Underwriters Laboratories Inc. Impact Resistance Testing

UL Standard 2218

       This test, developed and standardized in 1996, was designed for use in evaluating resistance to impact by replicating the impact energy of large hailstones on roof covering materials.
 

      
        The free falling steel balls from these distances generate kinetic energies consistent with similar  diameter hailstones falling at terminal velocities.
 

       Steel balls, while different in composition and geometry than hailstones, were chosen for their test-  consistency and controllable accuracy of impact point. Tests attempting to employ frozen water rather than steel balls generally suffer from lack of control and repeatability and are, therefore, less reliable for comparative analysis use.

       After the test deck assembly has been subjected to all 12 drops, the roof covering is carefully removed and examined on top and bottom surfaces. If the assembly consists of multiple roof covering layers, each layer is examined separately, under 5X power magnification looking for any signs of tearing, fracturing, cracking, splitting, rupture, crazing, or other evidence of product failure.

       Roofing materials vary widely in this test. More fragile materials such as concrete fare very poorly and seldom survive a drop of a 2 inch steel ball from more than 2 to 3 feet. Rubber products and metal products fare quite well against penetration or fracture but rubber is fire susceptible and metal sustains cosmetic damage.

       Ce·DUR Shakes were one of the first products tested in 1997 to the then new impact resistant roofing material performance standard, UL 2218.  Ce·DUR Shakes were classified as a Class 3 material at that time. Improvements since then indicate that a Class 4 rating will be achieved for Ce·DUR Shakes produced in new manufacturing facilities now in the planning stage.

       While UL2218 is the best and most repeatable test for impact resistance of materials, it is not necessarily an accurate measure of impact protection of all roofing configurations. Roofing system designs that employ multiple overlapping layers of material (like wood shakes and Ce·DUR Shakes) provide substantially greater protection from hail than do single layer designs.  A 24 inch long, 1 inch thick butt Ce·DUR Shake installed with normal 10 inch weather exposure provides  1¼ inch of material thickness at the thinnest point and 2 1/8 inches of total material thickness at the thickest point.  Conversely, roof covering materials of cement and metal, provide only one layer of protection and, therefore, no redundant or back-up protection between the weather and the all important underlayment.

        These additional layers of Ce·DUR Shake protection assist in absorbing the impact of falling hail (or, incidentally, the weight of someone walking on the roof) with almost all the impact force being absorbed by the exposed outer layer and very little transmitted to the layer(s) below thus leaving the bottom or lower layer undamaged by penetrating forces.

        In addition to this bonus “protection” provided by multiple layers, a Ce·DUR Shake’s irregular surface assists in breaking up hailstones as they strike the roof, thereby spreading impact forces away from the point of contact and in so doing, reducing roof damage from hail.

Typical Ce·DUR Shake profile