Our December 2006 column titled, “Using a Gypsum Board Membrane System for Fire Resistance” generated a couple of phone inquiries. The gist of both calls indicates that the column may have been a bit too brief in its explanation of membrane fire ratings.

The column focused on the use of a membrane system for providing fire-resistance and explained the situations where the use of such a system might be appropriate. It also described three membrane protection systems that are published in GA-610, Fire Resistance Provided by Gypsum Board Membrane Protection, a two-page document that is available from the Gypsum Association.

A topic the December column did not address, an issue that was brought to light by the phone inquiries, is the use of a calculated method for determining the fire resistance of a gypsum-clad system. Both callers indicated that they had heard rumors about the existence of a calculated method for determining fire-resistance ratings, but neither knew where to find any information about it. Both were faced with a rating situation that was not answered by a standard fire test report or the information in GA-610.

Calculating fire-resistance

Specifically, a method for calculating a fire-resistance rating for a gypsum board clad system has existed in model building codes for many years–most notably in the Uniform Building Code–and was integrated into the International Building Code (IBC) when it was first published in 2000. Unlike a fire rating established by a fire test, a fire rating derived through the use of a calculated method uses “specific materials or combinations of materials” to establish the fire-resistance of a system. No ASTM E119 or equivalent test is involved in the process; rather, individual materials or systems are assigned standardized fire ratings based on established precedent. The individual ratings typically are summed to create a system rating.

Section 721 of the IBC summarizes a calculated process and describes a series of methods for calculating the fire-resistance of systems incorporating materials such as concrete, masonry, steel, and clay masonry. Specific methods for calculating the fire-resistance of a wood assembly that is clad with gypsum board are written into Section 721.6 of the IBC along with language that provides direction on calculating the fire-resistance of exposed wood members–specifically timber beams and columns with a 6-inch minimum nominal dimension.

The method described in Section 721.6 of the IBC provides another approach for calculating a fire-resistance rating for a gypsum-clad assembly beyond that described in GA-610 or a published E 119 (or equivalent) fire test. Caution is suggested, however, when using the process, for it can be somewhat confusing in application. Both of the individuals who inquired about the calculated method and its relationship to the December 2006 article quickly focused on two specific issues, both of which immediately pinpointed some of the common confusion with the calculated method.

A common misconception

First, the calculated method is applicable for use only when calculating a rating for the fire-exposed side of a system. Section 721.6.2.1. is clear in this regard when it states “(t)he fire-resistance rating of a wood frame assembly is equal to the sum of the membrane on the fire-exposed side, the time assigned to the framing members and the time assigned for additional contribution for other protective measures such as insulation. The membrane on the unexposed side shall not be included in determining the fire resistance of the assembly.”

This is no different than the intent of GA-610; nonetheless, there seems to be a common misconception that the individual material ratings in the IBC can be added together to create a full system (i.e., a wall clad on both sides) rating. Section 721.6.2.1. clearly prohibits that and while a fire rating certainly could be calculated for a wall that is clad on both faces, the calculated rating could not use the membrane on the non-fire exposed face of the partition as a part of the calculation.

Second, unlike GA-610, the information in the IBC is applicable for use only with wood frame systems that are installed with studs spaced not more than 16 inches on center. Again, Section 721.6 is very clear in this regard in both its charging and specific language. The three systems described in GA-610 do not contain the same restriction and may be applied to standard wood or steel framing members spaced as required by the document.

Section 721.6 also allows for the addition of time to the calculation when mineral fiber insulation is installed in the stud cavity. GA-610 does not. Both methods require system joints to be finished with compound.

Mention of the calculated method typically generates a significant amount of discussion (and angst) among gypsum industry technical gurus, for the results obtained through the use of the calculated method don’t seem to dovetail all that well with those obtained from a standard fire test. As an example, the calculated method assigns a 10-minute fire rating to an individual layer of 3/8-inch-thick regular gypsum wallboard and a 20-minute rating to the wood studs to which the wallboard would be attached. Using the calculated method, and summing the parts, one could “calculate up” a 60-minute rating by applying four layers of 3/8-inch wallboard (40 minutes) on one side of a wood stud wall (20 minutes). While that would result in a healthy 1 and 1/2 inches of gypsum on one side of a row of studs, you would be hard pressed to find the fire test documentation that would support the calculation. In addition, while Section 721.6 does contain some information on fastener type and spacing via reference to other sections of the code, it provides no guidance on joint spacing or stagger between the individual layers of installed board. A system installed with a “through joint” clearly would have suspect performance abilities.

Our experience is that caution should be exercised when using the calculated method since it can be misapplied and misinterpreted, and it may not be fully supported by conventional fire testing research. However, it is a part of accepted code language, and it does have some limited applications as an approach of last resort. When all other options have failed, the calculated method can be used to resolve a specific problem.

As always, the Gypsum Association recommends the use of a system tested in accordance with the ASTM E 119 method or its equivalent. A variety of resources–including GA-600, Fire Resistance Design Manual–exist where design information for fully tested systems may be obtained.