2010-11 Design Guidelines for Retrofitting Wood Roof Sheathing Using Closed-cell Spray Applied Polyurethane Foams

Graduate Student:
Kenton McBride

Background

During severe wind events such as hurricanes and tornadoes, the roof is often the first element of a residential building to fail. Very often, plywood or oriented strand board (OSB) roof sheathing separates from roof trusses. Previous research conducted at the University of Florida established the benefits provided by using closed-cell spray-applied polyurethane foam (ccSPF) to increase the wind uplift resistance of roof sheathing (Prevatt 2007). As a continuation of that research, this project will investigate the long-term structural capacity of roof systems retrofitted with ccSPF when subjected to a warm, humid climate during the wet season.

Figure 1: Project Setup

Our experimental setup for this project will include two independent studies of four full-scale attics situated outside in Gainesville, Florida. On each roof, we will simulate several roof leaks (potential locations are highlighted in green in Figure 1) that would be undetectable from the outside by a homeowner. The underside of the roofs will be sprayed with ccSPF at the connections between roof trusses and sheathing panels (Figure 2). Spray racks mounted above the roofs will simulate the wet season (July to September) in Gainesville by spraying the equivalent daily rainfall in 15 minute increments.

Figure 2: Interior Cut of Roof

The primary concern that this project addresses is the effect of water intrusion on a ccSPF-retrofitted roof over time and how that affects the structural capacity of the roof. The “closed cells” in the foam prevent water from penetrating the surface of the foam into the interior of a house or a roof. What this also means is that the foam also acts as a vapor barrier, and could potentially trap water against the sheathing that it has been sprayed to. Our hypothesis is that there are other modes of egress for water through the roof system.

Throughout the project, temperature, humidity, and moisture content of wood members will be continuously monitored. A variation of moisture sensors developed by Dr. Christian Brischke et al. (2008) will be used to monitor sheathing moisture content, which is a critical variable in the uplift capacity of the attics’ roofs.

Should this research yield positive results, ccSPF could gain significant backing as a structural retrofit. If the retrofit is indeed a viable long-term solution, the economic and personal gain (or lack of loss) could be very significant for coastal housing across the United States.

Advisory panel members:
• Mr. Rick Dixon (Florida Building Commission)
• Mr. Michael Sievers/Mr. Eric Banks (BASF)
• Mr. Xuaco Pascual (Honeywell)
• Mr. Jason Hoerter (NCFI)
• Mr. David Brandon (Brandon Construction)
• Mr. Edward Keith (APA – The Engineered Wood Association)
• Dr. Jinhuang Wu (Huntsman)
• Mr. William L. Coulbourne, PE (Applied Technology Council)
• Mr. James L. Buckner, P.E. SECB (CBuck Engineering)
• Mr. Peter Nelson /Mr. Jason DerAnanian (Simpson Gumpertz & Heger Inc.)

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