We have been experiencing drain flies for couple months. Had plumber come and he took toliet off and stated we had drain flies coming from toliet pipe. He cleaned pipe out. We also had an exterminator come and spray area. Week later now, and the drain flies are back. Called plumber again who came back. He now stated that the floor underneath our cermaic tile floor around the toliet is rotted out and this is causing the drain flies to still appear. He stated we have two options. One take up new ceramic tile and put in new sub floor and retile again. Or, take toliet back off and but a porcein base which would sit on ceramic floor and then place the toliet over the basin. We have spent alot of money on plumer and exterminator. Help please; is this plumber just giving us good information. thank you