Having built a papercrete barrel vault in 2000, there are some things we would do different next time around. Although, it is still standing strong, the aesthetic design could have been better.
It's very important that the walls support the vault roof so it doesn't push out on the top of the side walls. There are a number of ways to do this: a high pitched cantenary design that distributes the weight straight down on the wall, thick end walls, butressing, vigas or beams connecting the side walls attached to a bond beam, or vertical steel pipe enclosed in the walls set into a concrete footing.
There's plenty of information out there for adobe vault building but papercrete is very similiar, and has the advantage of being lighter and stronger if done correctly.
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