of Project Here
Selecting a Location
1.) If your adding to an existing structure, it will be necessary to
drill and dowel
into the existing foundation. Call your electric or gas company and find out who to call to have
your property checked for underground utilities if you don't know already.
2.) Refer to these illustrations
for how to setup a batter board to level this type of foundation. Allow 1-1/2" for the thickness
of form when securing the string to the batter board. Local codes will vary. Some areas will require
a deeper footing due to frost lines. For those, we will add a section for basements and crawl space
later. You will find monolithic slab type foundations more in the southern regions where there is
very little to no frost line.
3.) Once you've setup the batter board and the string is at the proper height and square, drive
2x4 stakes every 3' feet around the outside of the perimeter of the string. On areas where
the string is over 18" from the ground, space your stakes every two feet. Using a level to hold the
stake plumb as you start driving it will help keep the bottom of the foundation plumb with the top.
For the most part, driving the stake just below the top of the string is more desirable for ease of
working on the surface of the slab and setting the forms using the string.
4.) Breaking the ends of the form boards on a stake, extend the first form several inches beyond the
first corner and use the string to set the height. To secure, hold a sledge hammer against the inside
of the form and fasten forms to the inside of the stakes with two #10 duplex head nails driven through
the stake from the outside. Tip: string tends to sag in the center, allowing for this will help keep
your forms true and straight. If more form boards are needed to reach the ground, stagger the ends of
the boards so the joints don't land one over the other and the ends break on a stake. Begining at the
same corner, set the form for the adjoining side in the same manner. Always measure and mark the forms
when making another corner rather than relying on the string just to be on the safe side. The string
should be used for height and for square only. Install all forms and secure to each stake and at the
corners. It may be necessary to rip the bottom form for a snug fit to the ground. Also backfilling dirt
against the bottom of the form after bracing will help hold the concrete in and reinforce the form.
However, remember you need to remove this board at a latter time. Too much dirt means you may be doing
some digging to remove the form.
5.) Refer to these illustrations
to get an idea of the many concrete slab configurations. Dig your beams according to your plan and the
specifications of your local codes. Any plumbing or electrical needing to be installed under the slab
should be done after beams are dug including any beams required on the interior or center of the foundation.
6.) Using a string along the top of the form, remove any earth within 5 to 6" then backfill with sand so
your concrete is a minimum of 4" thick.
7.) Slab foundations usually require a vapor barrier to be installed under the concrete. Use a suitable 6 mill
polyethylene plastic sheathing. Depending on your local codes, vapor barrior if required need only
be installed over the surface that supports the floor of the slab and need only extend down into the beams
but is not necessary to line the entire beam.
8.) Measure 12" from a corner and drive two #3 rebar stakes cut 6" longer than the height of the
form measuring from the bottom of the trench. Drive the first at 2-1/2" away from the outside form
and the second at 7-1/2". Repeat this every four feet around the perimeter and down every interior beam
starting and ending 12" from a corner or end of abeam. Drive rebar stakes 1-1/2" below top of
form, 3-1/2" where there is a brick ledge.
9.) Using 20' lengths,
bend a piece of #5 rebar about 4' from the end at a 90 degree angle. Set this piece in a corner
and lay between the outside rebar stake and the trench wall, attach it to the steak with wire ties
about 3" off the bottom of the trench. Bend another #5 rebar at 4' and install in the same corner directly
above the first but because the ends of the rebar should be staggered and not end with a joint one over the other,
install with the 4' section facing the opposite direction and secure to the rebar stake 3" from the top, 4" if
there is a brick ledge above it.
10.) Bend two more pieces, this time at 8' and install on the inner stakes at the same height as the
first two bars. Again, do not break the ends one over the other. Install rebar around the perimeter
overlapping bars a minimum distance equal to 40 diameters of the bar, or 25" for #5 (5/8") rebar.
Steel should be within 3" of the bottom and top, or 4" from the top where there is
a brick ledge. If the paired rows of rebar will be farther than 20" apart, (i.e., it's more than 26" from
bottom of trench to top of form) install another pair of rebar between the first two pair. Any brick ledge or
doorway forms needed may be installed now. A brick ledge is typically a 2x6 nailed flush to the inside
top egde of your outside forms. This creates a 1-1/2" recess below the finished floor.
Local codes will be a determining factor.
11.) Lay #3 rebar 18" on center both directions with the perimeter bars within 1-1/2" of the form.
Stagger the ends and overlap a minimum distance equal to 40 diameters of the bar. This will be the
reinforcement for the floor and can be referred to as the mat. Install concrete chairs 4' on center
both directions. Install them as you go as it maybe difficult to lift the mat later on. Install the
chairs that are tall enough so the mat is located in the center of the concrete. Secure each bar where
is crosses another with wire ties, including the beam rebar.
12.) On areas where the form is more than 6" above the ground, drive a 2x4 stake 2 to 4' out from each
form stake, drive till it is very firm and deep into the soil.
13.) On areas where the form is 18" or more above the ground, following
this example, measure for a
2x4 brace to lay directly on the ground between this new stake and the form stake. Next, measure for
a 2x4 to set on top of the first brace and extend to the top of the same form stake. Nail securely
with #16 nails.
14.) I will explain what I think is the easiest way to do this. First you will need a temporary
center form which will be removed after you pour. This form will be used to screed the concrete. (see Pouring
below) Setup a string from one end of the form to the other to set the height of the center
form. Drive some short 1x2 stakes every 3' until the top of the stake is just below the string
lay a 1x4 next to the stakes and using the string as a height gauge, fasten the 1x4 to the stakes
from one end of the foundation to the other.
Tip: strings tend to sag in the center, allowing for this will help keep your center strait.
15.) A pump truck is the best tool for pouring large slabs. Smaller ones can be easily poured with
just a truck. If access is not possible for a truck, several wheel borrows and a lot of help is an
option. Other options include machines such as concrete buggies or a tractors with a bucket. However
you decide to do it, dump some concrete near the center between your forms about 4' from the end.
Using rakes or tools called comealongs, move the concrete till it is evenly distributed throughout
from one form to the other. Use a long 2x4 or 2x6 called a screed and drag across the top of the forms
to make the concrete even with the top. Pour more concrete as needed and push into the low areas or
drag concrete out of the high areas as you drag the screed. After pouring beyond a section of center
form you will remove that section, stakes and all, then toss some concrete into the depression left by the
form. For example, if your 1x4 form is in 12' lengths, pour about 13 to 14' all the way across the slab,
then pull up that 12' section of form. You will have trouble finishing if you don't pitch some concrete
every couple of feet into the depression left behind.
16.) After you move down and before the concrete begin to set, use a tool called a bull float, which
is a large adjustable metal trowel about 3 to 4 feet wide with a long multi-sectional handle. Assemble
the float and starting back at the beginning of the pour.
17.) Run the float back and forth from one side to the other in an area till all the pits have disappeared.
Then move down a few feet and start again. Have someone following behind you and install the foundation bolts.
Locate bolts 18" from the corners and every 6' around perimeter. Set bolts back from form or brick
ledge 1-3/4" for 2x4 walls or 2-3/4" for 2x6 walls. Do this till you reach the end of the
pour. Water can be sprinkled on the surface if necessary to help remove the pits. Once the surface
has all the pits and imperfections filled by the bull float your ready to move on.
18.) Once the surface is firm enough to support a person standing on it, set a troweling machine on
top and try to run it. If the blades turn without digging into the surface, start troweling, two machines
would be better than one. Have one or two persons hand trowel around the plumbing pipes and foundation bolts
ahead of the machine. To help in the removal of forms, you can lightly run an edger around the perimeter.
Keep running the troweling machine till it is no longer needed. Water can be sprinkled lightly to help take
out ridges and imperfections if the concrete drys to fast.