The building sewer shall be cast iron soil pipe, A.S.T.M. Designation A74, the latest revision; vitrified clay sewer pipe, A.S.T.M. Designation C-200 (extra strength), the latest revision; or other suitable material approved by the Loudoun County Sanitation Authority. Extra heavy (XH) must be used in all road rights of way; service weight (SW) may be used from the right of way to the building. Joints shall be watertight and rootproof. Any part of the building sewer that is located within ten feet of a water service pipe shall be located at least one foot in elevation below the water pipe; constructed of corrosion-resistant cast iron soil pipe with leaded joints; and tested with a ten- foot head of water or equivalent and found to be tight. Cast iron pipe with leaded joints may be required by the Authority where the building sewer is exposed to damage by tree roots. If installed in filled or unstable ground, the building sewer shall be of cast iron soil pipe, except that nonmetallic material may be accepted if it is laid on a suitable concrete bed or cradle as approved by the Authority.