Poured Perimeter Foundations

The most widely used concrete foundation in this country is the poured perimeter foundation. This type of foundation can be used on level or sloping ground, and has the advantage of keeping the house above the ground.

A perimeter foundation also pro vides a crawl space under the house for easy access to plumbing, wiring, and heat ducts.

The poured perimeter foundation can be built in several ways. In one style, not very common today, the footing is poured first and allowed to set before the foundation wall is poured. That means the concrete truck has to make at least two trips, which costs more. The method widely used by professionals today is to pour the footing and the foundation wall all at once. This style, known as a stem wall, is described here.

The stem wall is formed with lumber you will later use for the floor joists and rafters. This means you don’t have to buy special form material. The joists and rafters will be stained slightly by the concrete, but this doesn’t matter because they are hidden from view.

To construct a stem wall, you suspend the foundation wall forms in the footing ditch. As the concrete is poured, it fills the bottom of the ditch, then comes up inside the wall forms.

Constructing the Forms

The perimeter wall is laid out in the standard fashion using batter boards and twine to find the outside of the foundation wall, as described.

The stem wall is suspended over the footing ditch by hanging the form boards on either 2 by 4 stakes or steel support posts. The steel support posts, which can be rented, are easier to use, particularly if you have to drive them into hard ground.

Support Stakes

Place the outside form boards first, starting with the end wall at the highest corner of the layout. With your perimeter strings in place, begin driving stakes about every 4 feet into the ditch. Since the form boards are 1½” thick and the stakes will be nailed to the outside of the boards, use your tape measure to place the stakes 1½” outside the perimeter line. When you nail the boards to the stakes, the inner edge should be directly under the string.

One trick for getting steel stakes nearly vertical is to suspend the stake at the top between the tip of your thumb and forefinger before you hammer it into the ground. After the form boards are nailed to the stakes, the entire wall will be straightened and braced in position.

Ill.90a Stem Wall Foundation: Drive steel support stakes into the footing trench 1½”outside the “outside of-foundation-wall” string.

Ill.90b A poured perimeter foundation requires sturdy, well-braced forms to hold the concrete. However, the finishing work is easy, since there is no large surface to smooth.

The Form Boards

With the stakes in place, begin nailing up the form boards. This will require two people, one at each end of a board. Start the first board at one corner, with one end of the board extending a foot or so beyond the corner of the string line. The other wall will butt up against this first wall. Pull the board up until it's just a hairbreadth away from the string, but not touching it. Nail through the stake holes into the form board. Keeping the top board perfectly in line with the string, put at least two nails through each stake into the board.

When necessary, drive additional stakes to support the end of a form board and to start the next one that butts up against it. After the top boards are placed along the first end wall, start the second course. Space this course so the ends don't fall on the same stakes as the top course. Pull the second-course boards up tight against the top course and nail through the stakes. The number of courses you must put up will depend on the height of your wall and the width of your boards. The last course should extend into the ditch to approximately the top of the footing. This means the bottom of the lowest form board will be about 6 inches from the bottom of the ditch, which allows a 6”-thick footing.

When you come to the first corner, allow the ends of the form boards to extend beyond the string line. Butt the ends of the next wall against the first wall, and nail the two walls together. You will have to cut the bottom board that hangs in the footing ditch, but by not cutting any of the others, you save most of your joist and rafter stock.

Continue placing stakes and boards along each wall in this fashion until the exterior wall is complete.

Straightening and Bracing Walls

At this point, the wall may be up, but it will not be at all straight. You straighten and brace at the same time. Bracing the wall thoroughly can not be overemphasized. You have never really felt sick to your stomach until you have seen a wall suddenly give way, and tons of concrete begin to run out onto the ground.

Braces are made from 2 by 4s. The bottom should be 2 feet long and the upright leg about the same height as the wall. Cut the ends of the diagonal piece at a 45-degree angle, and nail the brace together. You need a brace every 4 feet and at all the corners.

Use these angle braces to straighten the wall. This is done by pulling the wall into line directly under the string, nailing the brace to the wall, and then driving a stake in the ground just behind the end of the brace. Then nail the stake to the brace so it won’t slip off the stake under the weight of the concrete.

Additional bracing must be done wherever form boards are butted together without a steel support post on each end, which may happen if you run short of posts. The wall can be braced here by nailing a length of 2 by 4 over the butt joint to prevent the concrete from forcing it apart.

When the exterior wall has been straightened and braced, the next task is to hang the reinforcing rods before you do the interior walls.

91a Building the forms

91b Form braces: Stake, Angle braces staked to the ground hold forms in line with strings.

Hanging Steel

Steel reinforcing rods must be suspended in the footing and in the foundation wall. One layer of reinforcing rod (normally 1/2 inch diameter, but check with your building inspector) should be placed in the center of the footing, and another about 2 or 3 inches from the top of the foundation wall. If the wall is more than 18 inches high, a third layer of steel should be hung between the top and bottom pieces.

Reinforcing rod must be bent to go around the footing-ditch corners. To do this, put the rod on the ground, place one foot on it where the bend is to be, then pull the rod up toward you. Or you can drive two wooden stakes firmly into the ground spaced about two inches apart, put the rod between them, and then bend it.

Wherever one length of reinforcing rod meets another, the two must overlap by 24 inches and be tied together with metal tie wire, available in most hardware stores.

If you must cut reinforcing rod, cut about halfway through with a hack saw, then bend it. It will readily break at that point.

To hang the steel in the forms, drive 16d nails every 3 feet into the forms, about 3 inches down from the top of the forms. Start hanging the steel from the top down, wrapping the wire around the reinforcing rod, hanging it from the nail, and allowing enough wire to tie the one or two more layers of steel below that. Keep the steel away from the forms or they will show through once the form boards are removed.

The Interior Wall

To form the interior wall, first string a line from the batter boards 6 inches inside the exterior wall line. Place the support stakes 1 1/2 inches beyond that line, then begin hanging the form boards in the same manner as you did the exterior wall. More boards will have to be cut for the inside form wall because they can't overhang like the exterior wall.

Once the interior wall is hung, it's easier to straighten than the exterior wall, because you tie it to the outside wall with spacers. These spacers, cut from 1 by 4s, should be 9 inches long for a 6”-wide wall. The additional 3 inches allow you to nail the spacers to the top of the inside and outside form boards.

A ¾” hole is drilled in the center of each spacer and the anchor bolts are suspended there during the pour. Find the center of a pattern board by drawing diagonal lines connecting the corners. Use the pattern board to drill out the other braces. Cut enough of these braces to allow you to place one 6 inches from the end of each mudsill and not more than 6 feet apart.

These anchor bolt supports not only tie the top of the wall securely together, but also space it precisely 6 inches apart.

At the mid-level of the wall, use steel tie straps to further tie the wall together. These straps are narrow steel bands that slip through the joints in the form boards and are held in place by pegs on the outside. After the concrete has hardened, remove the pegs, strip the forms, and snap off the short piece of tie strap sticking out of the concrete.

In addition to all this bracing, still use the triangle braces every 4 feet along the inside wall.

92a Hanging the steel: Suspend re-rod from forms with wire hung on 16d nails.

92b Placing spacers and tie straps: 1 by 1/4 spacers with anchor bolts suspended in center holes; Steel tie straps with tapered pegs holding them taut. The inside form is placed in the same way as the outside form. The inside and outside forms are then tied together with spacers and tie straps.

[[ Some Concrete Tricks: Since you need access to the crawl space under the house after the foundation has been poured, you must form an opening in the wall. Using redwood or treated lumber, make the forms from 2 by 6 material for a 6- inch-wide wall. As shown, it's a three-sided box that's dropped into the formed wall where you want the opening. The bottom of the box should be positioned just above ground level. The open top of the box will be covered by the mudsill. Hold the access form in the wall and nail it in place. Drive a few nails into the uprights to hold the boards in place when the concrete is poured. When pouring, be sure to work the concrete with a shovel so it flows under the bottom of the opening. If girders will be used to support floor joists, the inside of the concrete wall can be “keyed” to accept the ends of the girders. This is done by cutting a 2” length from the end of one girder, or using a similar piece of scrap, and nailing it to the inside form precisely where the girder will fit. Use batter boards set for the interior posts and piers as your guide. The top of the key must be flush with the top of the mudsill, not the top of the wall, so be sure to raise the block 1 1/2 inches above the surface of the wall. When the forms are stripped, the block will have left a 2”-deep indentation in the wall where the girder end will fit.

If you are planning on running water, electricity, or heating ducts underground into the house, you will have to form holes in the footing.

This is easily done by placing a length of electrical conduit or plastic drain pipe across the footing ditch before the pour is made. Dig a trench so the pipe extends about 12 inches beyond the footing on each side and can be readily located after the pour is complete. In addition, remember to wrap any conduit or drain pipe in burlap or roofing felt paper before the pour. This provides a cushion around the conduit and prevents the expansion and contraction of the concrete from eventually cutting through the conduit or pipe.

Place the plumbing waste line across the footing ditch in a similar manner before the pour is started.

Additional pipes can be dropped into the footing if you feel it may be necessary to drain water from under i house during the wet season. ]]

Ill.93a Access to crawl space: To provide crawl-space access, insert a three-sided box of 2 by 6 boards into the forms. Anchor bolts within 6 inches of opening

Utility access: Holes through the footing far utility access are made…

Ill.93b Key for beam: Nail a 2” piece of beam lumber to the inside of the form. Additional batter board marks position of beam.

Ill.94 Pouring the concrete: Let the concrete form 2 or 3 inches above the bottom of the forms. The first pour is to fill the footing trench. Helpers must poke and prod the concrete to settle it and remove air pockets. Continue poking and prodding. Be sure forms are rapped to seat the concrete against the wood.

- Making the Pour -

Order the concrete as discussed. Then, before that big concrete truck arrives, go over your form work one more time, checking that everything is square, the steel is hung, and the access form plus any openings for water and electricity are in place. Then make sure that every thing is well braced. Lean over your forms and push out on the walls as hard as you can. If it moves, it isn’t braced enough.

Spray or paint the inside of the forms with old crankcase oil shortly before the pour so the forms will pull away from the concrete without sticking and defacing the foundation wall.

You should have a minimum of two and preferably four other helpers on hand. One person must control the chute, working with the truck driver to go around the forms placing the concrete. The other two should be working closely behind the chute operator with long stakes, prodding and poking the concrete to make sure it settles. Another can work behind them with a hammer, rapping on the form walls to chase out air pockets.

Filling the Forms

The chute operator should take the concrete around the forms once to fill the footing ditch until the concrete runs 2 to 3 inches up inside the forms. Don’t carry it much higher than this or the concrete will force its way outside the footing ditch.

Once that first pass is made, start the second pass that places the concrete in the foundation wall. If the wall is 18 inches or higher, fill it in two passes. Again, this allows the first layer to harden and reduces the pres sure on the forms that would result if you filled the wall all at once. Don’t, however, permit any undue delays in passes with the concrete. If one course is allowed to harden completely, the next course will not mix with it and properly adhere at this point, resulting in a weakness known as a cold joint.

As the chute operator works, the helper directly behind should be watching that the concrete does not force the reinforcing rod against the form boards, which would result in the steel being visible after the forms are removed. If the steel is out of line, it's also not doing its reinforcing job.

The Final Steps

Once the concrete reaches the top of the form, a helper should flatten and smooth the top flush with the top of the form. The helper can use almost anything for this, such as a short length of 2 by 4 or a wood float. A smooth, flat top means the mudsill will lie that much flatter.

Let the forms remain in place for at least 48 hours, and preferably for 72 hours. When you strip the forms, remove any concrete stuck on the boards with a flat-bottomed shovel, so it will not dull your saw blade when you cut them for joists and rafters.