I have recently purchase a new toilet. The base of the toilet seems to be uneven. When placed on an absolutley flat surface the toilet base only touches across the middle from side to side and there is a 1/8 inch gap at the front and back. Is it normal for the toilet itself to be uneven or is this a defect? What is an appropriate material to shim the toilet with so that it wil be properly supported and will not shift when installed?