Drying Classes 1–4.
How wet, how hard, how long.

Two completely different dimensions of the same loss. Category is about CONTAMINATION (Cat 1 = clean, Cat 2 = gray, Cat 3 = black water) — it determines whether you can save the materials, what PPE the crew needs, and what the disposal protocol is. Class is about HOW MUCH IS WET (Class 1 = least, Class 4 = specialty / deep cavity) — it determines how many air movers and dehumidifiers you need to deploy and how long drying will take. A single loss has both: e.g., a Cat 2 Class 3 loss is gray water that has saturated walls + carpet + ceiling. Both classifications are logged on day one.

The least-extensive Class — only part of a room is affected, and the wet materials have minimum moisture absorption (low-porosity surfaces like sealed concrete, hardwood that hasn't been saturated, or vinyl flooring). Limited evaporation load means relatively small equipment count: typically 1-2 air movers and one small dehumidifier. Drying timeline 2-4 days in a typical case.

An entire room is affected — walls and floors carry significant moisture. Wet carpet and pad, drywall to typically 12-24 inches up the wall (the visible wet line), and possible material absorption into structural elements. Higher evaporation load means more equipment per square foot than Class 1. Drying timeline 3-5 days with proper psychrometric setup.

The greatest evaporation load — water reached overhead. Saturated ceilings, walls, insulation, and flooring throughout multiple rooms. Often results from a supply line failure on an upper floor or a roof intrusion. Requires significantly more equipment density (high air movement, high dehumidifier capacity) and longer drying time. Class 3 is also where building materials begin to be unsalvageable due to swelling and structural compromise — flood cuts on drywall and removal of saturated insulation are typical.

Specialty drying situations — water has saturated low-porosity materials that are slow to release moisture: hardwood floors, plaster, brick, concrete subfloor, structural wood. The materials look fine on the surface but hold trapped moisture deep in their core. Requires specialty equipment (heat drying, low-grain refrigerant dehumidifiers, sometimes desiccant systems) and extended timelines (often 7-21+ days). A loss can be Class 4 alongside Class 1, 2, or 3 — they're not mutually exclusive.

S500 publishes minimum air-mover and dehumidifier counts based on Class plus measurable variables (wet wall linear feet, wet floor square footage, room volume in cubic feet, and AHAM-rated dehumidifier capacity vs the calculated humidity load). DRR's psychrometric calculators at /authority/calculators implement the formulas. Adjusters pull these from the field log to confirm equipment matched the loss — under-equipping is a leading cause of failed dry-outs and re-mediation supplements.

An IICRC WRT-certified (Water Restoration Technician) field tech measures: linear feet of wet wall, square feet of wet floor, room volume, and uses pin / pinless moisture meters to assess depth of penetration into structural materials. The combination dictates the Class. Determination is logged on day one alongside the Category — both classifications are first-day field-log entries that the carrier expects to see.

Class is essentially a proxy for evaporation load. Class 1 has a small load and dries fast. Class 4 has a load that resists evaporation no matter how much equipment you throw at it because the moisture is locked deep in low-porosity material — it has to migrate to the surface before it can evaporate. Higher classes don't just need more equipment — they need different equipment (desiccants, heat injection) and longer time. "Why won't my floor dry" complaints almost always trace back to a Class 4 condition that wasn't recognized on day one.

Yes — it's reassessed daily as part of the standard drying-log entry. A loss can drop from Class 3 to Class 2 as upper materials dry, or escalate from Class 2 to Class 4 when meters reveal trapped moisture in structural wood that wasn't caught initially. The carrier expects daily psychrometric readings to reflect the current Class so equipment count + scope adjust accordingly. Failure to reassess is a documentation flaw that leads to scope reductions on review.

Drying-equipment line items (air movers, dehumidifiers, dehu days) map directly to the Class + room measurements. Antimicrobial line items map to the Category. Together they should produce a scope where the equipment count looks reasonable for the visible damage. An adjuster who sees 4 air movers on a Class 3 multi-room loss will flag the under-equipping; one who sees 50 air movers on a single-room Class 1 will flag the over-equipping. The Xactimate scoping reference at /authority/xactimate-scoping covers the line-item discipline.