3 feet trench for single storey rear extension - materials to be used and how

Hello . Well it depends on the way the pipe is running , but if it is running through trench not in full length of ; you will need to go slightly under pipe thus ensuring concrete carries through then encase in normal white polystyrene keeping concrete from direct touch then when concrete laid lay concrete lintel or two depending if double leaf brick / block over pipe slightly higher so settlement doesn't happen to crush it . Then get inspected and just carry on with your build Hope that helps Ben

Hi. If it is going along trench the best solution can be replace the pipe. Make sure the pipe is in use because you can find many pipes which are not in use. If she coming across the trench maybe better if you bridge it over with concrete/steel lintel (make sure lintel not touching pipe). Polystyrene which we using for pipe protection in concrete slab is 75mm. That should be enough in this case as well if ds required it. Regards Martin