In the picture below you can see that outside the walls I stepped out a horizontal brick and then made vaults on top, then again and agin. This allowed me to get a 30 cm overhang over my walls.
Now both buildings have small cracks as you see in the first picture, all in the middle of the wall and ending in the vaults above the wall. Not every one, and only two go into the brick and then not all the way through.
Most tend to crack where the brick is stepped out in the midle of each side.
Either:
the stepped out vaults add weight and further down and so change the line of thrust
-The plaster behaves differently than the bricks, and it is very thin, so the line of thrust could move out of the plaster
-the wall makes different dynamics.
Any thoughts?
It seems odd to me that the crack runs horizontally and not vertically. Maybe they crack in the middle of the overhang course because it doesn't carry the load above down to the bottom of the vault and there is nothing to serve as a buttress thus evening out the pressure?
ReplyDeleteYeah Matt I agree. has to do with overhang. what do i do about it?
DeleteI'll have to give this some thought...but if what I said is true, then there needs to be something incorporated into the overhang to transfer the load down....I think.
DeleteNote that I mentioned. The crack did not happen till the waterproof cement/sand plaster was applied, and doesnt seem to affect the bricks with exception of a few bricks. I think there has to be mesh in the plaster.
Deletewhich i probalby should be doing anyway
DeleteMaybe the cladding is the issue. I'm thinking the cement isn't flexible enough to move with the brick. Since the brick you use is a little on the soft side this is a good possibility. Lime plaster might be a better option?
ReplyDeleteAre the cracks on the more sunny side of the vault or shady side? I'd guess that this is caused by differential thermal expansion between the plaster cladding and the brick substrate. Similar thermal differential between outside vault overhang and inside.
ReplyDeleteJeff
They dont seem to have a pattern, and as we are near equator and this is roof they all kind of get hit the same albeit at different time. I roofed this way but I kind of knew it was wrong.
DeleteYeah Matt, i think it is moving from vibrationg and thermal differences. I have to rethink roofing these vaults.
I think you could still use cement but you would need an uncoupling layer between the cladding and the brick. This way both surfaces can expand and contract without effecting each other but still be part of the same structure. 30# felt would work or even a thin layer of sand, dampened during application.
DeleteThanks. I think you are right, and i thought about it. at same time i think i shoud put a waterproof membrane. live and learn
ReplyDeletereally appreciate you thinking about it.
the engineer/ architect who has helped me told me that once. dont try to tie the brick roofs into a cementious material
It might be because of the differences between the compression/decompression of the bricks and cement. I just saw a video of how you can make breatheable yet 100% waterproof plastering with lime and animal fat such as tallow or lard, quite interesting: https://www.youtube.com/watch?v=_vl7hdqEeFs
ReplyDeleteThanks. that is interesting. HOw come i didnt come accross a year ago. This seems to be water proof. I also now heard that lime and clay works.
Delete