19 Mar. 2001|
Plywood used for roofing,cabinet makers supplies melbourne,wood playset design - Try Out
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. While watching the builders next door do their thing, I became curious with their technique to install plywood on edges neighboring the next townhouse, and OSB for the rest (which for these narrow interior units is a single panel in the middle, but they use OSB all the way to the non-attached side on the end units).
Follow up: I'm guessing this is related to OSB's tendency to expand on the edges when wet, possibly for the flashing and the plywood side of the firewall that would rest on top of this edge of the roof.
Perhaps OSB is cheaper at the moment for this builder than ply and wants to use OSB as much as they can.
They're required by 2012 IRC to use a fire retardant treated plywood on the roof within 4' of the UL assembly (the firewall that's between each unit). I'm not sure of any difference at all - but I could swear I remember Mike Holmes saying that he liked OSB for roofs because it's stronger and a bit more resistant to water (more glue, I guess).
This is a major project with fresh materials being delivered for each stage of the project, so it doesn't appear to be a reuse of leftovers. Our photo (left) shows the success of FRT plywood in preventing fire spread from one portion of a flaming building to the next. As a substitute for vertical through-roof firewalls on multiple-dwelling buildings, the advent of FRT-plywood permitted omission of those more costly fire-walls that had to be built up extending through roof design. On entry into the attic space of a multiple-dwelling structure such as attached town homes or condominiums, look first for the fire-wall that should have been built between the abutting units. In that design, typically FRT plywood roof decking was used for four feet on either side of the firewall between building sections, and the firewall terminated just below the roof decking. In more recent construction as well as in retrofit jobs intended to improve building fire safety you may see a double layer of fire-resistant drywall installed for as much as four feet against the roof sheathing on either side of the party wall or fire-wall between adjacent living units. But on older buildings where FRT plywood was used, attic heat and age was found to lead to deteriorated roof decks even where no actual fire had ever occurred. The following is adapted with permission from an original Fire-Retardant-Treated (FRT) Plywood article in Professional Roofing by Tom Bollnow, Professional Roofing, May 1999 p.62. FRT plywood's structural strength changes from 10 percent to 20 percent after an initial pressure treatment procedure.
FRT plywood treatments are divided into three categories: Exterior, Interior Type A and Interior Type B. Building code authorities, such as the Building Officials and Code Administrators (BOCA) International Inc., have specific requirements for treatment processes and labeling. In addition, each plywood piece must be labeled properly with its performance rating and design-strength adjustment values. Chemicals that produce low-level acids causing fire-retardant effects also cause premature FRT plywood degradation at lower temperatures.
Roofing professionals should note that there are construction alternatives available that can eliminate the use of FRT plywood. Fire retardant treated FRT plywood, while it is still a combustible material, has been chemically treated to provide a lower flame-spread rate than un-treated plywood used in building construction. The plywood industry states that the flame-spread rate of FRT plywood is at least as low as gypsum wallboard (although without specifying which fire-rated wallboard was used for comparison). According to the APA, and in accordance with the International Building Code (IBC), noncombustible buildings Types I and II (usually built of steel and concrete), allow fire-retardant-treated plywood and heavy timber construction in limited uses.
For a detailed, industry-provided and current description of Fire Retardant Treated FRT Plywood, see Fire-Retardant-Treated (FRT) Plywood, American Plywood Association (APA), representing the engineered wood industry, publication NO. This document includes the types of construction where FRT plywood is used, specifies the proper type of fasteners used for FRT plywood, describes the burn-through resistance and design capacities of FRT wood, provides the FRT plywood treating process and test standards, outlines code-approved applications for FRT Plywood, and explains how to identify fire-retardant treated plywood. I was told that a newer TH with OSB roof sheathing and no FRS plywood was OK because a sprinkler system is installed in the house. Trying to educate my real estate due diligence staff on problematic fire retardant plywood. Eric, I've added another photo of the identifying stamp found on FRT plywood as it might be visible in an attic. 3If you can’t measure the roof directly, or are faced with a gable roof having intersecting wings and other complexities, apply a formula that will convert the floor area covered by the roof into a rough estimate of the plywood required to cover the roof. Given the significant increase of lighter-weight plywood cost, most people eventually get beyond their hatred of OSB.
That would seem to indicate that it was leftover OSB that did not have an indoor space for storage, so needed to be used up or be lost to the weather.
My best guess is that the flashing around the firewall is a common location for leaks, or they may need a higher fire rating at that junction.
During the late 1980s, there was an outbreak of structural roof deck failures directly related to degradation of FRT plywood used as roof sheathing. For example, plywood must be manufactured according to American Wood Preservers Association (AWPA) standards, and the treatment process must be evaluated by BOCA Evaluation Services, National Evaluation Services or an AWPA-approved, independent agency. Roofing professionals should use caution and precise documentation when confronted with FRT plywood roof decks to avoid repercussions if failures occur. Non-combustible exterior walls (required for building types IIIA and IV) are required however.
Many of the examples especially near the end are good practice for the California PE Special Seismic Exam design questions.
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If sheathing the roof with plywood, there are basic formulas for roughly estimating the number of 4 foot by 8 foot sheets of plywood you will need to cover your roof. For example, if your house has a main area of 20 by 40 feet and a wing that is 15 by 20 feet, multiply 20 by 40 to get 800 square feet for the main area, and multiply 15 by 20 to get 300 square feet for the wing. If it makes any difference, those firewalls get cut about 1' above the roof, framed with plywood, and covered in flashing. The concrete block wall on the left of the photo is the fire-wall, and you can see that the plywood roof sheathing adjacent to the fire-wall looks different (like plywood) than the remaining roof sheathing (that is OSB in the right portion of the photo). Because the potential for FRT ply- wood degradation still exists, roofing professionals should be knowledgeable about FRT plywood properties so the likelihood of degradation occurring can be reduced. Roofing professionals should note that achieving fire retardancy at the expense of structural integrity is not desirable.
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Add 20 percent to that figure to allow for the roof slope, which results in 1,320 square feet of roof area. When a flame is removed from FRT plywood's surface, the plywood will char but not burst into flames.
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