Both the International Residential Code (IRC) and the Uniform Plumbing Code (UPC) require that whenever a water heater is installed in an attic space or over a wood floor it is required to have a drain pan. (IRC 2801.5 or UPC 510.7)
1. The Drain pan must be 24 guage galvanized steel or it may be listed and approved plastic pan.
2. The pan must have a minimum 3/4" dain line IRC 2801.5.1 (UPC 510.7)
3. The drain must terminate by means of an indirect waste or outdoors 6" to 24" above grade (IRC2801.5.2 or UPC 510.7)
Although not specifically listed in the IRC or UPC, your local code may also require a drain pan under the washing machine, if so, it should meet the same standards as the water heater drain pan. I would strongly recommend a drain pan under a washing machine). The exception would be if the entire room is constructed as a shower pan type floor with a floor drain.
The drain pans and water heater T&P valve must have individual drain lines, they may not be combined.
4. The pan must be a minimum of 1 1/2" deep (IRC 2801.5.1)
5. The water heater T&P (Temperature & Pressure relief valve) MAY NOT terminate into the pan (IRC 2803.6.1 UPC 510.8)
The water heater T&P valve must run undiminished in size and terminate in an approved indirect waste receptor or outside and terminate 6" above finish grade (IRC 2803.6.1) ( not less than 6" nor more than 24" above finish grade UPC 608.5)
The lines must be pitched downward at 1/4"/ft and there may be no traps on the discharge line (IRC-2803.6.1 or UPC 608.5)
The end of the pipe must be clearly visable and there may be no valves, fittings or threads on the discharge end of the lines (IRC 2803.6.1 or UPC 608.4 + 608.5)
Line must be Galvanized Steel, hard drawn Copper or CPVC..(IRC2803.6.2 or UPC 608.5)
The line must be protected from freezing or terminate into an approved indirect receptor in a heated space (IRC 2803.6.1 or UPC 608.5)
An electric water heater would be permitted, but a solid, liquid or gaseous fueled heater may not be placed in a bathroom, bedroom, or a closet or utility space open to the same. (IRC 2005.2 or UPC 509.0)
The washing machine standpipe waste receptor must be a minimum of 2" diameter.
A laundry sink may drain into the washing machine standpipe if the sink drain center is within 30 inches of the standpipe.
The standpipe must have a p-trap not less than 6" nor more than 18" above the floor UPC 804.1
Standpipe must be a minimum of 18" but not greater than 42" above the p-rap wier (IRC 2706.2) (18" to 30" UPC-804.1)
The washing machine may discharge into a Laundry Tub (IRC 2706.3.2 or UPC table 7.3)
The waste line must be sized for 3 drainage fixture units..
NO trap below the floor (UPC 804.1)
