Construction Concerns: Wood Frame Corners

Article and photos by Gregory Havel

When most of us think about the intersection of the framing of an exterior wood-frame wall with an interior wood-frame partition wall, we visualize something like the walls in photo 1, with the studs nailed between the top and bottom plates. This configuration goes back nearly 200 years to the earliest use of load-bearing wood-stud walls instead of wood-stud curtain walls built as fillers between posts and frames.

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Photo 1 shows an exterior wall framed of 2 x 6 lumber and its intersection with an interior partition wall framed of 2 x 4 lumber. This method creates a sturdy place to nail the wood lath, or today’s gypsum drywall board, on the exposed framing of both walls. After plastering (or drywall taping), it makes a tight, solid joint that is unlikely to crack. This construction method requires careful laying out of the exterior walls, so that the space between the double stud at the intersection with the partition is just the right size and in the right place to support either the ends of wood lath or the edges of the sheets of drywall board. This is the typical framing method used for partitions listed by Underwriters Laboratories (UL) and other laboratories as fire-rated. See the Fire Resistance Certifications Directory at www.ul.com/database for construction details of fire-rated wall assemblies; especially the U300 and U400 series.

Since the 1940s, this narrow and inaccessible stud space in an exterior wall would be stuffed with insulation by quality builders before the partition wall was raised and nailed in place. The other kind of builder would leave this void space uninsulated. The intersection of two interior partitions is similar, without insulation.

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Photo 2 shows a type of wood-frame wall intersection that has developed over the past 20 years and is now most common in many parts of the United States. This photo also shows an exterior wall framed of 2 x 6 lumber and an interior partition framed of 2 x 4 lumber. Instead of adding two studs to the exterior wall at the partition wall, three or four pieces of stud material are nailed between the studs that are already in place, to which the partition wall will be attached.

This method is popular because it saves lumber (two full-length studs), allows the use of some of the leftover scraps of lumber as blocking between the studs at the intersection with the partition wall, requires a less precise layout when constructing the exterior wall frames, and allows the entire exterior wall to be insulated at one time. In addition to using less material (at reduced cost), this framing method requires less skill and labor to assemble than the traditional method, further reducing construction costs.

The disadvantage of a corner constructed as shown in photo 2 is that one of the sheets of drywall board is supported only where it is in contact with the wood blocking, and not along its entire edge. Even with modern drywall tape and joint compound, this makes a weak joint that can be opened by the vibration of traffic in the street, slamming doors, and by seasonal expansion and contraction.

Firefighters assume that a fire starting in a room will be confined to that room until there is a failure of a wall, door, or window. With a fire in the room, the drywall board joint in the corner shown in photo 2 is likely to fail more quickly than the drywall in the corner shown in photo 1. Once the corner joint fails, the fire can run around the end of the partition wall in photo 2 and into the next room more quickly than if it were constructed as in photo 1. And once the fire is into the stud space, it can run vertically into the joist space of the floor above or into the attic or truss loft.

Although neither of the construction methods shown are part of fire-rated assemblies, the method shown in photo 1 is more inherently fire resistive, since the gypsum finish has better support and because the extra studs in the wall provide some fire-stopping.

Since the finished product of both methods will look the same, especially when viewed under stress through smoke or through a thermal imaging camera, we can no longer assume that this fire will behave as so many previous fires have behaved. We must assume that fire spread from room to room can be more rapid than we expect.

Gregory Havel is a member of the Burlington (WI) Fire Department; a retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, 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 30 years of experience in facilities management and building construction; and has presented classes at FDIC.