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Lightweight Home Construction

Recently the NFPA (National Fire protection Association) has raised concern with lightweight floor construction in residential homes and firefighter safety. The main concern is with the use of prefabricated wood joists; often referred to as “I-Joists”, in place of standard solid wood joists. These joists are very commonly used in all new home construction. The NFPA’s claims have raised the attention of the APA – The Engineered Wood Association which is a trade association for the engineered wood industry. The NFPA has expressed this concern because I-Joists do not perform as well as solid wood joists when exposed to fire. Being constructed of lighter material and often with more surface area, the I-Joist will burn more rapidly than a solid wood joist. Due to a more rapid burn and less base material, the structural capacity of the I-Joist is compromised at a faster rate than the solid wood joist, resulting in an earlier failure rate. The main area of concern is with fires that start in unfinished basements that result in a rapid floor collapse. The NFPA’s claims have been substantiated by tests performed by the National Research Council of Canada which proved that exposed I-Joist floors fail between 5 and 8 minutes as compared to an average of 12 minutes for solid wood joists. To be fair, in these tests it was found that the toxic smoke would have made survival by an occupant impossible prior to collapse. The toxic smoke however is not a threat to a firefighter wearing a SCBA, a floor collapse is. The APA has countered these claims with studies of their own, in a report released in November 2007 they came up with the following findings; when a standard lightweight floor assembly, covered with one layer of 5/8” thick gypsum wallboard was exposed to fire, failure did not occur until after 32 minutes as compared to between 8 and 12 minutes for an exposed solid wood joist floor. This report is not a fair comparison, gypsum board is a standard material used for construction fireproofing, by adding this factor to one system and not the other gives the lightweight floor assembly an unfair advantage. It also does not reflect the common reality that a large number of basements remain unfinished without gypsum ceilings.

 

The NFPA is not advocating that prefabricated wood joists are removed from use, as the APA’s tests have shown; if they are properly protected they can perform very well. Prefabricated joists offer designers and homeowners great advantages to span greater distances and save on construction costs. The concern is with the way they are being used without adequately informing the homeowner about the risks associated with these products. Floor collapses in lightweight construction buildings are becoming more commonplace in house fires and pose a great threat to firefighters. Some municipalities in the United States have resorted to a placard system for buildings that use lightweight construction, similar to the hazardous material placards used on vehicles transporting dangerous goods. It seems to me that a good fire sprinkler system could solve many of these problems, construction costs can remain low and adequate protection is increased. Unfortunately there does not yet seem to be the political resolve to push this solution.





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