contribution of the imposed moisture loads that usually result in the performance issues affecting durability. Buildings located in the urban-wildland interface (UWI) can also be exposed to the environmental load of wildfire. Testing has recently been conducted at the University of California Fire Research Laboratory whereby exterior building components and assemblies were exposed to simulated wildfire conditions. One of the results of these tests showed that construction details commonly used to protect a structure from moisture were often in conflict with those which would more effectively protect the same structure against the flame impingement and burning brand exposures typical for homes located in the UWI and subjected to wildfire. Examples of conflicting moisture-wildfire design issues include attic and crawlspace ventilation and roof overhangs. Traditional vents are vulnerable to flame and ember entry, but depending on the climate, are considered important from a moisture management perspective. Similarly, wide roof overhangs are considered a good design feature to protect cladding from rainfall, and can be good from a solar gain (energy conservation) perspective depending on location, but are a poor design feature from a flame impingement perspective.

Performance Issues:

Design of Components and Assemblies

Similarities and differences were noted between design features that would be used to protect the exterior envelope from moisture/rain and those that would be used for wildfire. The similar design features for both moisture and wildfire design is the importance of proper detailing at the joints and penetrations. Obtaining adequate moisture and wildfire protection in the field of a given material or assembly (i.e., away from the edges) is the easiest to accomplish. Penetration of moisture and fire typically occurs at joints. This is why flashing details are so important when considering moisture management issues, and the same is true for fire penetration. The conflicting design issues deal with the “gross” design features and in the selection of materials . Examples of these gross design features include the width of the roof overhang, use of attic and crawlspace ventilation, and the spacing of deck boards in attached, spaced-board decks.

Roof Overhang and Ventilation

Roof overhang performance issues related to wild fire exposures are two-fold, one related to ventilation of attics, and the other related to flame impingement on the wall. Attic vents are frequently located on the underside of eaves, and have proven to be vulnerable to both entry of flames (flame impingement exposure) and glowing embers. Our research has shown that all forms of vents on the underside of the eaves (strip vents, frieze block, etc.), in both boxed and open-eave construction , are almost immediately penetrated under flame impingement exposures. From this perspective, incorporation of metal or plastic strip vents with a noncombustible soffit material eliminates the material advantage in a fire safe design. The vulnerability of eave vents to fire has led to their elimination in some areas. The addition of through -roof vents on the roof surface can compensate for the loss of vent area at the eave, but it is questionable whether the attic area is being as effectively ventilated.

The second performance issue is related to flame impingement on the wall. The flame height on a wall is dependent on the entrainment of air into the flame plume (ASTM 1997). Because the flame is blocked on one side when it is against a wall, it will climb higher than one that is not in contact with a wall. Flames will climb higher yet at a corner. The flame plume will spread onto the surface of the eave (soffit) of an overhang exists due to the reduction of entrained air as the flame turns on the sloped surface (ASTM 1997). Results from research conducted at the UCFPL Fire Research Laboratory showed that flames would enter soffit vents located in an open eave (frieze block vent), and strip vents installed in boxed eaves almost immediately after a flame source was ignited at the base of the wall. If a combustible soffit material is used, wide overhangs can be more vulnerable to a flame impingement exposure even if vents are not used because more material is exposed, In the same study, failure in combustible soffit material occurred at joints in tongue & groove boards and at knots and core gaps in plywood soffits.

Crawlspace ventilation issues are similar to attic ventilation issues. Some level of crawlspace ventilation is required by code for moisture management, but again the effectiveness of ventilation has been questioned by some building scientists (Rose and TenWolde 1994). Crawlspace vents are often in close proximity to landscaping vegetation, which increase the chances of flame entry into the crawlspace should a wildfire reach the structure. Other construction materials and details can be used to compensate for reduced venting that may be required by some codes. The use of a plastic ground cover in a crawlspace can reduce the need for ventilation, but not eliminate it (Quarles 1989), and the appropriate use of an air barrier can reduce the amount of moisture movement into the attic and building envelope. Slab on grade construction can also be used to avoid the crawlspace ventilation issue altogether.

Competing priorities create another conflicting design issue between aesthetics, “energy efficiency” and “firewise” constructions. As indicated by Wilson (2001), vegetation surrounding buildings can provide energy savings for a building, typically by improving the shading of a building. However, homeowners usually prefer vegetation surrounding homes, regardless of the other benefits and dangers.

For additional information, visit the UCFPL Website at www.ucfpl.ucop.edu. They have a great site with some of the visuals used in the presentation.