Wall Heights and Fire Ratings

Donald E. Smith, CCS

September 2008

On a recent road trip my traveling companion asked me what appeared to be a simple question. He raised the question after watching two drywall hangers perform a demonstration of hanging drywall. He pointed out that they were using scraps of the 5/8 inch thick board to pick the bottom edge of the board off the floor. His question was this: Does the height of the drywall off the floor affect the fire rating of a partition? Being without my reference library, I was stumped. When I got back to the office I started digging for the answer. ASTM C840 says, "7.1.3.1 Gypsum board applied to walls shall be applied with the bottom edge spaced not less than 1/4 in. (6 mm) above the floor.”

OK, so we have set the minimum. What about the maximum? That’s just a case of simple math. The fastener has to be a minimum of 3/8 inch from the edge of the board, and it has to engage the supporting member. If we are using 2x4 wood framing, we know that the face of the stud used a plate is 1 1/2 inches, or it could be 1 3/8 inches. Based on 1 3/8 inches, the maximum height of the bottom edge could be 5/8 inch so as not to split the wood and give reasonable support to the bottom edge of the board. If you are using cold-formed steel framing you may well end up with less space to secure the board to the framing because some runners only have 1 1/4 inch flange.

How does all of this impact the fire rating or a sound rating? Let’s deal with the fire rating first. When a panel is assembled for a fire test conducted in accordance with ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, the edges of the panel being tested are sealed with a cementitious based fireproofing material. The concern is that the wall assembly is the more important item being tested. Also, the wall is under a negative pressure. That is, the bottom of the wall is surrounded by cool air while the top of the wall is surrounded by warm air, hence the reasoning behind the importance of the head-of-wall design. The head-of-wall is a separate assembly design and is very important to the wall assembly design.

It is a different story when dealing with a sound rated partition. According to GA 600, Fire Resistance Design Manual, published by the Gypsum Association, the first essential for airborne sound insulation using any system is to close off air leaks and/or flanking paths by which noise can go through or around the system. It goes on to say that failure to observe special construction and design precautions can reduce the effectiveness of the best planned sound control methods. The entire perimeter of a sound-rated assembly has to be made airtight to prevent sound flanking. This can best be accomplished with the use of flexible sealant or an acoustical seal between the sound rated system and dissimilar surfaces, and also between the sound rated system and similar surfaces. Taping gypsum board wall and wall-ceiling intersections provides an adequate air seal at these locations. Remember: When in doubt, consult the description of the rated assembly. You have to install the assembly as described in the design description.

You may or may not have encountered a requirement for field testing of sound-rated assemblies. There is an ASTM Standard for just this purpose. It is ASTM E336-07, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings. If you are ever questioned about the actual sound rating of an assembly, this is an optional test that can be performed. Of course, it would have to be a critical element in the overall design of the space.
GA 600 is an excellent source for fire-rated and sound-rated assemblies. Many of the assemblies are non-propriety and can be suggested as alternates for the specified design assembly specified in the contract documents.

Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to smith@awci.org or call him directly at (703) 538.1611.