Construction Concerns: Wood Frame Concealed Spaces

By Gregory Havel

In wood frame (Type V) single-family residences, we can usually assume that the top floor’s ceiling is attached to the underside of the roof trusses (photo 1) or, if rafters were used, to the underside of the ceiling joists that also serve to tie together the ends of the rafters where they join the tops of the load-bearing walls (photo 2).

(1) Photos by author.



The ceilings in modern residential construction are often of gypsum drywall board that is screwed to the underside of the trusses or rafters. In older construction, ceilings are more likely to be of plaster supported by wood lath, expanded metal lath, “rock lath,” or another system. In all of these instances, the ceiling functions as a barrier to fire between the occupied rooms in the structure, the dry structural fire load of wood trusses or rafters, and the wood roof sheathing in the attic or truss space.

In wood frame (Type V) commercial construction for assembly, business, educational, mercantile, and other occupancies and in Type III (ordinary or masonry and joist) construction, this is not always true. The height of the largest room on the floor (an assembly area, a retail display area, or a classroom) determines the height of the ceiling joists or trusses from the floor (photo 3). Building and fire codes usually require the installation of a gypsum drywall board firebreak on the underside of the trusses or joists as shown in photo 3.



Gypsum drywall board is installed on the wood studs to a point a few inches above the height of the suspended ceiling that will be installed in smaller rooms (offices, break rooms, washrooms, and corridors). The space between these studs above the suspended ceiling may be filled with sound-deadening insulation or may be left open. This can create a large concealed space between the suspended ceiling and the fire break above, which are often located heating, ventilating, and air conditioning (HVAC) ducts, pipes, electrical conduits, cables, recessed ceiling light fixtures, and heating units. In some buildings, this is considered a “plenum space,” and is used as the return air duct for the HVAC system.

Drywall board can be fastened to one side of the wood studs above the suspended ceiling to form a draft stop, to subdivide the concealed space, and to slow the spread of smoke if there is a fire in the building (as shown in the background room in photo 3). These are required in most occupancies to subdivide these into areas not exceeding 1,000 square feet each unless the building has an automatic fire sprinkler system that protects both occupied and concealed spaces.

Review the International Building Code, Sections 701-718, in the 2009 and 2012 editions. Limited free access is available online at and at

Review National Fire Protection Association (NFPA) 5000, Building Construction and Safety Code, chapters 6, 7, 8, 16-33, and 55, 2015 edition. Limited free access is available after log-in at

Note that although most states and cities adopt either the International Building Code or the NFPA Building Construction and Safety Code, they may not have adopted all sections or chapters of that code. Some states and cities use their own codes, whose requirements may not match the requirements of these codes. Also note that many structures were built before the current codes were developed, that the codes allow them to remain compliant with the codes in effect at the time they were built, and that the codes do not usually require that buildings be brought up to current code until they are extensively remodeled or change occupancy.

The author acknowledges the valued assistance with the research for this article by Robert Kaebisch, Instructor in Construction Sciences—Architectural/Structural, at Gateway Technical College’s iMET Center in Sturtevant, Wisconsin.


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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.




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