Tips on Load Bearing Straw Bales
Abstracted from the ideas of Andrew Morrison.
Load Bearing Straw Bale Requires
Special Design Considerations
It is important that you pay attention to some basic
design rules prior to building your load bearing straw bale structure. This is
particularly important when it comes to planning out the door and window openings. Here’s what you need to know about
the 3 basic ratios of load bearing:
1. Due
to the use of the bales as the structural component of the home, the openings
in the walls are limited to 50% of the
wall surface area. Although this sounds somewhat harmless, when applied, it
means that you have to be very precise with the type
and placement of your windows.
2. You also have to
pay attention to how high you make your walls. There is another important ratio to pay attention to:
The ratio between wall height
and wall thickness.
The height of the structure is limited by a width to
height ratio of 5.6:1 under most codes.
In
construction terms, this means that a 23” thick wall
has a maximum wall height of 10 ‘8.” In
addition,
load bearing structures are limited to single story;
however, a loft design can be utilized if the roof
pitch is steep enough.
3. Another ratio in
load bearing construction that needs to be considered is the
“unsupported wall length” which cannot exceed a ratio of 13:1 for a 23 inch wide wall. This ratio basically means that
you cannot have any more than 25 feet of unsupported wall length in any direction.
This is fine as long as the interior partition walls are spaced appropriately and used to offer perpendicular support to the
bale walls.
Foundations
There are basically 3 different foundation options open
to you:
a. Slab Foundation
b. Concrete Stem
Wall
c. Raised Floor
System
Slab Foundation
If
you’re building a structure on a slab foundation, very few differences exist
between a post and beam and load bearing system. But
pay attention to this:
Create the foundation with enough depth and
room to handle the width of the bales as well as
the foundation bolts placed through the toe ups.
Stem Wall Foundation
If, however, you are building on a stem wall foundation
with a separate slab floor (often the system used when placing an earthen
floor), you will have to form the foundation wide enough to support the full
width of the bales you use. That can
mean a concrete stem wall that is 18” - 24” wide.
Raised Floor System
Another
option is a raised floor system like that used over basements. It should be installed
in the same way that one would in a post and beam structure. Be sure to account
for the extra weight of the bale walls in the floor joist engineering.
Pay Attention to Your Toe Ups
Toe ups are used to raise the bales off of the floor as
well as to secure the bales to the foundation. By raising the bales off of the ground, they are safely removed
from any minor floods caused by leaking appliances or plumbing. The second job of the
toe ups is to provide nailing for the welded wire mesh. I typically use 4 X 4
dimensional lumber for my toe ups. The toe up can then become part of the shear system of the
building if engineered as such. Once in place the toe ups are bolted to the concrete
foundation It’s important that you know the width of the bales you
will be using before you install the toe ups.
I add one inch of width to the bale width when determining my interior
toe up lay out. In other words, if the
bales are 18” wide, I mark the layout so that the interior face of the toe up
is 19” from the exterior face of the mudsill.
I then snap a chalk line to represent the interior edge of the toe
ups. This yields a straight and precise
line for the edge of the toe up, making installation quick and accurate. Be sure to account for the door openings when laying out the toe ups!
Bale Compression - Using Compression
Straps
Unlike
Straw Bale Infill Construction, it is necessary to add special straps
underneath the toe ups before they are tightened to the foundation. The straps are used to secure the bales to the toe ups and ultimately to the
foundation.
When
the straps are tightened, a box beam (more on this later) placed on top of the bales will act as a cinch and will compress the bales into their
final position. If the bales
are not pre-compressed, they will have a tendency to sag over time under the weight of the roof assembly. So this step is essential!
Because the straps tend to get in the way of the
construction while raising the bales and straightening the walls, I run a short
section under the toe ups, typically extending about 3’ up each side of the
wall, which is then rolled up and secured by rubber bands at each
location. I lay the straps about every 2’
along the length of the wall making sure that I am
not laying them in window or door openings.
Once
in place and perpendicular to the toe ups, I tighten down the toe ups to hold
the straps securely in position, and add either clean gravel or insulation in
between the toe ups.
The Box Beam
A box
beam is used on top of the bales to provide a surface to mount the roof
assembly and to help evenly compress the bales before the roof is
installed. Here’s a tip that you must pay attention to:
The box beam should be built immediately
after
the toe ups are installed. Build the box
beam as
a mirror image of the toe ups. Build it
to the
exact dimensions so that when placed on top of
the bales, it will be easy to find the plumb
location of the box beam and ultimately to
tighten up the walls in a plumb position.
The
only place where an exact copy of the toe ups is not used is in door openings.
Because the door openings need to be spanned by the box beam, do not stop the
construction of the box at the openings that are created in the toe ups.
It is
important that the box beam be built in sections so that it can be easily
lifted into position and assembled on top of the bales. Once the bales are in place and checked for
plumb, raise the sections of the box beam to their respective locations and
nail the sections to each other at every union.
Of course, since we have yet to raise the bales, all you
can do now is build the box beam
with precision, label each piece for its respective location, and place the
pieces to the side.
Window and Door Bucks
In a load bearing building you will have to build special
frames for all your windows and doors. These are called “bucks” and need to be created before you being the
baling process. A buck acts as the framing
member that anchors the window or door firmly to the building. Without the buck, there is
no way to secure the windows and doors to the bales. It is important that the bucks be square to
ensure proper window and door operation.
I always build bucks that are deeper than the window or
door they will support so that I have space on the buck to attach wire mesh to
during the shaping phase of the construction. A simple 2x6 box is often
enough to create a window buck and a 2x8, 3 sided box (no bottom) is adequate
for a door buck. More elaborate bucks
can be created in order to aid in the bale shaping
process if so desired.
Be sure to add braces that keep the buck
square.
You can either use bracing inside the
buck itself or use plywood scraps nailed to the
face of the buck at the corner to hold it in place.
Whichever you use, be sure to place the bracing
in an area that will not impede the baling
process.
The buck should be fully anchored to the building with
mesh and dowels before the braces are removed and the window installed.
For that reason, I usually install the bracing just inside the exterior plane of the buck where the window or door
will eventually sit.
Once your window bucks are built and braced, number them
so that you know where they go in the structure and place them around the building in the general area
of their final position. The door bucks should be
installed at this time. Stand them on
the floor system and nail them to the toe ups. You
may need to temporarily brace them in position until enough bales are installed around them to firmly anchor them in
place.
Keeping Your Load Bearing Structure
Plumb
Contrary to popular belief, load bearing structures need
to built as plumb and square as a wood framed building. Consider that the bales are the wall
system. Now consider that the wall
system, if built out of plumb, is not actually bearing directly on the points
below which have been designed to hold them.
Instead, the walls are pushing out to the sides and placing stress on
the corners and other supporting areas of the structure like any partition
walls, which were never designed to carry such loads.
In order to build a load bearing structure plumb,
reference must be given. Without it, a series of bales stacked on each other will eventually slip out of plumb. This is because the eye, although very competent at maintaining a plumb or level line is fooled
by the odd shape of the bales and the bulges that are commonly found on the ends of
bales.
To compensate for this, the eye tends to tell the brain
to build out rather than straight. The result is a wall that gradually grows longer and a corner that gradually
flares out as you go up in courses. Be sure to plumb
your corners with wood braces before you start stacking bales.
Building Your Walls
a. Safely Securing the Bales Together
The bales need to be stacked in what is called running bond. This means that the bales are stacked like
bricks so that no two courses have joints that line up with each other (remember the legos™!)
Start in one corner and work in one direction all the way
around the structure while building the initial corner in two directions or build
the walls out entirely in two directions from the starting corner. If you have
several people helping with the bale raising then build out in two directions so that more people can actively place bales;
however, if your labor force is small, it is best to work in one direction and focus on
completing the home in sections.
As you approach another corner or turn in the wall, be
sure that a full bale is used on the bottom course. The full bale should either run in line with
the current wall or perpendicular to it. Do not use small
sections, less than ½ of a bale, in the corners, as a weak corner will develop.
If
you find that a full bale or half bale will not fit in the corner and a small
piece will need to be added, place the full bale in the corner first and then
add the small stuffing behind it, towards the bulk of the wall. In that way, the running bond will still
apply, and the full bale will maintain the strength and integrity of the
corner. Be sure to pay attention to the corner braces so that the walls go up plumb from the
beginning.
b. Stacking the Bales
Stacking bales is very satisfying. In fact, it can be so exciting that you might
not want to stop, even when you are supposed to install a window buck. Make sure that everyone involved in the bale
raising is aware of the window locations and that the bucks get
placed in the wall before the bales get
stacked too high.
The
easiest way to add bucks is to place them on top of bale courses in the proper location. Once in place, secure them by
adding bales on either side of the opening while
maintaining the running bond pattern. If
the bucks do not stay in place, nailing a wide piece of plywood to the bottom
of the sill will help it stand up. The
plywood can be left in place once the window buck is
secure.
Installing Your Box Beams and
Keeping Your Walls Plumb!
The walls may be weak at this stage, especially if they
reach the maximum height for bale
walls per the width to height ratio given earlier. The bracing in the corners will help
strengthen the walls, but the major push at this point is to install the box
beams on top of the bales.
Because the sections were labeled and properly laid out
around the structure this stage of construction is a simply assembly line. Hoist the sections of box beam to the top of
the wall and assemble them with nails and plywood. Be sure that as you nail them together, the corners remain square.
Because the box beams are a mirror of the toe ups below,
keeping them square will insure that the whole building stays square and,
ultimately, plumb. Once the box beam is
fully assembled, line up one edge of the box beam perfectly with the section of
toe up directly below it. This is used
as the baseline for the building, and confirms that the walls, if in line with the toe ups and box beam, will be
plumb.
To make minor adjustments in the walls, use
a soil tamper or home made “persuader” (large
wooden sledge hammer) to hit the bales into position. The weight of the box beam
will help secure the wall; however, be careful not to hit the wall too hard and knock it over. It
is best to
do this in pairs with one person on the
inside of
the house and one on the outside. Choose
a side that will represent the perfect plumb line and stick with it. For example, if the
person on the outside of the house holds the plumb stick, then all checks for plumb shall happen on the exterior of the building, all the way around. Work the bales from one corner, around the
building and back.
When satisfied with the position of the bales, add the
remaining strap material up and over the box beam and attach it to the small sections
previously installed at the toe ups.
Strapping, Compressing, and Wire
mesh
a. Strapping and Compressing the Bales
The strap material serves two purposes. The first is to compress the bales. This is important to reduce cracking in the final plaster and to
increase the overall strength of the
structure. In most cases, the bales will
not compress more than a couple inches. Be sure to compress them as far as they
will go so no further settling will ensue.
The other purpose of the straps is to adjust the box beam
into a level plane all the way around the structure. This is important for the roof framing. If the box beam is out of level, the roof framing will be difficult, even for a seasoned carpenter. Therefore as the straps are tightened watch
the level of the entire box beam. This
is where working from one corner out is important.
After the majority of the compression has been made, the
final adjustments can be made incrementally.
By starting in one spot and marking it as the baseline for the rest of
the house, the box beam can be lowered evenly creating a perfectly level base
for the roof assembly.
The
strap material may have a tendency to stretch over time when exposed to direct sun. For that reason it is important to check the
straps and check the box beam for level one
last time before the welded wire mesh is installed.
This
is also the time to adjust your bales one last time and have at them with a
weed whacker. The smoother your walls
are now, the easier they will be to plaster.
b. Attaching the Wire Mesh
Welded wire mesh is a very strong and versatile
product. It is used as the lateral shear
strength of the building, to shape bales, and to attach the bales to the toe up
and the roof. Once the mesh is
installed, any adjustments to the strapping will cause the mesh to bow and
buckle which makes the finish plaster difficult to place so it’s important to
do that last check on the strapping prior to
installing the mesh.
Start installing the mesh on the exterior of the
structure by nailing it to the box beam and then stretching it to the foundation toe ups.
Be sure to install the mesh tightly running from the top of the building to the bottom.
Wrap the mesh around each corner to strengthen the corners. Don’t forget to
remove the temporary corner bracing first!
After the entire exterior has been installed, adjust the window and door bucks to
insure they are perfectly plumb. Check them for
level once again and then attach the mesh to the face of the bucks.
Once the entire exterior is wrapped in mesh, focus should
turn to the roof assembly. A detailed
explanation of roof construction is outside the scope of this report. It is vital that the roof be installed as
soon as possible to fully load the walls and to protect the bales from inclement weather.
After
the roof is complete, move to the inside and perform any of the necessary
interior work, such as electrical, before adding the
interior mesh. With
all of the interior work complete, the shaping of the bale openings and the
interior mesh work is priority number one.
Once the mesh and shaping are complete, use bale needles to sew the mesh
through the wall to the mesh on the other side.
Tying the two mesh planes together strengthens the walls
and ties the entire structure together. This is a vital step for
anchoring the bales to one and other and for the overall strength of the building. Use baling
twine to tie the mesh every 24” on center, both directions, or more if required by your local inspector. Use a Miller’s Knot to tie the twine tight to the bales. After the mesh
is fully secured, the structure can be plastered. The sooner the structure is plastered, the sooner it is protected from fire and
weather.
