I'd like to install a cast iron shower pan in my basement. Do you think I can set the pan right on the concrete floor, or do I need to place a sheet of plywood underlayment underneath?

You can put it directly on the floor. The pan should have small "legs" or "ribs" + the curb apron that will hold in in place and give it proper slope. However, even when it is Cast Iron pan, we still set it in light weight concrete. This takes care of potential unevenness of the floor. Hope that helps. Milo

Yup... as suggested above, set the cast iron base in structolite or lightweight concrete and set/level the base on the concrete floor... best overall job this way, for sure!

Good luck!

Mark

Thank you both for your replies. Just so I understand; you would recommend a thin layer of concrete on the cement floor throughout the area under the pan so that the "legs" settle in the concrete and limit any lateral movement.

Bed the entire "footprint" of the pan... not just the legs.

Here, let's say that you have 2" between the bottom of the pan and the floor...you'll want to put say 3 1/2" of structolite in the footprint of the base and then set/squeeze the pan down to the floor as you level the base from front to back and from side to side until you have the base perfectly level in all planes (secure properly while the bedding substrate hardens overnight). This is particularly important if you are installing shower doors... ;)

That make sense?

PS: Don't forget to take the pitch of the pan into consideration when distributing the structolite (or similar).. if 2" at the outside may be 1"at the center... pitch structolite accordingly, OK?