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Creating Code-Compliant Designs

Advertising supplement provided by
National Gypsum Company

What follows is an analysis and discussion of the details of one of the UL's fire-resistant assemblies from a seminar presented by Rich Picolo and Missy Merfeld. Use the UL Directory's Numbering System for Fire Rated Assemblies Chart.

Question: What is an assembly number for a wooden floor system using drywall?

In the first column of the Numbering Systems for Fire Rated Assemblies chart, select the type of construction and then look across for the series with gypsum board. You should come up with the letter designation L or N for wood joist, and 500 to 599 series. Let's pick assembly number 558 as an example to review. Refer to the drawing of the assembly and the information that is typical of the detail you will find in the fire systems manual. We will talk briefly about things important to note in specification, and in subsequent construction.

L558 is an unrestrained assembly. Depending on which part of the country you are from, some of the codes, for example the uniform code, are very particular about where you have to use restrained or unrestrained systems. BOCA does not delineate it, but as a rule of thumb, when you specify an assembly, you must specify whether you want to construct it as a restrained or an unrestrained assembly. This may affect how an assembly performs during a fire; sometimes you will need additional drywall in some assemblies.

The assembly has a finish rating. We have a 1-hour assembly but the finish rating is only 23 minutes. Here is what a finish-rating means: When we test assemblies we look for a couple of critical temperatures---an average temperature of no more than 250 degrees and hot spots no greater than 325 degrees. The finish rating is the point at which those temperatures are reached on one side of the assembly. BOCA or IBC never discuss finish ratings. If you go to NFPA 101, they do talk about finish ratings. So depending on what code the municipality uses, finish rating may or may not be critical.

Let's continue to go through the assembly L558. It starts with the flooring system. The finished flooring may consist of any one of the four systems listed in the UL design . We will 1 floor system to study.

From L558: The nominal 1 x 4 tongue and groove is laid perpendicular to the trusses, 15/32 in thick wood structural panels, minimum grade underlayment, or single floor. This tells us certain types and thickness of material we must use that must be laid perpendicular to the trusses. It does not say plywood , this is a relatively new assembly; it says wood structural panels. Some other designs still say plywood; some say wood structural panels. The long dimensions of the panel are to be perpendicular to the truss with the joints staggered. The staggering of the joints is common construction procedure. It's even more important, however, in a fire-rated assembly, because if you put all the joints in one spot there is a bigger chance of heat spread, and a bigger chance of failure during the course of the test.

A vapor barrier is optional, but if a vapor barrier is used, it must be a commercial asphalt-saturated felt, not Visqueen. That is a horrible mistake that is made in the field, whether it is specified that way or done by the contractor. That is why it is important that the UL details are put directly on your plans. The contractor will then know what he is supposed to bid and what he is supposed to build.

Sub-flooring: 23/32 in-thick wood structural panels installed perpendicular to the trusses with the end joints staggered.

Nails: spaced 12 inches on center along each truss. You can use staples but they have to be equal or greater withdrawal and lateral resistance and strength. So you can use staples but the burden of proof will be on you that it is equivalent to the nails specified.

Trusses: Parallel chord, so we can't use this as a floor-ceiling assembly. Maximum 24-in OC with fabricated nominal 2 x 4 lumber. 'Can I go with a spacing wider than 24 in.?' No you cannot. 'Can I go less than 24?' Yes you can. Minimum truss depth is 18 inches. You can go deeper (suppose you have mechanical equipment or duct work to install), but you can not go less than 18 inches. That has to do with heat transfer. It says no less than 18, you can't use12-inch trusses. Make sure that the truss manufacturer who bids this job knows about the truss plate requirements. Batt and blanket insulation: This assembly does show insulation: glass fiber, maximum of 3 ½ in. You can't use six inches. There are some exceptions, but basically you must follow what it says. Fitted in the concealed space, draped over the resilient channels, and gypsum wallboard membrane. Can I use a sprayed cellulose insulation? No, it says fiberglass. The insulation also has to bear the UL classification mark. That means it has passed certain surface-burning and certain fire-resistance rating. There is an exception where you can use loose fill insulation, but the loose fill has to have the UL classification mark. Air Duct: You can use any UL Class 0 or Class 1 flexible air ducts. They have to be installed according to the manufacturer's installation instruction. That is critical, since when they run these assembles through the test, everything is installed according to the manufacturer's instruction. Ceiling damper: …is not a fire damper. It has no hourly rating. It's just listed as a ceiling damper, maximum size 324 inches, no more than 18 x 18, maximum height 14 inches, and installed per the manufacturer instructions. But, it is critical if you are doing a ceiling damper, no more than 324 sq. inches in any 100 sq. feet. You can't put two of them next to each other.

Furring channels: gives you the size of the furring channel, depth of the channel, the space of the channel, types of screws to be used, how far to overlap, this is critical in the installation. If you don't follow this, the assembly may fail in a fire.

Gypsum board: Here it tells us that we use nominal 5/8 inch gypsum board, 48 inches wide, installed in the long dimension perpendicular to the resilient channels. It tells you which way the drywall has to run. It tells you 1-1/8in- long, type S screws every 12 inches. Also, it tells you how far from the side joint and how far from the end joint you have to put the screw. If you don't follow this, it does not meet the assembly specifications. The drywall may fail sooner into the fire. Once the drywall fails, it exposes the assembly to the fire and the assembly will fail quickly. When insulation is installed, it must be draped over the channel, over the sitting member and the screws shall be installed 8 inches on center, and here it says we must use a certain type of drywall. It says FSW-C. This is a Type C drywall. You can't use Type X drywall with this assembly. You must use Type C. When the directory specifies a product, it's the only one that can be used.

Finishing system: It is not shown in the assembly, but it has vinyl, dry or premixed compound applied two coats to the joints. In this particular assembly, even though page 1 says a minimum of 1 coat, this assembly says 2 coats applied to the joints. So this is more specific, and the specific always overrules the general. So if an assembly has a specific requirement, it will always overrule the general requirement.