Dear James: A dark spot is forming on the ceiling below my master bathroom with a tile floor. My newer house is just past the warranty. Where should I look for the water leak? —Amy F. Dear Amy: It seems like problems always occur just after the new house warranty expires. The basic plumbing is probably not the source, or the leak would have started from day one. The two most common causes are a leaky wax ring underneath the toilet or faulty waterproof membrane under the shower. If the toilet was not bolted down tightly, it could wobble a little each time it is used. This can deform the wax ring over time, and it will leak a little with each flush. Try to wobble the toilet. If it wobbles, tightening it will not help. You will have to replace the wax ring, which is a simple project. If …