Civil Engineering Reference
In-Depth Information
Single-storey dwellings with walls over three metres high may need additional lateral
stability. This will depend on the placement and frequency of internal walls. You will need
to employ the services of an engineer that understands straw bale construction to design
suitable stabilising for your house.
Laying the bales
How straw bales are made
It is useful to know how straw bales are made and why one side of the bale is thicker than
the other. All bales have one cut face and one folded face. The folded face will have a fluffy
appearance. When the bales are being formed the straw is picked up from the ground and
fed up to what basically appears as a large fork, called a magpie. The magpie picks up a
batch of loose straw and forces it down into a chamber causing one side of the straw to be
folded back on itself. As the magpie retracts a ram pushes the straw forward, compacting
it. While one side of the straw is folded, the other side is cut flat with knives giving it a
relatively clean-cut surface. The folds of the straw are likely to run back into the bale for
about 100 mm, which results in the folded or fluffy side of the bale being slightly thicker
than the cut side. If you have ever stacked a pile of lever arch files you will have noticed
that they tend to slide off one another. You will get the same effect if a system of alternate
laying is not done.
Straw bale bulging
The most common cause of a straw bale wall bulging during compression is incorrect
laying. Incorrectly laid bales are inclined to lean one way and bow in the middle during
the compression process. It was recently suggested that the removal of 20 mm from the
folded face of the bale was desirable prior to installation in the wall. If, however, this were
done it may be more difficult to differentiate between the fluffy side of the bale and the
cut side of the bale. This may lead to the bales being installed incorrectly thereby
compromising the structural integrity of the wall. It would be much more likely that the
wall would bulge and collapse during compression, or worse still, after completion. As the
depth of the folded straw exceeds 20 mm, cutting this amount off would do little if
anything to negate the variance in thickness in the bales.
Building a straw bale wall
Windows and doors
Provision for the installation of windows and doors needs to be made before laying any
bales. Install any window or door bucks that extend to the floor prior to installing the
straw bales. Mark the position of any windows that do not extend to the floor on the
bottom boxing before laying any bales. Try to avoid vertical joints of bales lining up with
the stile of window bucks that are resting on the bales, as there is a significant possibility
that the buck sill will drop unevenly into the joint between the bales during compression.
Position loose bales on top of the bottom boxing around the house to ascertain the ideal
layout of the first row of bales relative to windows and doors.
