If hail impacts a roof covering and the effects of this impact result in some loss in the material’s overall integrity or functionality, this material is classified as “hail damaged.” More specifically for cedar shakes and shingles, valid hail damage is typically a new fracture of the cedar corresponding to a clean spot or scuff mark from the hailstone impact.
While the definition of hail damage appears to be very straightforward, the physical characteristics and inconsistent nature of wood shake roof coverings are conducive to misidentification of damage (i.e. splits, missing shakes, and age-related deterioration) for this type of roof covering by even the most veteran of investigators. Hail damage to wood shakes can be identified by a hail impact on or near a fresh split in the shake itself. The size of the hailstone that can cause damage to wood shakes can vary dependent upon the condition of the wood shake. However, independent testing results have identified hailstones less than 1¼ inches in diameter generally lack the kinetic energy required to cause functional or structural damage to sound wood shakes.
A commonly misidentified cause of distress on wood shakes is damage due to foot traffic, often damage incurred during investigation into whether the wood shakes have been damage by a hail event. Damage as a result of foot traffic often has the same physical characteristics (i.e. fresh split in the shakes) as damage caused by hail impact, less one very important component, lack of evidence of a hailstone impact on the distress shake or evidence of impact by hailstones not capable of damaging sound wood shakes. Wood shakes are inherently susceptible to damage (splitting or fracture) due to foot traffic. This is the result of the exposure to ultraviolet (UV) rays and the natural curing process of the wood shake. This process removes the natural oils (tannin) from the wood shakes causing the shakes to curl or cup. This also produces a dry, brittle material that when an external force is applied (i.e. pressure from one’s foot) can and will result in a fracture.
Other commonly misidentified blemishes include clean spots (area on stained wood shakes where hail has temporarily cleaned the area), dents and gouges due to hail impacts, hail impacts resulting in butt-end crushing, poor installation practices, and age-related deterioration.
Clean spots occur as a result of hail impact. These are temporarily cleaned areas on stained wood shakes where the hailstone has impacted. Generally speaking the size (or diameter) of the clean spot will correlate to the size of the hailstone.
Hail impacts resulting in dents or gouges are common after a weather event. These dents and gouges (that do not result in a split in the shake) are cosmetic blemishes that do not compromise the functionality or structural integrity of the wood shake. Additionally long-term natural weathering studies following impacts have confirmed that impact marks without initial splitting do not create a potential for future splitting.
Hail impacts resulting in crushing of the butt end are very commonly identified as “hail damaged”. Despite their unsightly nature and unappealing appearance these blemishes are also considered cosmetic in nature and do not detrimentally affect the structural integrity or functionality of the wood shake.
Installation and general maintenance of wood shakes roof coverings are difficult and costly. When spot replacement is completed as part of a general maintenance practice it is common to see the wood shakes being installed to be face nailed. As a result of the improper installation the shake will split, and consequently the functionality and structural integrity of the shake has been lost.
Finally, weathering and, age-related deterioration and natural splitting is inevitable with this type of roof covering. As the wood shake sheds water the run-off pattern will weather some areas of the shake more than others (red). This results in a thinning of the material and ultimately loss of functionality. Additionally, as the wood ages the natural grain of the wood results in a split and deterioration of the ends of the shake (white). This natural deterioration is often misidentified as the result of hail impacts.
Hail damage to wood shakes can be difficult to identify, but if one knows the indicators to recognize it can easily be distinguished from the number of other causes of distress to wood shake roof coverings.
 “What are the effects of hail on residential roofing products?” Jim D. Koontz, P.E.