Designing metal ceilings for exterior soffits

UL ratings
Per UL’s classification for “Roofing Systems, Uplift Resistance” (i.e. TGKX):

Roof Deck Constructions Classified for Uplift Resistance have been investigated for damageability from both external and internal pressures on the deck associated with high velocity winds. …ANSI/UL 580. The ANSI/UL 580 test method subjects a 3.05 x 3.05-m [10 x 10-ft] test sample to various static and oscillating air pressures to index performance under uplift loads imposed on roof decks.

The magnitude of the wind velocity across a roof deck and the resulting uplift pressures on a roof deck are dependent upon many factors such as wind gusts, the shape of the roof deck, edge configuration and the landscape surrounding the roof deck installation. (Search “TGKX Guide Info Roof Deck Constructions.”)

A UL 580 Class 30 designation indicates a system has been tested and will resist a 1436-Pa (30-psf) wind pressure (positive and negative) when installed in accordance with ASTM C636-04, Standard Practice for Installation of Metal Ceiling Suspension Systems for Acoustical Tile and Lay-in Panels, and the manufacturer’s instructions. In addition to laboratory testing, UL audits the company’s manufacturing process to ensure the products are made as tested.

As an example of a Class 30 UL-rated exterior metal soffit system, UL Construction No. 630 details the following components (asterisks mark those components bearing the UL classification mark) (This information was taken from UL Construction No. 630, “Roof Deck Constructions.”):

Steel framing members^*
The system used to support the soffit consists of the following components:

• main runner: ^* nominally 3.6 m long by 19 mm wide (12 ft by 3/4 in.) spaced 610 mm (24 in.) on center (oc) supported by hanger wire (Item 2) spaced 1220 mm (48 in.) oc;
• cross tees: ^* nominally 610 mm (24 in.) long; spaced 1220 mm (48 in.) oc; installed perpendicular to main runner;
• perimeter channel: ^* channel-shaped 63.5 mm (2 1/2 in.) deep with 19-mm (3/4-in.) legs; 0.6-mm (0.02-in.) thick aluminum; fastened to walls with 25-mm (1-in.) long steel screws spaced maximum 610 mm (24 in.) oc;
• main runners (Item 1A) and cross tees (Item 1B) attached to top leg of channel using one #7 minimum size, 11-mm (7/16-in.) screw into bulb of component;
• hold-down insert: channel-shaped hold-down insert 63 mm (2 15/32 in.) deep with 19-mm (3/4-in.) legs, 0.6-mm (0.02-in.) thick aluminum, nested inside perimeter channel (Item 1C) to hold down perimeter panels;
• inserts fastened to wall using two 25-mm (1-in.) long steel screws spaced 305 mm (12 in.) oc; and
• downbrace: as shown in Figure 1, downbraces are spaced 1219 mm (48 in.) oc along main runners and fastened to main runner with two 4.2-mm (0.17-in.) 8-gauge by minimum-sized 12.7-mm (1/2-in.) self-drilling screws; the downbrace can be notched to fit over bulb of main runner as an alternative to method for uplift applications (Item 1Eb), and the optional downbrace clip used in conjunction with downbrace and main runner must be fastened with six 3.8 mm (0.15 in.) 7-gauge by 11.1-mm (7/16-in.) screws.

Hanger wire
Located directly over main runners spaced 1219 mm (48 in.) oc, the hanger wire is fastened to supports (Item 3) with an eyebolt or other appropriate connector the diameter of wire is to be 2.7 mm (0.12 in.), 12 gauge. Hanger wire must coincide with compression post (Item 1E) locations.

Supports
Supports are any structural steel, wood, or other type member capable of supporting soffit construction at a spacing as required to support soffit load and withstand uplift load.

Panel^*
Panels must be 610 x 610 mm (24 x 24 in.), 0.5-mm (0.02-in.) thick steel, or 0.7-mm (0.03-in.) thick aluminum. End-joints are perpendicular to main runners (Item 1A). Side-joints are centered on the face of the main runners. Panel side-joint legs snap into face of main runners.

Panel retention clip^*
The panel retention clip must have a minimum diameter of 1.6-mm (0.06-in.) steel wire. Clips fasten panel end-joint legs to main runners and occur at all panel end-joint leg intersections with main runner.