In keeping with the "How" theme of this forum I thought I'd post the sequence of steps it took to rebuild the arched roof trusses and roof of an underground shelter/root cellar that was on my place when I got it. These trusses are quite unique, simple, and effective and I have never seen anything like them before. You'll see the load they easily carry and by following along with the rebuild you can see how to construct these roof trusses for yourself. I think they'd be especially good for an underground shelter that was away from any road/driveway because the components could be packed it and assembled without heavy machinery.
Anyhow, there was dirt falling in from the roof inside the shelter when I purchased and a flashlight examination showed much rot and decay in the ceiling. This cellar is where the well pressure tank resides and also an irrigation pump plumbed to the immediately adjacent in-ground 3000 gallon cistern. Not wanting it to weather another winter, I began excavation:
That dirt's about 30" thick on top
Roof exposed. It consisted of two layers of planks and a thin layer of roofing felt (or what was left of it). I don't know how long this had been there, but judging by the condition of the outside of the concrete blocks and the adjacent building, I'd say at least 50 years!
Some of the deterioration where dirt was falling inside
After removing the planks, the trusses were exposed. They weren't in very good shape either