Head of Wall (HOW) - Overview Head of Wall (HOW) Bulletin


Head of Wall - Fire Resistive Construction Bulletin

 

 

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Background

 

Metal buildings are a popular choice in the US and abroad for various low-rise commercial, industrial, assembly and institutional uses. In today’s marketplace they make up approximately 40 percent of the low-rise non-residential building market and are a cost-effective solution for many owners and occupants. Their noncombustible structure consists of rigid and braced steel frames which are comprised of both hot- and cold-formed steel shapes, plates, sheets, roof deck and/or panels. A typical, partially erected structural framing layout during construction is shown in Figure 1.  

 

 

 

 

 

Figure 1: Partially Erected Metal Building During Construction

 

 

Many elements of single-story metal building construction are commonly not required by the governing code to contain passive fire protection, in accordance with the Type IIB Classification of the International Building Code (IBC).  This IIB type of construction designation requires non-combustible framing materials and compliance with specific allowable heights and areas for the design occupancy, per the building code. 

 

As a result of this classification, the steel roof is often permitted to remain unprotected-unrated for fire resistance.  However, there are some situations that require fire-resistive walls even in this Type IIB construction. This may be due to interior area or occupancy wall separations, exit distances and/or building proximity limits to an adjacent property line. The walls may be rated interior separation, corridor or exterior walls.    The building code requirements and their application to any specific Project are intended to be considered on a case-by-base basis.  

 

The subject of this Summary Bulletin and the following UL Test Report is a tested fire protection detail for the HOW joint at the intersection of a fire resistive rated wall (gypsum board on steel studs) with either an unrated or a rated roof.  Given that the rated walls of metal buildings may need to be located anywhere within the building, they could align to be parallel, perpendicular, or in-line with the roof purlins, as illustrated in Figure 2 for the three basis cases.  The developed HOW joint details, therefore, needed to cover all of these possible construction conditions.  

  

 

Figure 2: Diagram Showing Three Basic Orientations of an Interior Fire-Resistive Wall Relative to the Roof Purlins 

 

 

This work was undertaken and successfully completed by MBMA to answer specific questions of regulatory and fire safety officials relative to metal building construction compliance with the 2006 International Building Code (IBC) Sections 713.1-713.3, and similar provisions in other codes. Even though IBC Sections 713.1-713.3 explicitly cite “joints installed in or between fire-resistance rated walls, floor or floor/ceiling assemblies and roofs or roof-ceiling assemblies” (i.e. joints between two fire-rated assemblies), these sections are commonly interpreted to also include joints between fire-rated walls and non-rated (unprotected) steel roofs in metal buildings.  This interpretation has led to enforcement of IBC Section 713.3, which requires joint testing in accordance with ASTM E1966 or UL 2079 even with unrated roofs.  However, there is no current standard test method for fire resistance evaluation of HOW joints between a rated wall and an unrated roof.  A more recent Feb. 20, 2009 letter from the International Code Council (ICC)  provides an official interpretation of these IBC sections, which is supplemented by additional explanations and advisory guidelines offered by MBMA (available here).       

 

 

The purpose of this Bulletin is to further explain and highlight the results of this MBMA Project relative to such metal building construction applications with both rated and unrated roofs.  It supplements and supersedes the earlier fire resistive information developed on this subject in 2000-01, as provided in MBMA Insurance Bulletin No. 19.

 

Under the auspices of the MBMA Committee on Fire Protection and Insurance, two wall joint fire tests were successfully conducted in mid-2007 at Underwriters Laboratories, Inc. (UL) and three new UL assembly listings were issued.  These head-of-wall (HOW) joint tests were conducted in accordance with the existing UL 2079 (ASTM E1966) standard for a head of wall joint in a fire-resistive 1-hr wall, except that a non-rated, non-combustible steel roof assembly, typical of single-story metal building construction, was used in-place of a rated assembly required by these standards.           

 

The joint test assemblies replicated common metal building construction conditions for an unrated (unprotected) steel roof with purlin inside the wall, as well as being parallel and perpendicular to the wall. These two HOW joint tests successfully met all the UL 2079 temperature and joint integrity acceptance criteria for a 1-hr fire rating period after Class II movement cycling for 100% compression and extension of the maximum joint width (opening) of 2-inches. 

 

At the end of each test, the unexposed side wall temperatures were substantially below the UL 2079 acceptance limits, the ASTM E119 fire did not penetrate the wall or HOW joint, and the joint successfully passed a hose stream test. The UL Test Report with supporting data is available in the section that immediately follows this, entitled UL Report.  

 

 

Unrated Roofs

 

The most common situation for low-rise metal building construction is that the roofs are not required to be fire resistive; i.e. they are unrated. 

 

For this condition, UL was unable to issue an explicit listing for the actual assemblies as tested, i.e. with an unrated roof, because the current UL 2079 (ASTM E1966) test standard only covers joints between rated wall and rated floor or roof assemblies.  Instead, the three new UL listings issued (HW-D-0488, HW-D-0489, and HW-D-0490) all reference roofs protected with a ceiling membrane.  An explanatory UL letter, dated Feb. 5, 2008, is available in the section following the test report in order to more fully document this application.

 

 

The section drawing of the representative HOW joint detail for a 1-hour fire resistive rating is illustrated above, as given in the 2008 UL Fire Resistance Directory, for HW-D-0488 with the roof purlin being inside the wall.  UL Designs HW-D-0489 and HW-D-0490 similarly show the construction details for a 1-hr HOW joint for the cases wherein the purlins are oriented parallel and perpendicular to the wall, thereby enveloping all possible single story metal building design conditions. Except for the 1-hour protective ceiling membrane identified as “1H” below, this HOW joint reflects the assemblies tested and the commonly occurring field condition for metal buildings. Readers are referred to the 2008 UL Fire Resistance Directory itself for these specific joint assembly listings and all of its construction details. 

 

Several noteworthy aspects of these HOW assemblies include the following:

 

1.       The wall joint detail is representative of current and typical metal building construction practices.

2.       It is applicable for use with a 1-hr rated gypsum board and steel stud wall assembly in any U400 or V400 series design

3.       The roof insulation is continuously draped over the top of wall; it need not be cut and re-attached to the sides of the wall, as had sometimes been required in the past. 

4.       This HOW joint has now also been qualified for vertical joint movement of up to 2-inches up and down from the installed neutral position.  The steel deflection channel within the wall, together with other salient wall construction features that allow for this movement, are fully described in the UL listings.   

5.       Firestop caulking, or another brand bearing the UL Classification mark, as required, is important to the intended fire performance of this joint. 

6.       All the components of these assemblies are generic, except for the fire caulking product.

7.       The UL joint listings cover all the possible design alignments of the roof purlins relative to the rated wall:  within the wall, parallel and perpendicular to the wall.     

 

 

 

Fire-Resistive Rated Roofs

 

In some cases, metal building construction may also be required to have a fire-resistive roof assembly.  For these applications, the HW-D-0488, HW-D-0489, and HW-D-0490 listings in the UL Directory may be directly applied for the rated HOW joint, with inclusion of the “1H” protective ceiling membrane, for conformance with IBC Sections 713.1-713.3.  Readers are referred to the 2008 UL Fire Resistance Directory itself for these specific joint assembly listings and all of its construction details. 

 

These UL HOW listings allow any individual P200 or P500 series, 1-hr rated Roof-Ceiling Assembly to be used with a 1-hr rated gypsum board and steel stud wall assembly in any U400 or V400 series design.  Three available rated roof assemblies, in particular, have been developed through MBMA:  P516, P265, and P268.  P516 employs Type X gypsum board for the protective ceiling, while P265 and P268 use proprietary acoustical panels.

 

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Conclusion

 

In conclusion, a 1-hr. fire resistive HOW joint, which is characteristic of common low-rise metal building construction, was successfully tested at UL in accordance with current standards.  This information has been developed to enable compliance with building code and inspection requirements for both unrated and roof construction. 

 

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