It’s that time of year again for the Northern Hemisphere – Hurricane season runs from June 1 to November 30. As a supplement to our earlier articles dealing with preparing for severe windstorms (see December 2007: FM Approvals RoofNav Tool, July 2005: Windstorm Preparation Guide, May 2002: Severe Weather Watches and Warnings Definitions and July 1999: Hurricanes and Severe Windstorms) we’d like to discuss what could be the most important feature of a wind resistant roof: perimeter flashing.
The majority of roof covering failures resulting from windstorm involve improperly designed or constructed perimeter flashing. If flashing fails, a sizable amount of roof covering will likely be lost, regardless of the adequacy of cover securement. If roof cover securement is also inadequate, the expected loss is even greater.
What exactly is flashing? Perimeter flashing is a component or system of components used to weatherproof and seal the roof cover system at the building edges.
Perimeter flashing (coping and roof edging systems) must be well anchored and of sufficient gauge or thickness to resist anticipated rain, temperature and wind effects.
FM Global Property Loss Prevention Data Sheet 1-49, Perimeter Flashing, is a comprehensive source of information on the subject for industrial, commercial and institutional buildings. It includes 18 figures showing applicable flashing system designs for various roof cover systems and arrangements.
Differing flashing details are suggested for built-up and single-ply membrane roof coverings; however, the basic details are the same:
- Wood nailers should be securely fastened using appropriately sized bolts, screws, etc. depending on the wall type.
- Continuous hook strips should be secured to the wood nailers using appropriate fasteners (corrosion resistant, minimum No. 8 screws) spaced according to the anticipated wind uplift pressure. Hook strips should be at least one gauge heavier than the fascia metal. Fascia metal should then be securely crimped over the edge of the hook strip.
Retrofit improvements are also available for existing flashing/coping systems without having to dismantle the entire existing system. Many times it involves installing minimum No. 8, corrosion resistant, screws and neoprene washers through the lower third of the fascia into the wood nailer. The screws should be applied at a specific spacing (usually a maximum of 24 in. on center) and should penetrate the wood at least 3/4 in. This is just an example for a typical edge cant arrangement. Other retrofit improvements exist for alternative flashing arrangements.
In addition to the recommendations in FM Global Property Loss Prevention Data Sheet 1-49, FM Global also approves certain perimeter flashing systems. These perimeter flashing systems are FM Approved for use with the wind uplift classifications noted.
All FM Approved roof assemblies must be installed using a perimeter flashing system capable of withstanding the anticipated loads. When an FM Approved perimeter flashing system is unavailable, the flashing must be designed to withstand the following minimum loads:
Uplift Load (psf)
Outward Load (psf)
If you would like an analysis of the level of windstorm preparedness and protection at your site, you may contact Risk Logic Inc. for a professional consultation. Risk Logic can also review plans and drawings for new construction and roofing systems to ensure they are properly designed with regard to windstorm resistance. These are services offered to all locations under contract in Risk Logic’s property loss control program.
FM Global Property Loss Prevention Data Sheet 1-49
FM Approvals RoofNav