CATEGORY ONE water losses originate from a clean water source. For example, if a basement supply line leaks the water travels through the dog poop on the floor that the homeowners were too lazy to pick up, is that a cat 1 loss? How clean were the floors that got affected by the "clean water"? There is more to determining the category of the loss besides where the water originated from.\r\nWater Damage Repair\r\nCATEGORY TWO water losses are approximately 75% of all losses based on my experience. Cat 2 losses include sump pump failures, toilet overflows, dishwasher discharge, and washing machine overflows just to name a few. But like others have mentioned category 1 losses can and usually do degrade to a cat 2 loss. Again look beyond where the water originated from because other factors including indoor temperatures, air circulation, and cleanliness of the indoor structure help determine what category of water you are dealing with. Cat 2 water losses usually require a disinfectant to be applied to affected surfaces.\r\nWater Damage Mitigation\r\nCATEGORY THREE water losses are grossly contaminated and require the use of PPE ( personal protective equipment ) and an AFD ( air filtration device ). Cat 3 losses require removal of contaminated porous building materials such as drywall, carpet, carpet cushion etc. although for the life of me I can't understand why you don't have to remove wood structural members?? Different topic for a\u00a0different day. Cat 3 losses usually require HWE ( hot water extraction ) and the use of a disinfectant on affected surfaces.\r\nToxic Water Damage Repair\r\nCATEGORY FOUR water losses are specialty losses that if you are a newbie and you're dealing with this then I assure you that you are incapable of handling this and you should exit stage left. And yes for you other responders there is a category 4. Category 4 losses are water losses that are mixed with fuel oils, oil or any other "out of the ordinary" chemical that is mixed with the water. These losses require environmental waste hauling companies. After removal of chemically contaminated water the loss is treated as a category 3 loss but requires MUCH more cleaning after removal of contaminated building materials.\r\n\r\nIn conclusion, the IICRC S500 guide for water damage restoration is just that, A GUIDE. Usually, it is a minimum guideline to be followed. For example in construction building codes are a minimum guideline. The best builders go way above and beyond minimum guidelines as do the best water damage restoration companies.