Large, Flat Roof Systems

For large projects such as grocery or big box retail stores—which can have grid dimensions of 45 feet by 50 feet or larger—panelized roof systems can be a cost-effective solution. According to one roof erector, panelized wood roofs on the West Coast can save $1.25 to $1.50 per square foot over conventional steel joist metal deck systems.² Savings can be attributed to, among other things, material costs, speed of installation, and insulation. This is also supported by the cost comparison described in the sidebar, Big Box Retail: Wood Saves Nearly $1 Million.

In North America, both all-wood and hybrid panelized roof systems are common. All-wood systems tend to be used for spans of 40 feet or more and are particularly well-suited for applications where conveyer equipment is hung from the roof structure or in food processing facilities that need to minimize dust from overhead joists. They are also a good choice for developers and designers who want to take advantage of wood’s aesthetic benefits for an exposed roof structure.

In a typical all-wood system, sheathing or wood structural paneling spans between sub purlins, which are 2-by-4 or 2-by-6 dimension lumber, depending on the span. Sub purlins span between purlins, which can be glulam, I-joists, metal-plate connected wood trusses, or wood flange metal web trusses, and the purlins span between girders, or girders and exterior walls. Girders are typically glulam beams, either simply supported or with cantilevers. That said, there are many possibilities for the design of large, flat wood roofs, including options with no sub-purlins but longer-spanning decking products, such as tongue and groove timber or laminated decking and mass timber.

Parapets are a common feature of many flat roofs, providing space for signage and hiding mechanical units. Framing options include:

• Tall stud parapets, where the wall framing extends past the roof frame so the parapet is a continuation of the studs themselves
• Built-up parapets, where the studs stop at the top of the wall and the parapet is built on the roof (requires bracing)
• Truss parapets, where the parapets are built into the trusses themselves during manufacture

For large, flat roof applications, unlimited areas can be achieved under IBC Section 507. However, designers of low-rise commercial and industrial applications may also design large roofs under standard provisions of IBC Chapter 5 using sprinkler increases and the sub-notes included with IBC Table 601.

The SDPWS includes information on high-load diaphragm design useful in panelized roofs. High-load wood diaphragm design with multiple rows of nails (accommodating ASD capacities up to approximately 1,500 plf for seismic and 2,000 plf for wind) is possible in configurations using tall and/or heavy wall systems, especially in combination with very large diaphragms.

End notes

¹“Big Box Retail: Wood Saves Nearly $1 Million,” WoodWorks, 2015, www.woodworks.org/wp-content/uploads/ Big-Box-Retail-Wood-vs-Steel-Oct-2015. Pdf
²“Faster, Safer, Lower Cost: Panelized Roof Systems,” WoodWorks, 2012, www.woodworks.org/wp-content/uploads/ IS-Panelized-Roofs.pdf