Metal ceiling systems can easily accommodate ventilation measures for cooling the plenum and subsequently the roof systems—the panels can be perforated or vents can be installed in them. In the case of linear systems, perforations can be used and/or the spaces between panels can be left open for ventilation.
Made of non-combustible material, exterior metal soffit systems are typically Class A fire-rated and rarely included in Underwriters Laboratories (UL) time-rated designs. While aluminum is inherently corrosion resistant, care should be taken when specifying materials and finishes in coastal areas. A metallurgist and manufacturer should be consulted before using these systems in warm, humid environments where salt spray occurs. Also, in areas where there are wide temperature swings, specifiers must consider thermal expansion in their design.
Different metals and materials expand and contract at different rates. For example, the linear expansion co-efficient of the International Alloy Designation System’s 3000 series aluminum is approximately 23.4 x 10-6 cm/cm – C (13.0 x 10-6in./in.- F). This equates to about a 5 mm (0.2 in.) change in length in a 15.2-m (50-ft) run of metal when there is a temperature change of 4 C (25 F).
Codes and criteria
Regardless of size, location, or occupancy, exterior metal soffits should meet necessary building codes and criteria. The 2012 International Building Code (IBC) and 2010 Florida Building Code (FBC) are two recent examples where wind load design requirements and related descriptions have been updated. These revisions are largely due to the 2010 edition of American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7, Minimum Design Loads for Buildings and Other Structures.
In particular, ASCE/SEI 7-10 revised the methodology and terminology used for establishing basic wind speed. These revisions include:
- Strength Design (SD), or ultimate, wind speeds replacing the former use of Allowable Stress Design (ASD) wind speeds (three difference wind speed maps are provided);
- deletion of Importance Factors, previously used to determine basic design wind speed;
- switching of Exposure Category for hurricane-prone regions from Exposure C to D (2012 IBC also notes an exception for Office of Statewide Health Planning and Development [OSHPD] projects to comply with Exposure C, unless justification can be made for Exposure B); and
- changes to the building classifications from the previous Occupancy Category to the revised
Risk Category.
The authority having jurisdiction (AHJ) determines the building’s Risk Category. ASCE 7-10 uses three basic wind speed maps to determine a buildings’ Risk Category. The category designations are:
- Category I: buildings representing a low risk to human life in the event of a failure;
- Category II: most residential, commercial, and industrial buildings; and
- Category III and IV: buildings where failure could pose a substantial risk to human life or essential service facilities.
A registered architect or professional engineer should review and determine the wind loads. Based on revised calculations and wind speeds shown in ASCE/SEI 7-10’s updated maps, these speeds can be converted into appropriate load requirements for components and cladding. Building codes’ components and cladding sections typically address exterior soffits.
Construction projects must meet a Seismic Design Category (SDC) as specified by an engineer or architect. Each category determines specific product performance and installation methods required by code to withstand certain levels of seismic activity. (For more on earthquakes and soffits, see “Specifying Seismic Ceiling Safety” by Tony Ingratta, in the September 2012 issue of The Construction Specifier. To read the article, visit www.constructionspecifier.com and select “Archives.”)
Photos courtesy Chicago Metallic Corporation
Positive and negative pressures
Specifiers should request metal ceiling manufacturers’ wind load technical data for the complete exterior soffit system, including suspension, panels, and structural supports. Also, laboratory test reports and assembly designs indicating both positive and negative air pressures should be requested.
When wind interacts with a building, both positive (uplift) and negative (suction) pressures simultaneously occur. These pressures also affect allowable carrier and strut spacing for metal ceiling systems in exterior soffits. Spacing requirements vary for different panel sizes, material thickness, manufacturers, and applications.
Collaborating with the manufacturer early in the design and specification process minimize confusion, ensuring long-lasting performance. By communicating the desired aesthetic, performance, schedule, and budget requirements early in the project, the manufacturer can ensure the product is engineered to meet these needs and can work with the installing contractor to guide them in proper installation, documentation, and inspection. UL conducts laboratory testing of metal ceiling systems for exterior soffits to verify a manufacturer’s claims meet industry standards for resistance to uplift forces.
