Classify Category vs Class First On Arrival Decision Tree

Why this matters

The first two decisions on a water loss are category (how contaminated the water is) and class (how much evaporative load there is). They drive entirely different parts of the response: category sets PPE, cleaning vs removal, and worker/occupant safety; class sets equipment quantity and drying-system design. Sequencing matters because category gates whether you can dry a material in place at all, while class only matters once you know what you are allowed to keep. Classify category first: a Category 3 loss can make a low-class drying plan irrelevant because porous materials must come out regardless of how fast they would have dried. Reversing the order leads to crews setting air movers in contaminated water and to scope that the standard will not support.

The decision flow at a glance:

  On arrival - category or class first?
  |
  +-- 1. Clean, fresh, clear water? ----> CATEGORY 1
  |
  +-- 2. Gray / aged Cat 1? ------------> CATEGORY 2
  |
  +-- 3. Sewage / flood / Cat 2 aged? --> CATEGORY 3
  |
  +-- 4. Under 5% surface? -------------> CLASS 1
  |
  +-- 5. 5-40% surface? ----------------> CLASS 2
  |
  +-- 6. Over 40% / wet ceiling? -------> CLASS 3
  |
  +-- 7. Low-evaporation materials? ----> CLASS 4

Symptom presentation

The tech arrives to standing or absorbed water and must read two independent dimensions:

• Category indicators: clarity, odor, source, tide-line color, residue, and time/temperature exposure. Clean supply water vs gray appliance discharge vs black sewage/floodwater.
• Class indicators: how much wet surface area exists, how much water the materials hold, and whether low-evaporation (wet but bound) materials dominate. A few wet square feet of carpet is a different class than a saturated room with wet structural cavities.

Quick checks

Category, in order:

1. Identify the source. Broken supply line or rain through a clean roof points Cat 1; dishwasher, washing machine, or sink-drain overflow points Cat 2; sewage, ground/surface floodwater, or any wastewater points Cat 3.
2. Read clarity, odor, and residue. Clear/odorless vs cloudy/soapy vs turbid/foul with biosolids.
3. Apply the clock. Cat 1 that sits and contacts contaminants, or sits long in warm conditions, deteriorates toward Cat 2 then Cat 3. Time and temperature reclassify.

Class, after category:

1. Measure it, do not eyeball the source. Since the 2015 revision, S500 defines Class 1 through 3 by how much WET POROUS MATERIAL there is as a share of the combined floor, wall, and ceiling surface area of the affected space: under roughly 5 percent is Class 1, roughly 5 to 40 percent is Class 2, over roughly 40 percent is Class 3. Where the water came from does not set the class. Class 4 is the separate case: deeply held or bound water in low-evaporation materials (hardwood, plaster, concrete, masonry, multilayer assemblies), which is a specialty drying problem regardless of the percentage.

Isolation tree

• Source is clean and water is clear/odorless, freshly occurred. Branch: Category 1. Proceed to classify class; most materials are candidates for in-place drying.
• Source is gray (appliance, sink drain) or Cat 1 water that has aged or contacted soils. Branch: Category 2. Clean or remove based on porosity; carpet pad typically removed, structural materials cleaned and dried.
• Source is sewage, floodwater, or grossly contaminated, or Cat 2 aged past the point of control. Branch: Category 3. Porous materials are removed; non-porous cleaned and disinfected. Class becomes secondary because removal, not drying, governs porous materials.
• Category settled, now read class off the wet-porous-material percentage of combined floor, wall, and ceiling area. Under about 5 percent. Branch: Class 1, light equipment load.
• About 5 to 40 percent. Branch: Class 2, moderate load. Carpet and pad with walls wicked up a foot or two commonly lands here, but confirm it with the percentage rather than assuming from the picture.
• Over about 40 percent. Branch: Class 3, heavy load. Wet ceilings and insulation usually put you here, and top-down airflow is the usual design answer, but overhead water is a correlate of Class 3, not its definition. Overhead water in a small area is still Class 1 or 2.
• Low-evaporation/bound-moisture materials dominate (hardwood, plaster, masonry, multilayer). Branch: Class 4, specialty drying with low-GPP/desiccant strategy and longer timelines. Class 4 is called on the material, not on the percentage.

Confirming diagnosis

• Per S500, category is a measure of contamination at the time of assessment and can change with time and temperature; document the basis (source, indicators, exposure) and re-evaluate if conditions worsen.
• Class is determined by how much wet porous material there is relative to the space, and separately by whether bound-water materials dominate, which together size the dehumidification and airflow. Confirm class by actually quantifying wet porous surface area against the combined floor, wall, and ceiling area, and by identifying low-permeance materials. A class recorded without that number behind it will not hold up on review.
• The two are orthogonal: a Category 3 loss can be any class, and a Class 4 loss can be Category 1. Record both independently in the file with the evidence for each.
• Reassess category at every visit; a Cat 2 left without proper control will trend toward Cat 3 (see the clock-vs-condition standard guidance).

Remediation

• Set PPE and worksite controls to the category before any drying equipment is placed. Cat 2 and Cat 3 require escalating respiratory, skin, and containment controls.
• For Cat 3, remove porous materials per the standard before designing the drying system; do not run air movers across contaminated water (it aerosolizes contaminants).
• Size dehumidification and airflow to the class once category-driven removals are scoped. Class 4 materials get a low-humidity/desiccant or specialty approach, not just more air movers.
• Document category and class on the scope; both numbers justify the equipment, removals, PPE, and timeline to the adjuster.

References

• ANSI/IICRC S500-2021, Standard for Professional Water Damage Restoration, Section 10 (Categories of Water) and Section 11 (Classes of Water Intrusion), including time/temperature reclassification guidance.
• ANSI/IICRC S520-2015, Standard for Professional Mold Remediation (microbial amplification risk in Cat 2/3 and delayed losses).
• OSHA 29 CFR 1910.1030, Bloodborne Pathogens (PPE and exposure controls applicable to Category 3 sewage work).
• EPA, A Brief Guide to Mold, Moisture, and Your Home (EPA 402-K-02-003), moisture-control basis for category escalation over time.