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How to Build a House 2: The Concrete Foundation

The Excavation

There are two main reasons why we excavate (dig a hole) for the concrete foundation of our house. We need to protect the concrete foundation from the forces of nature, freezing and thawing being the major ones. Runoff water and protection from pests the others. The depth of frost is available for just about anywhere we plan on building on our planet. To prevent our concrete foundation from lifting we dig a hole deep enough so that the bottom of the concrete footing is below the depth of frost line. The soil under this concrete footing should be undisturbed soil or rock containing no top soil, organic or vegetable matter. The top of the concrete foundation wall should extend up above the grade to at least 8". This keeps our wooden framed house above the elements including surface runoff and above the pests that live in the soil ready to devour our home or at least enter it. To help in the layout of the excavation and concrete foundation we make use of a handy tool called the batter board.

Layout Your House Foundation

To layout a large area like the house foundation for a complicated house, a carpenter depends on batter boards.

batter board

A batter board simply consists of 2x4 stakes driven into the ground with a 2x4 ledger nailed to it about 2' above the ground. The stakes should be braced back to another short stake nailed close to the ground to prevent any movement. On the ledger is driven a nail which holds a line made of string or wire, which marks the outside edge of the house foundation wall, be it concrete or masonry. From the wire (rebar tie wire is a good choice) a plumb bob is hung to transfer the line from the height of the wire to the level of the ground or bottom of the foundation excavation. Depending on the depth of foundation excavation, we normally allow 3 or 4 feet beyond the concrete foundation wall to the edge of the back slope of the foundation excavation. This allows us room to work in forming the concrete footings and concrete walls and later on installing the perimeter drain and piping of the down spouts from the rain gutters.

Most jurisdictions require a survey of the house lot before the approval of house plans. Surveyors will drive iron pins in the ground to mark the property lines. These iron pins are easily found by looking for a white wooden post driven in beside the pin. The white posts may disappear in time, but the iron pin should still be in place. The surveyor will find these or drive in new ones and draw up a Plot or Site Plan with the property lines shown.

For each property line there is a setback, a distance from the property line in which the house can be built. These vary from side to side and front to back of the property depending on your jurisdiction and location of lot, either in the city or the country. This information is readily available at your town hall. On a simple box house, a surveyor will usually stake the corners of the house foundation walls right inside the trench for you, This practice virtually eliminates the need for batter boards, except for the more complicated floor designs. I've even persuaded my surveyor to shoot lines right on the batter boards to ensure the lines were square and at the proper set backs.

Mark out the excavation area, allowing for the extra room, with temporary 1x2 stakes or paint a line on the soil with a brightly colored spray paint. Make sure the batter boards are placed outside this excavation area so you can use the same ones to layout the concrete footings. It is important to note that batter boards should be installed at roughly the same elevation, on a level lot, enabling the measuring of diagonals to ensure the building layout is true and square.

House plans usually don't give elevations of the building lot, unless drawn up by an architect. The height of the top of the forms is usually decided by the house framer or builder who is putting in the house foundation. His concern is to have the building lot fit the house plan, according to the height of the existing grade and if the existing grade will be the same as the finished, permanent grade. This is one thing—as a new homeowner—to discuss with your house builder. If an architect is hired to draw up the house plans, he usually oversees the building of the house as well. The homeowner then discusses existing grade and permanent grade with the architect.

The Concrete Footings

Usually concrete footings are 6" thick and 16" wide running continuously around the perimeter of the house. Cross footings of the same size may be shown on the Foundation Plan for the house. These are concrete footings for wood framed house walls supporting a bearing wall in a basement or floor joists in a crawlspace. Since our concrete and masonry walls are 8" thick, the concrete footing extends 4" past the house wall line on each side. This is to stabilize the house wall as well as to add more surface area to bear the wall and house on the soil beneath it.

Some jurisdictions require the use of rebar in the concrete footings and walls, others don't seem to bother. Concrete has a strange property when it is reinforced with steel, it takes on the properties of steel. It becomes elastic like steel and has the tensile strength of steel, rather than a block of concrete which is brittle and not very cohesive. Required or not, I usually place rebar in my concrete footings and walls. Rebar is made from recycled steel, so the cost is not that high. The diameter of the bars is designated in eighths of an inch. In the concrete footings place two #4 rebar (4/8 or 1/2") spaced out, running the length of the concrete footings. Use rebar tie wire to tie the cross pieces together in the corners, as well as to join lengths of rebar together. Rebar comes in 20 foot lengths so not many splices are needed. If you need to extend the length of the rebar, the two pieces need to overlap each other. Overlaps for #4 rebar should be at least 15" with two ties.

The concrete footing forms are made from 2x6 lumber with a 1x3 cleat on the top and on the bottom of the concrete form, which keep the concrete from pushing the sides of the concrete form apart when the concrete is poured. The cleats are spaced about 3' apart and the bottom ones are buried in the concrete. Movement to the side is restrained by 1x3 stakes driven into the soil. The concrete footings are not leveled if the excavation is fairly level. Rebar is placed into the concrete footing and hung from the cleats about 3" off the soil. The concrete footings can be poured at this stage, in which case a key is formed along the top of the concrete footing to allow the concrete wall to be poured into the key. The vertical rebar, if any, would be in place before the concrete is poured. Generally, if the concrete wall is only 2' high, no vertical rebar is required. Two horizontal rebar are required within 6" of the top and bottom. For a 3' to 4' high concrete wall, both vertical rebar and horizontal rebar are required at 18" on center (o.c.). For 4' to 6' high concrete walls both rebar should be placed at 16" o.c. and for 6' to 8' high concrete walls the rebar should be every 12" o.c. Over 8' high concrete walls, better check with an engineer or building inspector.

The House Foundation Wall

If the site requires the use of a concrete pump, the concrete footings and walls can be poured together. The cleats on top of the concrete footing will hold the bottom plates of the concrete wall forms. The location of the concrete wall is marked on each end of the concrete footing on top of the cleats, 3/4" is allowed for the concrete wall form plywood. A line is snapped between the two ends and the 2x4 bottom plate is nailed in position along the line. Both plates can be laid out and fastened at the same time, leaving a distance of 9 1/2" between plates for an 8" concrete wall.

Depending on the height of the concrete wall from the concrete footings, a decision should be made to order the concrete forms. Draw up a little sketch and take it to the concrete form rental outlet. Here you will receive the necessary plywood and concrete form ties and tie bars to complete the work.

If the exterior sheathing on the concrete walls of the house are 3/4" boards, these can be used as concrete forms, as well. In my area we use 1x8 re-sawn boards that are less costly than renting concrete forms, but more involved in labour. In this case the concrete tie bars will go vertically every 16" in place of studs.

With plywood concrete forms the tie bars go horizontally every 16". The ezze snap ties go in a 16"x16" pattern. With the 1x8 boards the concrete ties lay in between the boards starting on top of the bottom board and continuing up every other board.

Usually when you rent the plywood concrete forms the plywood is pre-drilled out for the snapties. Since the plywood concrete forms are pre-drilled, the snap ties can be inserted after the plywood concrete form walls are up.

2x4 studs are used every 8' to help tie the concrete forms together and hold the plywood or boards in place. If the concrete tie bars are installed in the horizontal position, the studs are eliminated and a horizontal top plate is used to hold the plywood together. Every 8' or so, a brace is nailed to a vertical stud or toe-nailed to the top plate. The brace is usually 8' long and is staked to the ground to prevent the form wall from moving and keep it straight when pouring the concrete.

For straightening the concrete form wall, a string line is run along the inside of the outside form wall. To keep the string away from the concrete form wall, insert a piece of 3/4" cleat or stake at each end. Carry a third piece with you while checking the line. Have a partner move the brace back or forth until the distance between the string and the concrete form wall equals the thickness of the 3/4" piece.

Once the outside concrete form wall is up and fairly straight, braced to keep it from falling down, the top of the concrete form wall is marked with points shot on with a builder's level. This is a very accurate way of making sure the top of the concrete form wall is level. Years ago we used to use a see through plastic garden hose filled with water. Since water finds its own level, a worker would hold one end of the garden hose near the top of the wall while the other would walk around the forms placing level marks at the top of the bubble. This worked great unless the worker tripped and lost some water out of the hose. We would then have to start over. It also was time consuming for the movement of water to settle down inside the hose. Spend an extra couple of bucks and rent or borrow a builder's level.

Lines are snapped between level lines all around the top of the concrete form walls. A 1x2 levelling strip is fastened to the line, with the strip below the line. This strip becomes a useful aid to levelling off the fresh concrete with a float.

When the concrete forms are stripped the pour strip leaves a 3/4" x 1 1/2" depression. Any block outs should now be put in place. A block out is a wooden form built to keep the concrete out of a certain area of the wall or footing, When the concrete forms are stripped or removed, the block out is removed leaving a hole or pocket formed by the cement. Plumbing pipes, electrical wires, etc. can be inserted through these holes after the concrete is poured and the mechanical trades appear on the job site at a later date.

Rebar (Reinforcing Steel)

Rebar is now installed in the wall. Here are some guidelines:

Concrete footings should always have rebar—two continuous lengths of rebar spaced out evenly about 3" above the soil.

For a 2' high wall no vertical rebar is required, one horizontal rebar within 6" of the top and one horizontal rebar within 6" of the bottom.

For 2' to 4' high walls place vertical and horizontal rebar every 18", within 6" of top and bottom.

For 4' to 6' high walls place vertical and horizontal rebar every 16", within 6" of top and bottom.

For 6' to 8' high walls place vertical and horizontal rebar every 12", within 6" of top and bottom.

For over 8' high walls check with your local authority having jurisdiction or an engineer.

To tie the rebar in place on the concrete form wall, nail a 3" nail into the form wall, leaving about 1 1/2" projecting. Tie the rebar to this nail to keep it away from the concrete form as well as straight and plumb. The vertical rebar should have a 90 degree bend on it to be tied onto the concrete footing rebar. The horizontal rebar can lay on the snapties and be tied to the vertical rebar.

The other side of the concrete wall form is then placed. The tie bars are slipped into the oval holes in the snap ties. Try to keep the tie bars opposite each other to ensure that the snap ties are square across the concrete form. This ensures that the concrete wall will be 8" when the form is filled with concrete and the concrete dries. Usually we put a spreader across the concrete form at the top to keep the concrete wall at 8".

At the end or change of height of the concrete wall form we put in a bulkhead. This is a 2x6, 2x8 or 2x10 ripped down to fit the thickness of the concrete wall. Make it loose, like 7 7/8" or it will be tough pushing the bar through the snap tie. Another option for bulkheads is to make them with a 2x4 on each side of the inside of the concrete form with a ripping of plywood between, to hold the concrete back. You'll notice that the snap tie has a notch on each end on one side. This is to be able to grab the snap tie with a hammer to pry the snap tie out of the concrete form a bit to help ease the installation of the tie bar through its hole.

When all the snap ties have been secured with tie bars, check that the concrete form wall is straight and strong. Once the concrete is poured it is too late to push the form wall around or take any kinks out.

The Concrete Pour

On concrete day get as many hands as possible to help. Get the anchor bolts ready to be placed in the finished concrete. Get enough concrete trowels for everyone. Even a 1x6 board about 10" long is good, nail a 1x2 handle on it. Talk to the pump operator if this is new to you, a concrete wall over 4' should be filled in layers, not right to the top.

When the concrete form wall is filled with concrete, have a helper go before the finisher and get the excess out of the concrete form wall or fill in to the approximate level, the top of the concrete pour strip. In my experience of forming and pouring concrete in heavy construction, working on industrial buildings, bridges, etc. we always are required to vibrate the concrete as it is filling up the concrete form. In residential construction more times than not they don't use a concrete vibrator (see it at Amazon). If the concrete walls are high, will be exposed later, or you just want a good job, rent a concrete vibrator. It is very easy to operate. Just turn it on and let it flow down the concrete form wall by itself. Immediately, you will notice that the concrete level goes down, as well. The vibrator actually compresses the concrete, filling in any voids. There are a couple of things to know before operating the vibrator. Concrete is extremely heavy, about 150 pounds per cubic foot with rebar. This means that every 18" in height in an 8" wall weighs 150 lbs. Just ask someone who has wheeled much concrete! So you don't want to leave the concrete vibrator in the wall too long. You could break the concrete forms if the wall is full and you vibrate right to the bottom every time. So vibrate only the fresh concrete layer you are pouring, going into the existing concrete layer just to provide good bonding. And the other thing to watch when vibrating is the air bubbles coming out. Air is entrained in concrete for freezing and thawing purposes, we don't want to lose it. Don't leave the vibrator in the concrete bubbling away. If using a concrete vibrator insert it into the concrete quickly, let it do its job and pull it out and move on. The concrete vibrator is cooled by the concrete so don't turn it on and leave it running in the dirt or on top of a scaffold, it will burn out.

When the concrete finisher has completed smoothing off the top of the form wall to the correct level, now is the time to insert the anchor bolts. (See it on Amazon.com.) These are placed in the center of the sill plate that will be installed later. This sill plate will be a 2x6 so make sure the anchor bolts are centered for either a 2x6 sill or wall plate or possibly a 2x4 wall plate. The bolts are required to be placed every 6'. Some jurisdictions require closer spacing. My engineer wanted our 8' concrete walls to have anchor bolts every 2'. What's the cost of adding a few anchor bolts to the price of a new house? The top of an 8' concrete wall is braced by the floor joists, against the backfill. So don't go cheap on the bolts. The bottom of the concrete wall is braced by the concrete slab as shown on this drawing.

concrete foundation and slab

Stripping the Concrete Forms

After a long and stressful day of pouring concrete and finishing the concrete foundation a good rest is well in order. I like to leave my concrete forms on for a couple of days, depending on the weather. I love it when I can pour a concrete foundation on a Friday and leave it until Monday to start stripping the concrete forms. After a couple of days the concrete is still soft or green as we call it. Be very careful removing the concrete forms. Start with the braces. I like to take my old hammer and remove any ridges left on the top of the concrete wall by the concrete trowels or floats. While the helpers are removing the concrete forms and pulling the nails out and scraping any concrete off, I start laying out the sill plates or wall bottom plates. Holes for the anchor bolts are drilled in the plates and the sill gasket is stapled on the bottom. If the bottom or sill plate is pressure treated, no gasket is required. I prefer a gasket since it not only keeps the wood plate from being in contact with the concrete (so it won't rot), it fills small voids keeping the cold air out.

As soon as the outside concrete forms are stripped and the concrete footing forms are removed, we can concentrate on the perimeter drainage piping and waterproofing the concrete foundation wall. I'll be discussing this phase of building our house in the next article of this series.


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Dave

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