Controlling thermal movement in metal panel roofs

November 10, 2016

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Photo © Birgstock.com

By Maciek Rupar
The term ‘thermal movement’ refers to a material’s dimensional changes resulting from temperature swings. A material experiencing a rise or decrease of temperature expands or contracts, respectively—this poses an interesting challenge when it comes to designing metal panel roof assemblies.

The net change in any one dimension is proportional to the net temperature change; this material-specific property is called the co-efficient of linear expansion (CLE). Figure 1 shows CLEs for metals commonly used in roofing.

Thermal movement may produce stresses in metal panel roof assemblies for several reasons. Some of these include:

The 2016 NRCA Roofing Manual: Metal Panel and SPF Roof Systems provides the National Roofing Contractors Association’s best-practice recommendations for designers of metal panel roof assemblies. This article is based on the treatment of metal panel roof system thermal movement found in the manual.

figure-1
This table lists expansion co-efficients of various metals that are commonly used in roof systems.
Image courtesy YMCA

Examples
Stresses produced when thermal movement is restrained can be significant. To appreciate the significance of the forces involved, it is helpful to go through an example calculation, step by step.

Example 1
The formula used to calculate a roof panel’s change in length over a temperature swing should be considered as the following equation:

ΔL = a x L x ΔT

In this equation, ‘ΔL’ is the net length change, ‘a’ is the coefficient of linear expansion, ‘L’ is the panel length, and ‘ΔT’ is the expected temperature change.

For a hypothetical scenario, a 12-m (40-ft) panel is formed from a 609-mm (24-in.) wide sheet of 24-gauge steel. The panel is expected to experience service temperatures from –18 to 120 C (0 to 160 F). The linear expansion coefficient for steel (a) is 0.0000067 in./(in.∙F) (or, 0.000012 m/[m∙C]). The net change in length for an unrestrained panel (ΔL) is then calculated as:

ΔL = 0.0000067 in./(in.∙F) x (40 ft x 12 in./ft) x 160 F
= 0.5146 in. (i.e. 13.07 mm)

This value is theoretical. It only works for a panel that is free to expand while supported by a substrate allowing for frictionless movement. If the same panel is prevented from moving by having both ends fixed, it will be under stress from an expansion force (F) proportional to ΔL acting at its fixed ends. The magnitude of F is calculated from the Young’s modulus (E) equation, which is a measure of material stiffness. The value for steel is 200 GPa (29 million psi).

E = stress/strain = (F/A)/( ΔL/L)

In this example, ‘A’ is the panel cross-sectional area. Rearranging the equation, we have:

F = (E x ΔL x A)/L
= (29,000,000 lb/sq.in. x 0.5146 in. x 24 in. x 0.025 in.)/
(40 ft x 12 in./ft)
= 18,654 lb (83 kN)

This value also is theoretical. The calculation assumes the panel is the only element affected by thermal movement. It does not consider thermal movement of the structure restraining the panel at its ends.

The net panel length change and expansion force values in this example reflect idealized conditions not encountered in practice. In real-world applications, the amount of thermal movement is less than the calculated theoretical value, as some of the expansion force is taken up by friction, binding of the clips, flexing of the panel, and rolling of the framing. Field studies have determined this reduction can be as much as 20 per cent below the theoretical value for most systems. Further, from the design perspective, the thermal movement of interest is the panel displacement with relation to the displacement of the substrate, rather than the total panel length change. Thus, calculations need to account for substrate movement in addition to panel movement.

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It is important to consider panel-to-structure or panel-to-substrate attachment, panel-to-panel connections, panel-to-flashing junctures, and flashing-to-structure or flashing-to-substrate attachments when determining thermal movement.
Photos © Bigstock.com

Example 2
For the purpose of this example, a construction where structural steel panels are attached to steel framing will be used. The temperature differential of interest is equal to the greatest expected difference between the panels’ and structure’s temperatures. In a conditioned building with blanket roof insulation, the roof framing is at design interior temperature while the panels are exposed to outside design conditions and subject to solar radiation temperature gain. For this scenario, the following formula provides roof panel thermal movement in inches:

ΔL = 0.0000804 x |T2 – T1| x L

In this case, 0.0000804 is the linear expansion co-efficient for steel reported in in./(ft∙F). This value is obtained by multiplying 0.0000067 in./(in. F) by 12 in./ft. The greatest expected difference between the steel panel temperature and steel framing temperature is |T2 – T1|. L is the panel length measured in feet.

For example, a metal panel roof on a conditioned building in Calgary uses 12-m (40-ft) steel roof panels attached to steel framing with blanket insulation draped over Z-purlins. The design temperature differential is calculated from the interior design temperatures and the minimum and maximum expected panel service temperatures:

Design best practices
The design of metal panel roof assemblies must accommodate thermal movement and sliding forces, typically called ‘drag loads.’ Thermal movement must be considered as it relates to:

Drag loads are caused by gravity and snow loads that force metal panels to move downslope. Metal panel roof systems need to be fixed to resist drag loads, but must allow for movement to accommodate panel expansion and contraction.

Panel thermal movement is typically allowed to occur in one of two directions: downslope or upslope. This is accomplished by providing a fixed point of attachment at either the hip and ridge or eave and valleys.

When panels are fixed at a ridge, they should not be fixed at valleys. When a metal panel is fixed at eaves and valleys, fasteners at the ridge cannot be attached to the structure to create a fixed point. Penetrations such as vent stacks and roof curbs should be installed so panels are not fixed to the substrate at these locations. Providing more than one fixed point for metal panels restricts panel movement, potentially resulting in deformation or damage.

In cases where site-formed curved panels span eave-to-eave, panels may be fixed in the middle with both ends free to move. In this example, maximum movement at each end of a panel will be half of what it would be if a panel was fixed at one end and all movement were directed to the other end. The fixed location should be a horizontal point shared by all panels so their movement is consistent from one to another.

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This side panel may be subject to different thermal expansions than the overlying top panel because of exposure to a greater temperature shift.

Movement along the length of a panel from ridge to eave is an important concern with most standing-seam metal panel roof systems. Long panels can move several inches or more depending on the type of metal, actual length, and temperature change. Clips must be designed to allow the panels to slide back and forth, or to accommodate the movement of the panels within their design by flexing, rolling, or sliding (i.e. with two- or three-piece clips). With expansion clips, the base portion of the clip is fastened to the deck or structural member and allows the upper portion of the clip—which is attached to the panel—to slide or ‘float’ with panel movement.

Horizontal movement of a panel is less of an issue because the seam-to-seam width of most standing-seam panels is relatively small. Some standing-seam panels have small intra-panel ribs, referred to as ‘stiffening ribs.’ The flat-pan portion of a panel can absorb some movement by bowing or distorting. However, significant movement through a pan is not a preferred method because of its effect on the appearance of a roof, and a possible loss of watertight integrity at the eave or ridge when the metal bows.

Flashing attached to a building or structure must be either securely fastened to restrain movement or designed to provide for movement without damaging the metal or fasteners. Flashings attached to metal roof panels or the structure must be installed with a slip-type connection or be designed with a multiple-bend profile allowing the flashing to flex and accommodate metal movement.

Rake-edge flashing can be detailed in one of two ways to accommodate thermal movement. It may be fixed to the substrate and an end-panel pan clipped to or locked to engage the flashing, or it may be fastened to the end panel and locked to engage a continuous cleat fastened to the substrate.

Detailing for eave flashing, gutters, and any other continuous longitudinal elements should be designed to accommodate thermal movement. If the structural substrate includes an expansion joint, the panels and longitudinal trim also should include an expansion joint at the same location.

Where panels are fixed at their low point, ridge and hip covers should be designed to accommodate movement as panel ends at the ridge and hips move closer together and away.

Where panels do not span from ridge to eave and end-laps are necessary, fastening should be sufficient for end laps to perform as continuous members with regard to thermal movement transfer.

Conclusion
Metal panel roof systems provide long-lasting coverage for industrial and commercial buildings, which is why it is imperative the panels be designed and placed with thermal movement and sliding forces in mind. Designers should calculate the expected expansion and contraction and account for this with their panel fixations.

maciekMaciek Rupar is the technical director for the National Roofing Contractors Association (NRCA) and has been with the association since January 2008. He holds a degree in materials science and engineering from the University of Illinois in Urbana-Champaign. Before joining NRCA, he had held technical positions with two roofing materials manufacturers for seven years. Rupar is responsible for responding to requests for technical assistance, maintaining and developing technical documents, and performing staff liaison duties for several NRCA technical committees. He can be reached at mrupar@nrca.net[1].

Endnotes:
  1. mrupar@nrca.net: mailto:mrupar@nrca.net

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