How big hail needs to be to cause roof damage depends on your roofing material. According to the International Association of Certified Home Inspectors (NACHI), three-tab shingles are the most fragile and can be damaged by hail 1 inch in diameter. Heavy wood shakes and thick fiberglass may get damaged when hail reaches 1.25 inches, or about golf ball size. Concrete tiles may get damaged when hail reaches 2 inches in diameter.