[GSBN] lime over hessien

Derek Stearns Roff derek at unm.edu
Thu Apr 17 10:47:14 CDT 2014

Hi, Sven and Sarah,

I share the concern with maintaining proper moisture levels for the lime curing.  However, the plan described with hessien, clay slip, etc, seems neither necessary nor sufficient for addressing the moisture curing issues and needs for the lime plaster.  It adds to the complexity, and particularly to the likelihood of inconsistent application around the entire surface of the two-story house.  Even if the plaster application can match the optimistic plan, moisture levels in the plaster will necessarily be highly variable, with both height on the wall, via water movement due to gravity, as EJ mentioned, and with the variable time since the last trowel stroke (or sprayer pass), as the plastering progresses.  The plan requires that the many steps be performed with great consistency, while gravity and drying to the air conspire to make the results inconsistent.

It seems to me that you need a simpler and more reliable moisture regulation strategy for proper curing of the lime plaster.  For the conditions that you describe, I would imagine that this might mean a system of tarps or tenting, that would enclose and protect the wall surfaces from the drying effects of the wind.  Remembering that lime plaster pretty much ceases in its curing below 5 degrees C/40 degrees F, I wonder whether you have the time left in the season to complete the plastering process before the weather gets too cold.  Having some kind of a tenting system will help protect against daily temperature swings and surprise cold snaps.  As you know, both lime and clay plasters can be destroyed, if they freeze before curing sufficiently.

I agree with you, that it is very risky to undertake a large project like this, using a plaster system that you have never tried before.  The goal is to increase the likelihood of success, and trying an experimental, complex system that is new to you and your crew is the wrong way to approach that goal.  Go with what you know.  Since you have experience with lime over clay, the final warning isn’t news to you.  Clay and lime are not particularly friendly, so it is important to test the specific clay and the specific lime, to make sure you get a good bond.  Lime over clay has worked many times, and has also failed miserably many other times.

Good luck,

On Apr 13, 2014, at 3:11 AM, Sarah Johnston <sol_design at yahoo.com<mailto:sol_design at yahoo.com>> wrote:

Greetings All,

We have clients who are in a predominantly very dry, windy climate with temperature extremes from 40C to -18C wanting to adapt the lime over clay plaster system we generally specify.

They are wanting to apply the 30mm 'body coat' (approximately 1clay;1sand;1straw) and immediately apply a clay slip soaked hessien/jute/burlap wrap around the entire home which is two story.  While still wet they are wanting to wire brush the hessian to create a key, apply another clay slip then 10mm of lime plaster.

The application of clay slip over the hessien seems like a bad idea to us but we can see the draw to being able to apply the lime over a wet substrate in their climate as trying to keep the lime from drying too fast is a real issue for them.

We have a colleague who has used this technique two times in their area but both projects have been up less than a year.  We have no personal experience with this technique and are hesitant to write a specification....

Can anyone give a history lesson?  I know there was an article in the Last Straw some time ago.  Anyone still using this concept?

Any input would be great!

Sven and Sarah Johnston

Sol Design, Ltd.
Geraldine 7930  New Zealand
03 693 7369
sol_design at yahoo.com<mailto:sol_design at yahoo.com>

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

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