Residential washing machines shall have a trap of what size and how many inches above the floor should the drain be extended?

• A. 1 inch trap, 6 inches above floor
• B. 3 inch trap, 8 inches above floor
• C. 2 inch trap, 12 inches above floor
• D. 4 inch trap, 10 inches above floor

Answer: (C)

Washing machines need a dedicated standpipe with a properly sized trap to handle the high flow and to keep sewer gases from leaking back. The standard choice is a 2-inch trap, which is large enough to accommodate the machine’s discharge and maintain a reliable trap seal. A smaller trap (like 1 inch) could clog or lose the seal under heavy use, while a much larger trap isn’t typical for residential drains and isn’t needed here.

The standpipe should extend a full 12 inches above the floor. That height gives enough clearance for the washer’s drain hose and ensures there’s a proper vertical portion for the waste to enter the standpipe without splashing back or overflowing, while also allowing space to install the trap and maintain correct venting.

So, a 2-inch trap extended 12 inches above the floor matches common residential practice for washer drainage, balancing flow capacity, trap integrity, and practical hose placement.