if you could use a membrane and lay the turf over it , the layer of turf could be thinned down a bit , maybe a hybrid mix of materials , first thatch to create a more attractive or even a practical inner roof surface to look at and touch ---easier to mount and mold to the round pole frame work---then a membrane --now hidden from inside view ---by the thatch--- then your layer of turf ---i have helped ----just with the labor---to build a wood shed from round pole and they wanted a turf roof on it -----a black heavy duty sheeting was used under the turf for water seepage control ---the roof was for the looks more so than trying to create the original concept ---but the product that he had want to use was developed in norway ---called PROTAN ----just at that time we had just come out of the lockdown , so it was nt available in time ---it looked good from the specs of the product