Civil Engineering Reference
In-Depth Information
garden hose to monitor any surface erosion. When applied at 5-10 mm thickness it
should dry without cracking and should be resistant to weather erosion.
An actual case of delamination of the top coat of render
I recently attended an owner-builder's property where the top coat of render had
delaminated from the second coat of render. Furthermore, the second coat of render was
failing under the pressure of weather exposure thereby exposing the first coat in some
places. In the excitement of getting the bales sealed the frustrating process of testing
renders was skipped for it was assumed that their theory was right, so why waste time. You
can only imagine the disappointment when the render started to come away from the
walls. The testing of the proposed render may well have eliminated this problem. Because
the render was failing, it was not simply a matter of applying another coat over the top, for
the strength and stability of the render is reliant on its adherence to the previous coat all
the way back to the straw. As the saying goes, 'a chain is only as strong as its weakest link'.
Fortunately the first coat was terrific so it was not quite as bad as it could have been.
I took the participants of a recent workshop to see this property and was somewhat
surprised at their response. I was showing them the property to press home the point that
it is essential to stick to the rules when doing the rendering or you will suffer the
consequences. Several people expressed to me that this was one of the key things to give
them confidence to build a straw bale home. They explained that in their mind one of
their greatest concerns was whether they could get the render right. This house was the
epitome of their fear and it would only take two or three days to fix. There were no walls
to be pulled down and less than $1000 (2005) to rectify the problem. In their mind, if the
worst that could happen could be overcome in a few days, then what was there left to
worry about.
The use of your soil for an earthen render
The soil on your property will vary
Many straw bale houses are rendered using the soil from the property. This soil may have
been recovered from the excavation of the building site, or even from digging a dam for
water supply. The primary thing to understand about your soil is that it may well vary.
The depth that the soil is below ground level and the locality on the property will both
have a bearing on the material you have to work with. This is not a problem, but you will
have to keep an eye on the results you are getting. While your tests may indicate that you
have a render recipe that does not crack or fail, it is possible that this could change with
unnoticeable variation in the material you put into the render. If you find that the render
is beginning to show signs of cracking, it is not necessarily that the initial render recipe
has not been followed or that it was wrong, but that the material used has changed.
Testing your soil for clay and sand content
When using the soil from your own property you will need to test it to ascertain the
percentage of clay so that you have a starting point for the development of the ideal recipe.
Once you have established the percentage of clay from sand, you will add washed sand to
the soil to give you a ratio of about four parts sand to one part clay. This is a starting point
