Do we need to lay a membrane under new topsoil?

Jony

just a thought but if some of you are relaying due to the lawn being very bad then have you considered something like greenthumb? they will kill of pretty much all your weeds in one feel swoop and recoer even the most tired old lawns.
if you dont have the lawn then try getting a large sprayer and get some liquid weed kill you have to dilute with water (best to use lawn weedkill but not the weed and feed) as then that should kill off all the weeds before you lay it.