How to Determine the Class of Water Damage (Classes 1 to 4)
The IICRC S500 classes of water damage, how to calculate the class from wet surface area, and what each class changes in an Xactimate estimate.
The class of a water loss is its evaporation load, measured as the percentage of wet porous material against the combined floor, wall, and ceiling surface area of the affected space. Under ANSI/IICRC S500-2021, Class 1 is less than roughly 5 percent wet porous material, Class 2 is roughly 5 to 40 percent, Class 3 is more than roughly 40 percent, and Class 4 is any loss where water is deeply held or bound in low evaporation materials such as hardwood, plaster, concrete, or masonry. The class sets the dehumidification target, the dehumidification target sets the equipment count, and the equipment count sets the equipment days on the invoice.
Class and category answer different questions
Category is about contamination. Cat 1 is clean, Cat 2 is gray, Cat 3 is black, and the category drives PPE, containment, disposal, and which tear-out codes hold up in review.
Class is about how much water the building absorbed and how hard that water will be to evaporate. Class drives equipment quantity and drying duration. A clean supply line break in a finished basement can be Category 1 and Class 3 on the same file. Writing "Cat 2, Class 2" out of habit on every job is how equipment gets undersized and underbilled at the same time.
The four classes
Class 1
Less than roughly 5 percent of the combined floor, wall, and ceiling surface area in the affected space is wet porous material, with minimal absorption into low evaporation materials. A contained spill on tile with one wet baseboard run is the typical file.
Class 2
Roughly 5 to 40 percent wet porous material. Water has wicked into carpet and pad, up into drywall a foot or two, or into the bottom of a cabinet toe kick.
Class 3
More than roughly 40 percent wet porous material. Water usually came from overhead, so ceilings, insulation, walls, and flooring are all wet.
Class 4
Water is deeply held or bound in materials that release it slowly: hardwood, plaster and lath, concrete slab, gypcrete, brick, stone, and structural framing. Class 4 calls for different drying methods rather than more of the same equipment, and it is the class restorers under-call most often. Restoration class surveys reported in Cleaning & Restoration found that restorers most often write Class 2 on projects where conditions support Class 4.
How to calculate the percentage
- Define the affected space room by room. The percentage is calculated per space, not averaged across the whole loss.
- Total the surface area of that space: floor, plus all four walls, plus ceiling. A 12 by 14 room with 8 foot ceilings has 168 square feet of floor, 168 of ceiling, and 416 of wall, for 752 square feet.
- Measure only the wet porous material. Carpet, pad, drywall, insulation, particleboard. Wet tile sitting on a dry slab is not porous material.
- Divide. If 100 square feet of carpet is wet plus 60 linear feet of drywall wet to 2 feet (120 square feet), that is 220 of 752 square feet, or 29 percent. Class 2.
- Check the low evaporation materials separately. If that same room has a hardwood floor reading 19 percent moisture content at the subfloor, or a wet gypcrete underlayment, the percentage stops being the deciding factor and the loss is Class 4.
Reclassify after tear-out
Equipment levels get set for the drying phase, so the class that governs the equipment count is the one that applies once demolition is finished. That is frequently a different number than the one from the first walkthrough.
A 2 foot flood cut removes the wet drywall and the wet insulation behind it, which drops the wet porous percentage. The same cut opens the wall cavity and exposes framing, sheathing, and slab, which are low evaporation materials that were holding water the entire time. A file that reads Class 3 on arrival is often correctly a Class 4 once the walls are open. Record the class on arrival, the class after tear-out, the reason for the change, and the equipment change that followed, all in the drying log.
What the class changes in the estimate
The IICRC publishes the initial dehumidification factors as a chart:
| Dehumidifier type | Class 1 | Class 2 | Class 3 | Class 4 |
|---|---|---|---|---|
| Conventional refrigerant | 100 | 40 | 30 | N/A |
| Low grain refrigerant (LGR) | 100 | 50 | 40 | 40 |
| Desiccant (air changes per hour) | 1 ACH | 2 ACH | 3 ACH | 3 ACH |
For refrigerant equipment, cubic footage divided by the chart factor gives the required pints per day, and required pints per day divided by the unit's AHAM rating gives the number of dehumidifiers.
Run a 1,500 square foot area with 8 foot ceilings, or 12,000 cubic feet, through both columns. At Class 2, 12,000 divided by 50 is 240 pints per day. At Class 3 or Class 4, 12,000 divided by 40 is 300 pints per day. With 140 pint AHAM units, 240 pints per day is two machines and 300 pints per day is three. Across a 5 day dry, the class call is worth 5 dehumidifier days.
Those days bill on WTR DHM per 24 hour period, and the code tier follows the unit's AHAM rating: WTRDHM for a medium unit near 65 pints per day (published around $54.59 per day), WTRDHM> for a large unit near 75 pints per day (around $73.13), and WTRDHM>> for an extra large unit near 125 pints per day (around $114.72). Rates move with the price list and the region. Class 4 also pushes toward desiccant equipment (WTRDHMD) and specialty systems such as wood floor drying mats, and it extends the drying period, which carries WTR DRY air mover days and WTR EQ equipment labor hours along with it.
Documenting the class
- Sketch dimensions in the estimate that match the cubic footage used in the dehumidification calculation. Reviewers recompute it.
- The per-room wet surface area measurements behind the percentage.
- Moisture readings against an unaffected reference for any low evaporation material, whenever the file is written as Class 4.
- The reclassification entry, with a timestamp, if the class changed after demolition.
Frequently Asked Questions
Can a water loss be Category 1 and Class 4 at the same time?
Yes. Category and class are independent. A clean water supply line that runs under a hardwood floor for two days is Category 1 by source and Class 4 by evaporation load.
Does the class change how many air movers are needed?
Air mover counts come from wet floor, wall, and ceiling area and from room geometry rather than from the class factor chart. The class affects air movers indirectly, because Class 4 drying takes longer and the same air mover count then bills across more days.
What happens if the adjuster disagrees with the class?
The class comes out of a calculation, so answer with the calculation. Provide the per-room surface area totals, the wet material measurements, the resulting percentage, and the moisture readings for any low evaporation material. An adjuster arguing Class 2 against a documented 46 percent wet surface area is arguing against arithmetic.
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