Basically there are two types of waste & overflow kits. If your finished tub will have a 12"x12" access hole,eihter through and adjacent wall or up through the floor from underneath you may use the common waste & overflow kit which has an overflow body with a long tube attached that makes the vertical riser from the overflow down to the Tee. The top of the Tee has a slip joint fitting so the tube from the overflow can be cut to length to fit tubs with different depths. The side opening of the Tee also has a slip joint fitting so the waste arm from the drain shoe can be adjusted for length. (Note: the drain shoe is the casting that fits directly under the tub drain hole) The Tee has an extended tailpiece which then drops down to the P-trap. The rough in fittings are universal however you will need to chose which type of drain stopper you want. They have the cable type, the wire rod type that connects to a lever on the overflow trim cap. a toe kick type which is opened or closed by tapping with your toe, or a lift & turn type that fits in the drain opening and requires lifting and turning it by hand. You will also need to chose which type of finish you want on the metal trim parts to match your tub spout and faucett handles. If your finished tub will be fully skirted and there is no access panel the code requires a "Glue In" type waste and overflow kit. The glue in type kits will have a cast overflow body, a tee, and a cast tub shoe. The installer then custom fits the connecting lines from standard schedule 40 DWV grade pipe and glues it in place. This type of waste & overflow also requires a glue-in type P-trap. These kits are also available with the different types of drain stoppers mentioned above as well as a broad choice of metal trim to match your faucett & spout. The advantage of the glue-in type is that all joints are glued permanent joints so there is little to no chance of a leak once it is installed. Although it is rare today, there still are some local codes that require all metal DWV piping. In those instances there is a tubular brass type waste & overflow kit available that is fitted the same way as the glue in type, but the brass joints are all soldered.

Pressure fittings are defined by the angle of the change in direction they make. I.E. a 90 deg elbow, 45deg elbow, 60deeg elbow etc. In order to help insure we don't mix them, DWV(drain,wste & vent) fittings are defined by the fraction of a circle the change in direction makes. A full circle is 360 degree so a DWV 90 deg elbow would be 360/90 = 4 or properly its called a 1/4 bend. 60deg = 1/6 bend - 45deg = 1/8 bend. A common fitting has a female hub on each end but a "Street fitting" has a female hub on one end and a male raw pipe size on the other end. To make the offset you would begin with a Street 1/8bend with the male pipe end directly into the TEE of the waste & overflow. This would change the direction to 45 degress. Install a length of PVC in the female hub of the 1/8 bend and run the offset down at 45deg to the point where your trap will be. On the bottom end of the 45 deg offset pipe you install another street 1/8 bend with the pipe into the female hub, This will change the direction from 45deg slant to vertically down and it would leave a raw male pipe end that will fit directly into the input of the P-trap. You will find the fittings in both ABS and PVC at any local hardware or home supply store. The choice of which material to use is determined by what kind of pipe you have now. Code prohibits glueing dissimilar plastic so if you have ABS you would use ABS, if you have PVC you would use PVC. In this case you have cast iron so you may use either material as long as all the pieces are the same material.

All they need to change is the overflow trim cover and the tub basket. Both of which are easily accessible from the tub side so there is no big deal in changing them out.

Greg, thanks for the perspective. If I may ask a clarifying question, please. I agree that under most circumstances the tub will be filled to or below the overflow drain. However, assume that the tub is unoccupied and the water fills to the overflow - now someone gets in and the water rises above the overflow. If it rises above the deck, then obviously it will spill over the side and onto the floor, but if it rises to a level just below the deck the tub, it will appear from the outside that nothing is spilling, yet below the deck water will be flowing out of the vent pipe. I acknowledge that under normal operation the water doesn't rise above the overflow - but hopefully you can agree that sometimes it may rise above, and when it does - this tub will leak underneath, in the sealed-off and invisible space. Is that considered an acceptable risk? Or is the sealed off space below the tub supposed to stay dry?