[Cob] Lime Render - FAILURE

[Shannon Dealy]

[snip]
> The images show the cracking of a lime render applied less than (1) 
> year ago over an earthen brown coat. The Brown coat was 3 parts sand to 
> 1 part - 90% indigenous site clay and 10% lime (NHL 3.5). The final lime 
> render was 2.5 parts sand to 1 part - 90% lime (NHL 2.0) and 10% 
> indigenous site clay. A pigment from Trans-Mineral USA was added for 
> color to the recommended acceptable amounts according to the 
> manufacturer of the pigment. (I think TM USA was the source, I know we 
> got our lime from them - St. Austier NHL).

Hi Bill,

Based on what I am able to see, it appears to me that pictures 1, 3 and 4 
are settling cracks. Number one could be due to foundation issues in that 
area, or issues with how the wall was constructed near that area. It 
doesn't look particularly bad given the amount of time that has passed, so 
it may be that everything has stabilized. Pictures 3 and 4 appear to be 
due to shifting/settling/shrinking around embedded wood (door/window 
lintels and embedded log - I'm assuming something is embedded in the cob 
above the door) note that the cracks originate and end at these points.
This is quite common if the wall and wood haven't had a long time to cure 
around these areas, since wood and cob shrink very differently as they 
dry.

I do not believe that any of these cracks are due to rainfall issues as 
the moisture from rainfall heavily favors the base of the wall and these 
cracks are not preferentially occuring at the base of the wall. Why is 
water more of an issue at the base?

   - The upper wall is better protected by roof overhangs
   - The upper wall receives greater airflow so it dries much faster
   - rain splash from water hitting the ground and rebounding onto the
     building is limited to the first couple of feet
   - rain splash from plants next to the building can be higher up,
     but the plants themselves are generally mostly at the base
   - the humidity is higher closer to the ground (where the water from
     recent rainfall is), slowing the drying at the base

Additionally, the finish render mix you describe should be highly 
resistant to water. If you have water issues behind this mix, I think it 
would be more likely to come from moisture within the house working its 
way through the walls (breathing, cooking, showering, ... give off a lot 
of water, I've discussed this issue previously), though I think it is 
unlikely given where you live.

I've seen and worked with similar exterior plasters on a number of 
buildings in Oregon with 40+ inches of rain each year. Some had problems, 
but it was not due to the rainfall in any case I am aware of.

The only picture that causes me any concern is number two and my question 
would be what (if anything) is embedded in the wall in that area as well 
as what is on the other side of the wall? Embedded wood, plumbing, 
shower on the other side, etc. Even this area is not a major concern to me 
unless the render starts flaking off (at which point you may be better 
able to assess what is going on behind it).


> A. I'll bet my bottom dollar the County will not pass my building w/ 
> cracks in the render - big ouch!

For this issue, I would simply make a small batch of plaster mix, matching 
as exactly as possible the original mix you used. Wet the area around the 
cracks, then fill them in with an excess of the new mix and refloat the 
surface of the wall around the crack to blend it in. I would start with 
the crack in the least visible area and get your technique down before 
doing the others. It is important to note here, that the color will not 
match for quite some time, even if your mix is exactly correct as the area 
needs to reach the same level of moisture as the surrounding wall, and the 
lime needs to react with CO2 in the air for a time to catch up with the 
state of the rest of the wall. I don't recall how long it took for my last 
patch job (jar window broke) to match the rest of the wall, but I think it 
was measured in months.


> B. The process of water expanding the clay underneath is progressing to 
> the point that the lime is delaminating and falling off in certain 
> areas. While this is not really a huge structural issue, it's a bummer, 
> and it's unsightly to say the least. Over time, it could start to wear 
> down the cob/earthen layer beneath it, which will create more work to 
> fix it.
>
> So, my question is this. . . What would you do next to remedy this 
> situation and create a functioning wall given my current situation?

None of your pictures show an area where the lime is actually falling off, 
so it is hard to say for that case. One of the problems that cracks can 
cause once they are present (due to settling or other issues) is they can 
allow water a pathway into the wall, and in some cases, even channel the 
water in. At this point, the greatest danger is usually from freezing as 
this causes the trapped moisture to expand and forcefully separate the 
plaster from the wall. If you have freezing temperatures in your area, I 
think this is more likely than water expanding the clay in the walls.

FWIW.

Shannon C. Dealy               |       DeaTech Research Inc.
dealy at deatech.com              | Biotechnology Development Services
Telephone USA: +1 541-929-4089 |      USA and the Netherlands
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