In this article · 6 sections
A hail alert that says "hail possible" gives a roofer nothing to estimate from. A size class does: it tells you which streets to inspect first, what the inspection is likely to find, and what record the adjuster will ask for.
The three classes Predictive Hail uses
Predictive Hail predicts a size class for every 1×1 km cell up to 55 minutes before hail reaches a site, refreshed every five minutes, and records the observed class as the storm passes. There are three classes, each a lower bound on the largest stone expected in the cell.
| Attribute | Severe | Damaging | Destructive |
|---|---|---|---|
| Threshold | 0.75 in and up | 1.00 in and up | 2.00 in and up |
| Size of | A penny | A quarter | A hen egg |
| Who Flash alerts | Roofing crews, agronomists covering crops | Fleet managers, claims adjusters, roofing sales | Everyone on site, plus horn and strobe relays |
Severe
- Threshold
- 0.75 in and up
- Size of
- A penny
- Who Flash alerts
- Roofing crews, agronomists covering crops
Damaging
- Threshold
- 1.00 in and up
- Size of
- A quarter
- Who Flash alerts
- Fleet managers, claims adjusters, roofing sales
Destructive
- Threshold
- 2.00 in and up
- Size of
- A hen egg
- Who Flash alerts
- Everyone on site, plus horn and strobe relays
A storm is rarely one size. Most hailstorms drop a mix, and the class describes the largest stones expected in the cell, not every stone that falls in it. That matters on the roof: a cell classed damaging will also have put a lot of smaller hail on the same shingles.
Where the National Weather Service draws the line
Since 5 January 2010 the National Weather Service has issued Severe Thunderstorm Warnings for hail of 1 inch or larger, quarter size. Before that the line was 3/4 inch. The NWS gave two reasons for raising it: research showing that significant damage to property generally starts at about 1 inch, and concern from emergency managers and the media that warnings for marginal hail were teaching the public to ignore them. The old 3/4-inch figure dated from a 1952 study of the hail that damages aircraft in flight.
So the damaging class lines up with today's NWS severe criterion, and the severe class sits below it, at the old 3/4-inch mark. That lower class is deliberate for roofing. Stones under an inch rarely make a warning, but they still reach your customers' roofs, and the next section explains why they still matter for an estimate.
What each class means for the inspection
- Severe (0.75 in and up)
- Below the NWS warning line, and easy to write off. IBHS testing found that sub-severe hail, stones under an inch, knocks the protective granules off asphalt shingles and exposes the asphalt beneath to sun and weather. Shingles that had natural weathering and two simulated sub-severe hail events became up to ten times more susceptible to damage from later large hail than new shingles. Damage from a severe-class cell is often a question of granule loss rather than an obvious break, and a roof that has taken several of these storms may perform worse in the next big one.
- Damaging (1.00 in and up)
- The size at which the NWS says significant property damage begins. Typical severe-storm stones of 1 to 1.75 inches fall at roughly 25 to 40 mph, according to NOAA’s National Severe Storms Laboratory. This is the class where a full inspection is worth scheduling, not just a look from the ground.
- Destructive (2.00 in and up)
- NSSL estimates that stones of 2 to 4 inches fall at roughly 44 to 72 mph, and that wind-driven hail can tear siding, break windows and blow into houses. At this size the inspection has to look well beyond the shingles: siding, windows and anything else facing the wind.
What an inspection checks, whatever the class
The Insurance Institute for Business & Home Safety (IBHS) publishes a short homeowner guide on telling hail damage from everything that looks like it. It is a useful checklist for a crew writing an estimate that an adjuster will have to accept.
- Find the collateral first. Dents in metal flashing, dents in the fins of the A/C unit and spatter marks on metal around the building are good signs that hail fell there. IBHS notes that spatter is not damage in itself: it is hail knocking oxidation or dirt off metal.
- Rule out the look-alikes. Installation marks, mechanical damage, blisters, manufacturer defects and natural weathering can all pass for hail. General granule loss along shingle edges is a common one.
- Look for the pattern. Real hail damage falls in a random pattern, and on shingles a bruise can usually be felt at the point of impact.
- Check the ridge caps. IBHS lists them among the most hail-vulnerable parts of a shingle roof.
For replacement work, IBHS recommends roof coverings rated Class 4 under UL 2218 or FM 4473, and warns property owners to be wary of unsolicited offers to inspect or repair. An estimate backed by a dated storm record reads very differently from a knock on the door.
After the cell
The class says where to look. The inspection says what happened.
The record that goes with the estimate
Adjusters ask when the hail fell, where, and how big it was. Flash keeps a timestamped, cell-level hail record for every address in the swath, updated every five minutes through the event. After the storm, the swath report exports as PDF and PNG with the cell, the size class and the timestamp, and the adjuster gets the same record you do.
The size class does not replace the inspection. It tells you which streets to inspect first and gives the claim file a record of the storm that nobody has to reconstruct from memory. How the classes feed alerts, canvassing and claims is set out on the roofing page.
Frequently asked
The National Weather Service treats 1 inch, quarter size, as the point where significant property damage begins. IBHS testing shows that hail under an inch can still strip granules from asphalt shingles and leave them more vulnerable to later large hail.
For National Weather Service warnings, 1 inch or larger, since January 2010. It was 3/4 inch before that. Predictive Hail’s lowest class starts at 0.75 in, so roofers see the smaller storms too.
No. It records how large the hail in a 1×1 km cell was and when it fell. Whether a particular roof was damaged is decided by the inspection.
Yes. The swath report exports as PDF and PNG with the cell, the size class and the timestamp for every address in the event.