| Fire-resistant Construction Protected Construction
Protected construction includes typical floor-ceiling, roof-ceiling
or wall assemblies with wood structural panels fastened to wood or steel framing, and a
fire-resistive material such as gypsum wallboard, plaster mineral acoustical tile added to
give primary protection to framing. The structural panels slow flame passage and
temperature rise while reinforcing supports against collapse under load.
Assemblies are rated in fire tests by Underwriters Laboratories
(U.L.) and other agencies. Over 40 floor-ceiling (and/or roof-ceiling)systems using wood
structural panels are described in the UL. Fire Resistance Directory. They are
accepted as rated constructions by most building codes. Two examples of one-hour-rated
floor-ceiling assemblies are shown in Figure 27; several two-hour-rated wood framed
assemblies also are described in the U.L. Directory.
Building Requirements
In many fire-resistant floor-ceiling assemblies, a two-layer floor
system (15/32-inch subfloor and 19/32-inch underlayment) is used, although several have
single-layer 5/8-inch or thicker combination subfloor-underlayment panels. Any finish
floor material may be used. Most codes permit omission of the top panel layer in roof
assemblies.
Plywood siding or wall sheathing in combination with gypsum
sheathing and wallboard on stud is recognized by code officials for one-hour load-bearing
exterior walls. A typical example - APA RATED SIDING over 5/8-inch Type X gypsum sheathing
attached to 2x studs 16 or 24 inches on center - is illustrated in Figure
28, page 63. Under two of the model building codes, the fire-resistive rating for
exterior wall applies only to the inside of the wall when separation to the property line
is greater than 5 feet. In this common situation, the gypsum sheathing can be omitted
under the siding as noted in Figure 28.
Fire-rated protected wall assemblies will qualify for the one-hour
rating if other materials are added over the fire-resistive materials. For example, APA
RATED SIDING panels or lap siding may be attached to the outside of a rated wall without
impairing the rating.
Roof Coverings
The fire resistance ratings of roofing materials re listed as Class
A,B, or C in descending order of fire protection afforded. Their use is prescribed by
building codes, and also affects insurance rates. Untreated APA RATED SHEATHING panels are
recognized as a structural roof deck substrate for rated roof coverings. For individual
requirements see the U.L. Roofing Materials and Systems Directory.
Wall and Ceiling Paneling
The Flame Spread Classification of materials used of interior wall
and ceiling finish (and occasionally for other applications) is usually limited by
building codes of certain occupations. Tests have shown that untreated APA wood structural
panels will develop flame spread index and smoke index values of 200 or less, which puts
them in a Class III ( or C)category.
Panels are therefore suitable as interior finish for most
applications. certain more restrictive locations, such as exit ways, require a Class I or
Class II raging which can be achieved by the use of fire-retardant-treated plywood (See page 15.) |
