[Cob] Lime Render - FAILURE

[Shannon Dealy]

On Wed, 22 Feb 2017, Bill Wright wrote:

> https://www.dropbox.com/s/2657yta5fpeyec2/Photo%20Feb%2021%2C%208%2000%2037%20AM.jpg?dl=0
> 
> DISCUSSION OF PHOTO:
> You can see in the photo that the lime-containing render is getting saturated 
> with the water from rains blowing onto the building where the prevailing 
> winds are doing their work. IS THIS PROOF that the mechanism at play here is 
> water making its way through the lime-containing render into the earthen 
> brown coat below, and the swelling action of the deeper middle layer cracking 
> the lime-containing outer render? It appears that way to me.

I'm afraid that all this really tells us is that the wet surface looks 
significantly different from the dry surface. It doesn't tell us anything about 
the depth of penetration. There are similar differences between wet and dry 
areas on the outside of three buildings I have which are just bare cob walls 
(over 10+ years exposure on all three buildings). Of course since I haven't 
plastered there is no outer layer for the water to get beneath, however, my 
bare walls are far more absorbant than the plaster mix you describe is likely 
to be.

I'm afraid the only way I can think of to get more information is to pick a 
spot in a wet area without any nearby cracks, dry the surface of the 
surrounding area with a towel, cut a hole through the plaster with an
old chisel and see how deep the moisture goes. You should also be able to tell 
how well the plaster is bonded to the wall in this area.

> The question I'm sitting with. . . Will lime-putty in the cracks, and a lime 
> wash on top of that be the best fix?

This would certainly be my approach, though I would use a lime-sand mix with 
fine sand rather than straight lime putty to fill in large cracks
(sand reduces/prevents shrinkage). Ideally you want your patch mix to match 
your surface as much a possible and refloat the surface to blend the patch in. 
The lime wash is a good idea, but will not hide any significant differences in 
the appearance of different areas of the wall (that takes time and multiple 
coats).

One thing I just realized, you didn't mention any kind of fiber in your plaster 
mix. While plasters can be done without fiber, using fiber can greatly reduce 
the chances of cracking, particularly fine cracks.


Shannon C. Dealy               |       DeaTech Research Inc.
dealy at deatech.com              | Biotechnology Development Services
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