Francis, I don't mind the criticism, but it is not warranted this time. Corbeled vaults are actually traditional. One very famous example is the Galloway oratory, an early Irish church that also happens to be dry laid.
My little building does not really compare, but there is almost four tons of stone in the roof. I think it would have collapsed during construction if it was going to.
The Galloway I found is only 10' wide and was lime mortared. It has 3' thick walls and when you add the two walls it deducts 6' from the span.Yes they lable it a corbelled valt but when looking at the walls they are really practically built like two leaning stone walls, especially the exterior. and very steep! When those two leaning 36'' walls [or so called corbelled walls ] meet at the middel of the 10ft.building 1/3 of of eadc 36'' wall is actually plumb over the spring point.
All that I read srtesses that thes type arches are not for spans to boast about......and they all stress large walls to counter act the corbel and bolster the side walls.
There are those corbelled vaults which are nothing more than large stones stepped in with a cap stone atop; there are the leaning type such as the building you specified ,.quite steep and laid with the stones pitched somewhat to shed water;and there are false corbells which are made with large flat corbelled stone and covered over with thin stones in a shingle like fashion.
Now if you are doing a circular vault the span and and roof pitch are key ans common sense would depict. They span is the mid point minus the wall thicknessand I don't remeber you mentioning it nor has anyone asked,.. before they cheerlead.
As it appears by looking at your roof it isn't very thick and the stones are very flat. It is suggested practice that you incline the stones so they not only shed water but form a circular arch and lock tight with gravity! To lay in this pitched circular fashion and acheive a wedged
circlular course on each course they need some thickness or they simply will buckle and miss meet i.e. like laying slate.
To lay such thin stones so flat with no counter weight [width of the corbelled roof stone] you are basically building a flat frizbee which can have a tin can effect, had it not been for the cement.
What is the span and how thick is the roof,.. and is it a dummy?
This is what any intelligent person would ask......Fundi you started asking about lap which was a start....but you dropped the ball with what is important.
Bill you are very polite and talented.......I can see what your doing and it's not a walk in the park with masonry...stone work is very trying and just when you think you have it licked it bites you in the butt....I used to go to disco's with nearly ten fingers taped....stop on the way home at 3 in the morning and joint off the stone work!
never did like gloves..
I'm sure your roof is bonded well enough ....tell me though why didn't you use a thicker stone.
Alot of the corbelled vaults use large cap stones....think about it...the perimeter of the large cap stone lays over the lighter ends of the inner corbels which have 1/3 of their tail ends nearly PLUMB OVER THE SPRING POINT "timeless teeterless". i.e the dummy thin stones over it can hide it all if need be. strength then beauty?