[Cob] arches

[Dean Sherwin]

arches are nice and make lots of sense in a mass wall.  rule of thumb 
is up to 4' wide for unreinforced arch in masonry.  For cob?  3' 
should certainly be OK.  Better to have significant amount of wall 
above it (makes structural & aesthetic sense too).
I've only built one in adobe, using a semicircular form.  That would 
be one way to do it.  (good to use form in 2 halves inside and out 
each tapered down toward center of wall so you can withdraw them 
after setting)(and or chocked up so form can be lowered in opening).
Traditional way one supposes would have been to cut the opening out 
of the wall once it had partially dried.
Too low a rise is asking for cracking.  Semicircular means rise is 
half of width, I would not go less than 1/3 even for a fairly narrow 
opening.  Also the wider the arch the more buttress support it needs 
from adjoining walls ie they cannot be too near ends or corners.
Dean Sherwin

At 03:00 PM 1/24/2011, you wrote:



>Arches are really nice. I saw a photo of an earth-sheltered house
>with two arches across the front above the windows. They were very
>wide with not much arc to them. I know this was done with concrete,
>but if I wanted to do this sort of thing with cob, how wide of an
>arch could I do? Anyone with experience or knowledge of large arches
>made of cob?
>Damon