A slip-on tub spout is so named because it slips onto the waterpipe extending out of the wall. You tighten one screw underneath the spout. Once you’ve removed the old spout by unscrewing the setscrew that is on the underside of the spout, gently push the new spout onto the pipe until the back of the spout touches the wall. Install it gently so that you don’t hurt the inner pipe or the wall pipe. Once you tighten the new setscrew, turn on the water to verify there are no leaks. If there aren’t, caulk around the back end of the spout.
Slip-on tub spouts don’t leak -- when working properly, of course -- because the spout’s inner pipe should fit snugly over the pipe that comes out of the wall. Water isn’t flowing out of a hole in the wall and into an empty sleeve. The inner pipe guides the water to the exit point of the spout.
It’s possible that the inner pipe actually is not an exact fit for the wall pipe, in which case you have to replace the portion of the wall pipe that’s sticking out. Debris, either in the water or already in the tub spout, can clog the spout’s opening, forcing water back along the inner pipe. If the pressure is great enough, water can leak out.
Checking for Leaks
It is always possible for something to go wrong and lead to a leak. Remove the spout and check for cracks in the inner pipe or clogs that could cause the water to build up and shoot backward. Also check the water pipe from the wall, as well as the rest of the shower pipes. The Family Handyman website notes that if the diverter, which is the mechanism that sends water to either the shower or the tub spout, wears out, then water can leak out the tub spout instead of moving up to the showerhead. Corroded shower pipes can also lead to water leaking in the walls and out around the tub spout, if there are holes in the caulking, making it appear like the tub spout is leaking.