Washer Standpipes

IPC 406.3 The waste from an automatic clothes washer shall discharge through an air break into a standpipe in accordance with Section 802.4 or into a laundry sink. The trap and fixture drain for an automatic clothes washer standpipe shall be a minimum of 2 inches in diameter. The automatic clothes washer fixture drain shall connect to a branch drain or drainage stack a minimum of 3 inches in diameter.

This means the washer standpipes must drain into a vertical 3 inch stack if you are building an Apartment or Condo, care must be taken to make sure the framing members are 2x6 walls in these areas.