Designing commercial low-slope roofing to resist the forces imposed by wind is critical to the roof’s longevity. This is especially true in regions frequently impacted by high wind weather events.
The building code lays out the requirements for roof wind design. The short-and-simple version is this:
- The designer identifies the loads expected to be imposed by the wind (e.g. pounds per square foot).
- The manufacturer tests assemblies made of its products to determine their capacity to resist applied loads.
- The contractor installs an assembly of products that has been tested (by the manufacturer) and demonstrated to be strong enough to resist the imposed loads.
This process usually results in specific installation requirements for the roofing such as:
- a certain number of fasteners per insulation board
- a certain type of adhesive
- a certain adhesive ribbon spacing
These requirements usually vary for different parts of the roof, since the wind varies across the roof.
One part of this process that is often overlooked, however, is the roof edge securement.
For more on roof wind design check out our podcast episode with the folks from GAF. We’ve also included a bunch of helpful links in the show notes.
The edges of the roof are usually secured with some type of metal fabrication, generally referred to as “edge metal”. The edge metal can be fabricated in a shop from coil stock or it can be a manufactured product made specifically for that purpose.
The edge metal is particularly important because roof failures often start at the perimeter.
…recent post-hurricane investigations by the Roofing Industry Committee on Weather Issues (RICOWI) and investigations of losses by FM Global consistently showed that, in many cases, damage to a low-slope roof system during high-wind events begins when the edge of the assembly becomes disengaged from the building.Roofing Magazine
Recognizing this risk, the building code requires edge metal to follow the same general process described above for the roofing in general. In Massachusetts, the applicable part of the code is 1504.5 in the 2015 IBC, which states in part:
…roof system metal edge securement, except gutters, shall be designed… for wind loads in accordance with Chapter 16 and tested for resistance in accordance with Test Methods RE-1, RE-2 and RE-3 of ANSI/SPRI ES-1…IBC 2015
The emphasis is mine, to differentiate between designer responsibility (italics) and manufacturer responsibility (bold).
There are many products manufactured that carry ANSI-SPRI ES-1 test ratings, and we generally recommend using one of these products for roof edge securement. Keep in mind: just being “rated” is not necessarily enough—the actual rating needs to meet the design requirements.
A complication can arise, however, when the job calls for shop-fabricated edge metal. How can one determine if shop fabricated edge metal meets the code requirements?
Luckily, the NRCA has performed testing on a variety of edge metal profiles and installation methods and publishes the results on their website. The NRCA testing and resulting edge metal details provide solid guidance when using shop-fabricated edge metal.
Roofing Edge Metal Summary
- Edge metal is an essential component in a roof’s ability to resist damage from wind forces.
- The edge metal should be designed and installed in accordance with engineering principles and building code requirements.
- Manufactured products exist that have been tested to determine their capacity to resist wind loads.
- The NRCA has tested various shop-fabricated edge metal profiles and installation methods to determine their capacity to resist wind loads.