In heating-dominated northern climates, roof designers should consider using black EPDM in fully adhered assemblies where the top layer of cover board is set in urethane adhesive—beads or full-coverage sprayed polyurethane foam (SPF). From a design standpoint, this assembly has many built-in safeguards, like the durability of non-reinforced EPDM without any hard insulation fastening plates or fastener heads directly under the membrane.
For cooling-dominated southern climates, a white EPDM fully adhered assembly would be the logical choice. Additionally, a ballasted roof assembly utilizing black EPDM is a great choice for hail resistance in both northern and southern markets. Further, roof cover durability in hail events is linked to membrane thickness. Design professionals should consider specifying greater roof membrane thickness.
From empirical observation by the authors and witnessed in the testing, there are several key characteristics of roof systems that are designed for hail resistance:
- employ non-reinforced roof covers with greater thickness and protection (e.g. 2286 instead of 1524 µm [90 instead of 60 mils];
- provide high-density support for the roof cover (i.e. do not allow the membrane to be depressed under hail ball impact);
- place any screw fasteners and stress plates below the cover board, not the roof covers;
- remember ballasted roof systems provide the greatest protection; and
- specify roof covers with a history of general in-situ performance (i.e. 20 years) to ensure performance as the membrane ages and is exposed to ultraviolet radiation and various climatic conditions.
Jim D. Koontz, PE, RRC, has been involved in the roofing industry since 1960 and began testing roofing materials in 1976. President of Jim D. Koontz & Associates, he has experience as a roofer, estimator, consultant, lecturer, researcher, and expert witness. Koontz has written numerous articles relating to roofing material/product research, including research on single-ply products and hail/wind impacts. He can be reached at jim@jdkoontz.com.
Tom Hutchinson, AIA, FRCI, RRC, is a principal in Hutchinson Design Group. A licensed architect and registered roof consultant, he specializes in roof design, contract document preparation, specifications, inspections, and the determination of moisture penetration and failure of existing roof system. Hutchinson is a former president of RCI Inc. He is also a Certified Energy Professional in Chicago and secretary of the Conseil International du Bâtiment/Réunion Internationale des Laboratoires d’Essais et de recherches sur les Matériaux et les constructions (CIB/RILEM) International Joint Committee on Roof Materials and Systems. Hutchinson can be contacted via e-mail at hutch@hutchinsondesigngroup.com.
