CONCRETE SLABS
Slabs are the concrete on which we place
buildings. In the old days,
concrete was not as readily available as it is now, and most floors were
wood. When making a decision for
either wood or concrete, keep in mind a few things.
If the grade is extreme, the slab is not cost feasible.
You should consider the wood floor
over a crawl space. With the wood
floor system, you have the added area under the house to put things. (Usually,
things you will forget and never see again.) Also, from a termite control standpoint,
it is easier to retreat later.
Plumbing is also easier to maintain with a crawl space. With slabs, plumbing pipes are sometimes
bumped and moved during the pour.
Those mistakes have to be dealt with after the concrete has set. With a wood floor, the majority of the
rough plumbing goes in after framing assuring less chance of error.
Disadvantages to wood floors are the inevitable
creaks and pops caused by curing, changing temperatures, moisture and gremlins.
If you have a water leak that you
are not aware of, as in a slow leak, the wood may rot. Even with plastic on the ground under
the house, temperature variations can allow condensation and decay.
If you have a level grade, slabs are the usual
choice because of cost. Floors
don’t squeak. One thing that I like
is the close proximity to the outside elevation. If you have to jump out a window, it’s
not that far to the ground. When we
get to the point in life that we can’t get around as easily as we would like,
climbing a lot of steps just to get in the house becomes a challenge.
There are two kinds of slabs that we will
discuss. The first is the
traditional slab where the blocks are lain on a footing the same way as you
would with a wood floor system. The
only difference would be the final block course which would be a header (or
form) block. Inside the foundation,
the ground should be approximately 8” below the top of the header blocks and
compacted to about 95%. Depending
on how the building will be framed, cross ditches may need to be dug for grade
beams beneath interior walls. If
you are working from plans, these will be indicated. Also, they need rebar and rod chairs the
same as the footings. This will
need to be done after termite treatment and poly have been installed.
After plumbing is roughed in, along with any
electrical or HVAC that may be necessary, 4” of gravel or sand should be
placed. All plumbing lines should
be at least 12” below the top of the block work to allow for cross ditches. If gravel is used for the fill below the
concrete, it just has to be placed and graded to 4” below grade. If sand is used, it should be tamped
with a plate tamper.
Always have your slab treated for termites, unless
it is going to be a very small temporary structure. Everyone tells me that the current
chemicals used are not very good.
However, if you don’t treat it’s the same as yelling “supper time” for
the little varmints. Also, remove
any wood inside the building area prior to installing the gravel or sand. After the pre-treat, you should not
disturb the material again.
Place 6 mil polyethylene over the entire area,
lapping the edges by at least 12".Holes that are cut for plumbing pipes should
not be excessive.
Now the decision comes for reinforcing wire or the
use of fiber in the concrete. I
have done both, but probably prefer the wire. If you are planning on the fiber at
least use wire where you will have exposed concrete, such as porches and
garages. The hairy fiberglass that
sticks out of the concrete is difficult to get rid of, and is obtrusive. I have actually gotten the stuff in my
foot by walking over it barefoot.
Place any other steel for cross ditches, and you
are ready to pour.
Watch the weather, and plan the pour. Be sure to have enough finishers to do
the work.
During the pour, you will need to place anchor bolts
around the perimeter to attach the rat sill (or bottom plate) of your outside
wall. How often they are placed is
determined by your area. Other than
actual bolts, there are some other alternatives that work well. Simpson tie makes straps that are poured
into the concrete and wrapped over the bottom plates.
MONOLITHIC SLABS
If the building is being placed on a level area,
the monolithic slab is a good alternative to the traditional footing and
block. The monolithic slab
originated in areas where the soil conditions were not good. The concept is for the footing and slab
to function as one unit and if settling or movement occurs, the whole will move
rather just a portion. There is
some truth to this. From a cost and
time standpoint, this is a good way to go.
And, you don’t need a block mason.
Usually the footings for a monolithic slab are
narrower than for the conventional.
Some designs call for a 12” wide footing. If the pad is graded level, most
footings only measure 20- 24” from
the top of the slab to the bottom of the ditch.
Monolithic slabs require carpentry skills to build
forms and set them to grade.
If you are bricking the outside of your building,
a form has to be built to allow the concrete to go under and past the line of
the exterior wall for a footing for the brick. The usual way to do this is to make a
form that is at least 8” high and 5” wide.
NOW, let me tell you how I prefer to do
it.
If the building will have masonry veneer, or if
there is a chance it might have sometime in the future, dig the footings 16” to
18” wide. The person digging the
footing should center the ditch under the string that is for the building
line. The reason for the ditch
being wider than the plans may call for is two fold. First, when the forms are set in place,
it is much easier to put the rebar in the ditches. Secondly, there is more room
for error if the ditch isn’t completely accurate.
Build the forms to be 12” high. Using 2 – 2” x 6” with 9” long blocks between thThen put 7/16”
OSB on one side that will face the interior of the slab. If the forms are made 8’ long, they are
easy to handle and set in place.
Blocks at each end and one in the middle are sufficient to hold the
concrete. Some people prefer the
forms to be only 8” high. I prefer
12” because that generally ensures that your brick will start below finished
grade, and the concrete beneath will not be seen.
With ditches dug and building lines pulled, you
are ready to set forms. The best
way to make sure that forms do not touch strings and cause them to be inaccurate
is to gauge the forms below the strings.
A small piece of ½” plywood works well for the gauge, as it can be
dropped in your tool apron.
Your building strings should be to the outside of
your exterior walls, so the forms will line up with the strings and be set to
the outside of the building. It
will take three stakes to hold up a form.
If you are building on firm ground, an 8’ 2” x 4” will yield 3 stakes 32”
long. Cut sharp points on end of
each.
Your stakes should be located strategically so
that the two on the ends will measure 8’ from center to center. This is done so that the next form that
abuts can be nailed to the same stake.
Turn the 3 ½” side of the stake parallel to the string line. Measure 6 ½” from the string line to the
outside of the building and drive the stake plumb into the ground. (Please, don’t call it level. Level is horizontal. Plumb is vertical.) Be sure not to drive the stake below the
top of your form.
Rip ½” insulation board wide enough to go from the
bottom of the ditch to above the bottom of the form. This is primarily to keep concrete from
going around the stakes, making it impossible to get them out later. It also acts as an insulator because
heat is radiated from the slab.
When the slab is being poured, one person needs to be on the outside
pushing dirt behind this insulation as the concrete goes in the footing to keep
the board from breaking through.
Some people use plywood for this, but that is something else that is hard
to wreck out after the pour.
Setting forms is best done with two people, but
can be accomplished by one. I
prefer to use double headed nails to attach the stake to the form. This makes it easier to wreck the
forms. Use the gauge to make sure
you are ½” below the building string.
Put a nail in the top 2” x 6” and also the bottom one. The form will not line up exactly with
the string, but we will handle that when we brace it off. It is better for the inside edge of the
form to be a little outside of the line, rather than to the inside at this
point.
After you complete a wall from one corner to
another, go back and line the forms to the string by driving a short stob on the
outside and cutting a kicker to go from the ground at the stob to the top of the
stake holding the form. Angle a
nail at the bottom of the kicker back into the stob, and then push the form in
line with the string and nail it to the stake. Be sure the stob is far enough from the
ditch so it doesn’t cave it in. Due
to the insulation board that is behind the form at the bottom, the form may not
be exactly plumb. This is not critical because the variance will be
minimal.
Set the other walls of forms in the same
manner. I suggest checking the
grades on the top of the forms just before the pour. It is best done with a laser level,
allowing one person to do the checking.
If there is much variation (more than ¼”) adjust all the forms to the
lowest place. This can be done
carefully with a sledge hammer by bumping down on the stakes.
Prep inside is about the same as for the
contemporary slab. You will be
doing the steel in the outside footings much the same as you would for standard
footings. The interior side of the
ditches is beveled slightly during the digging and prior to installing the fill
under the concrete.
After termite treatment, it is best to let the 6
mil poly go under the footings, then place the steel, and the reinforcing wire
(if you choose to use it) last.
Anchor bolts will need to be used the same as the
conventional slab.
Check the weather, and plan your pour.
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