Hail is a form of solid precipitation that grows inside thunderstorm updrafts, where supercooled water freezes around a tiny nucleus and builds up in layers until the stone is heavy enough to fall. Stones 5 millimeters or larger in diameter qualify as hail, and stones of that size routinely dent metal, bruise wood, and strip granules from shingles. When the ice finally melts, runoff can push moisture into flashings, joints, and underlayment, which is the kind of slow deterioration described in water-induced damage to buildings. Because roofs take the first hit, catching hail damage early limits both structural and water-related costs.
How Hail Forms and Why It Damages Roofs
Hail forms through an updraft cycle that repeats dozens of times inside a severe thunderstorm. Warm air near the ground rises, carrying evaporated moisture into colder layers. The droplets freeze, fall back toward the warmer air, collect another coat of water, and rise again to freeze once more. Each trip adds a new ice layer, which is why large stones show concentric rings when cut open. The process depends on strong updrafts, so the largest hail tends to appear in supercell storms.
Hail versus ice pellets
Ice pellets, often called sleet, form when raindrops freeze as they fall through a shallow layer of cold air. They stay smaller than 5 millimeters, stay roughly spherical, and rarely damage roofing because their mass is too low. Sleet bounces harmlessly off a roof, while hail arrives with enough energy to crush the surface of a cedar shake. Hail grows inside the cloud and can exceed 100 millimeters across. The distinction matters on a roof inspection: pellet-sized impacts usually leave no mark, while genuine hail leaves a visible bruise or fracture.
Stone size and impact energy
Impact energy rises sharply with stone size because mass scales with the cube of diameter. A 25-millimeter stone falls at roughly 30 meters per second, while a 75-millimeter stone can approach 50 meters per second. A single severe storm can drop millions of stones, so even a small percentage of direct hits produces hundreds of impact points on one roof. The table below summarizes common size classes and the damage each can produce.
| Stone diameter | Size class | Typical roof effect |
|---|---|---|
| Under 5 mm | Ice pellets | No visible damage |
| 5 to 20 mm | Small hail | Granule loss on asphalt, minor dents |
| 20 to 50 mm | Medium hail | Bruised wood shingles, dented metal, cracked tiles |
| 50 to 75 mm | Large hail | Fractured shingles, punctured underlayment, split shakes |
| Over 75 mm | Severe hail | Structural deck damage, holes through roofing |
Where the damage shows up first
Impact points cluster on slopes facing the storm, along ridges, and around roof penetrations such as vents and chimneys. These zones get hit from multiple angles, and they are also where water finds its way inside once the surface is broken. Inspectors usually work the windward slopes first and then check the leeward side for stones that bounced or rolled.
Roof damage and water intrusion feed each other. A hail-bruised shingle that looks fine in summer can leak in the next heavy rain, so repairs should happen before the wet season rather than after it. Keeping flashings sealed and gutters clear helps limit water-induced damage once the roof surface is compromised.
How to Inspect a Wooden Roof for Hail Damage
Wood shingles and shakes respond to hail differently than asphalt or metal, and a ground-level look is not enough. Hail bruises the soft grain of cedar and pine, leaving depressions that may not split until months later. Work in pairs: one person on the roof, one on the ground, so the inspector can see both faces of every slope. The inspection sequence below works for a first pass; a professional hail damage assessment adds moisture readings and under-deck checks that a ladder inspection cannot reach.
Signs of impact on wood shingles
- Bruised, slightly depressed grain where a stone struck
- Splits or radial cracks radiating from the impact point
- Discolored edges that look water-soaked
- Granule-free or fuzzy areas where the surface fiber was crushed
Step-by-step inspection
- Start from the ridge and work down each slope, checking the storm-facing side first.
- Examine shingles at eye level and feel for soft spots that indicate crushed fiber.
- Check flashings around chimneys, valleys, and vents for dents or lifted edges.
- Look inside the attic for daylight, water stains, or fallen granules.
- Mark every suspect shingle with chalk and photograph it with a scale reference.
Flashings and valleys
Metal flashings dent easily, and a dented valley can redirect runoff under the shingles. After hail, inspect valley metal for creases and check the fasteners holding flashing in place. Lifted nails are a common finding because the impact force flexes the metal. Valley metal should also be checked for granule buildup, since granules washed from the slopes collect where the flow slows.
Roof Types and How Hail Affects Each
Different roofing materials absorb impacts differently, and knowing the failure mode for each helps an inspector decide what is repairable. The same logic that leads engineers to choose flood-damage-resistant building materials for wet sites applies in hail country: pick a surface that matches the local storm risk.
Wooden roofs
Cedar shakes and shingles are among the most hail-sensitive surfaces. The impact crushes surface fiber, and the bruise can take weeks to become visible as the damaged grain dries and checks. Small stones cause cosmetic bruising; stones over 40 millimeters can split a shake in two. A simple field test presses a coin against the suspected bruise; crushed fiber feels soft where sound shingles stay firm. Splits open a direct path for water, and once several shakes are split, spot repairs stop making sense.
Steel roofs
Steel and aluminum panels dent rather than crack. A stone that would split a shake leaves a visible dish in the metal, and the dent can stretch the protective coating, which later rusts. The coating check matters most near dents, because exposed steel in a hail zone can rust through a thin panel within a few seasons. Thin gauge metal over solid decking dents less than the same gauge over open purlins, because the deck absorbs part of the energy.
Asphalt shingle roofs
Asphalt shingles lose granules on impact, and the exposed fiberglass mat weathers quickly once the granules are gone. Impact-resistant shingles with modified bitumen cores pass UL 2218 tests at Class 3 or Class 4 and survive stones that would bruise a standard shingle. Hail damage to asphalt is usually judged by granule loss in concentrated patches rather than by cracking.
What to Do If Your Roof Is Damaged by Hail
Acting in the first days after a storm changes the outcome. The steps below follow the same logic engineers use when they design masonry structures for accidental damage: identify the weak points, protect them, and plan the repair before the next event stresses the building.
Document the damage first
Photograph every impact zone from two angles, include a ruler or coin for scale, and note the date of the storm. Note the storm date from weather records and the local warning that named the storm, since adjusters compare both. Keep receipts for temporary repairs. Insurers evaluate claims on documented evidence, and a photo log taken before any work starts prevents disputes about what the storm actually caused.
Temporary repairs buy time
Tarps over split shakes and sealed flashing joints keep water out while you wait for materials or adjusters. Temporary work should be reversible, since permanent repairs will replace the damaged components anyway.
Repair or replace?
Spot repair works when damage covers less than about 10 percent of the roof area and the deck is sound. Beyond that, replacement is usually cheaper than a string of future repairs, because hail damage is spread randomly across the surface and the undamaged shingles are the same age as the failed ones.
Can Snow Harm a Wood Shake Roof?
Snow itself rarely bruises wood, but it exposes the weaknesses hail created. A shake split by hail absorbs meltwater, and freeze-thaw cycles widen the crack through the winter. Snow also adds load, and while a sound wood roof handles deep drifts, a hail-weakened one may not. The failure modes that matter here mirror those cataloged in types and causes of damage and collapse in reinforced concrete buildings: hidden cracking, progressive weakening, and sudden loss of capacity after years of service.
Snow load on wood shingles
Building codes assign a design snow load for each region, and a roof that passes inspection before a storm usually handles the load afterward. Drifted snow against a chimney or parapet can double the local load, and a hail-damaged deck under that drift is the worst case. The risk is not the load itself but the combination of load and damage: a split shake carries less, and the deck beneath it works harder.
Moisture and freeze-thaw cycles
Wood expands as it absorbs water and contracts as it dries. Hail bruising disrupts the surface so water penetrates deeper, and each freeze-thaw cycle peels another layer of fiber. Keeping the roof clear of heavy snow in spring, when daytime melting alternates with freezing nights, reduces the cycling.
Replacing a Hail-Damaged Roof and Choosing the Next One
When replacement is the right call, the choice of material should reflect the local storm climate rather than first cost. A roof that survives hail without leaking saves its owner the deductible, the tarp, and the adjuster visit.
When replacement wins
Replace the roof when decking is exposed, when more than a quarter of the shingles on any slope are damaged, or when the existing roof is within five years of its expected service life. Ask the roofer for the residual life of the remaining shingles; if it is short, replacement now beats two jobs in five years. Partial replacement of a wood roof is especially risky because new shakes stand proud of weathered ones and collect water at the joint.
Matching the roof to the climate
- Impact-rated asphalt shingles for hail-prone plains states
- Steel with a thick, dent-resistant gauge and a standing-seam profile for wind and hail zones
- Wood shakes only where hail is rare and the owner accepts maintenance
- Tile or slate where the structure can carry the added weight
Whatever the material, the repair sequence stays the same: document, stabilize, then restore. That discipline, borrowed from repairing earthquake damage in structures, keeps a post-storm roof from turning a single hailstorm into a season of water problems.