Let’s say this is a DIY project, and you just can’t get back to the formwork for the stemwall right away. This is when you’ll thank yourself for leaving the footing formwork in place.
Layout is much more critical now than was for the footer. The accuracy you achieve in laying out the stemwall will determine the squareness of your structure, which is very important. Start on one corner by measuring in 3-1/4″ from each side of the corner and making a mark. This will be the the outside edge of the formwork, which will be 3/4″ plywood. Now do the same for an adjacent corner that is the farthest away. This will give you a long “baseline” of which to square the rest of the building to.
Squaring your walls can be accomplished by several methods, none of which are too difficult. It will depend greatly on the equipment you have available. If you just happen to have, or know someone who has, a 2 beam laser you could layout your whole foundation in an hour. Set the laser over top one of the points you have made and line one beam with the other mark. Now you should have a perfect 90 degree angle between the two beams. Just mark the laser beam at the opposite corner, measure the distance that you need between points and set up on the mark. You will want to snap chalk lines between these points also.
Another method is the 3-4-5 triangle, which is just some basic geometry. Any triangle with multiples of 3-4-5 respectively will have a 90 degree angle between the 2 shorter legs. Start the same way as before by marking the two corners at 3-1/4″ from each side of the corner. Snap a chalk line between them, and double check the length between the two points to match what dimension you require. Now from one point measure towards the other point, along the chalk line. You will want a distance of at least 6′, mark it there. Now you will measure a distance of 8′ at 90 degrees to the chalk line. This measurement will need to be put down as an arc; with a little practice you should be able to mark the arc by using the end of the tape as a guide. Now measure from your 6′ mark toward the 8′ arc. You will need a measurement of 10′. Where the 10′ mark crosses the arc make a mark. This point and the first point you made creates a line that is a 90 degree angle to the original chalk line.
Continue on around the whole foundation; snapping lines, measuring, creating 90’s, measuring, and snapping.
After your layout is complete and you feel comfortable with the results you will nail a 2×6 into the 2×8 footing formwork with the inside edge lined up with the chalk lines. This is the part where you let it sit for 3 days before nailing the 2×6 down. If you had gotten to it the next day you could drive nails right into the concrete with out too much trouble, but we all know that sometimes it doesn’t work out that way. Nail your 2×6 down all the way around, along the chalk line. You are now ready to start the forming of the wall itself.
Formwork for your forms should be 3/4″ plywood. This will come in 4’x8′ sheets with no holes for ties. You will need to drill holes in all the sheets for the ties to go through. The ties will hold both sides of the formwork together at a set dimension while you pour the concrete. Typically, I will drill holes at 16″ on center and start 8″ in from each side. After drilling you will begin at a corner and nail the bottom of the plywood to the 2×6 “sill plate” with 8d box nails or roofing nails. Do the two pieces on each side of the corner first and nail the tops together, which will insure it doesn’t fall over while you are putting up the next piece.
After you get the outside of the form up you will start installing your vertical and horizontal rebar. Start with your 3′-9″ vertical #4’s by tying them to the stubs we talked about in the Concrete Footing article. After this is complete install the horizontal #4’s. On a typical 4′ tall wall you should have 3-4 rows, of which the first will be about 3-6 inches from the top and the bottom one about 10-12 inches from the footing. Make sure to wrap the corners and not just dead end it, and also to overlap each new piece at least 12 times the diameter of the bar.
Before you start on the inside form you will need to “Stuff Ties” on the outside form. For this operation you will need to purchase ties that will hold the inside and outside form at the correct distance apart. These ties will need to be placed in the holes talked about earlier and 2 – 2x4s (whaler) placed on the outside with a locking cam to hold it in place (you will probably need to rent the cams or borrow from someone). Now that you have all the ties stuffed you can begin to hang your inside form. You will probably need to rip 8″ off the inside corner form to line up with the outside form tie holes. As before, start at a corner and work around the foundation. After the inside form is set go back again and install your whalers and cams.
NOTE: What is described above is a typical method of forming a stemwall. Make sure to pre-plan everything. Did you make the wall a length that is divisible by 4? If not, where are you going to put the filler, and will the tie holes line up? How much do you need to rip off the ends to start a corner?
Pre-made forms come in a dozen different varieties and each has it’s own benefits and problems. When selecting forms talk to your local concrete rental store to see what would be best for you.
Prebuilt forms are fairly easy to set. The same principles apply as the plywood forms, except that you will probably have fillers for most typical widths and premade corners. Your concrete rental store should have installation directions and can answer questions. I would go into more detail but there are so many different systems that it wouldn’t be very beneficial. Just remember – PREPLAN.
The last thing to do for your formwork is plumb it and square the top. Again start at a corner and plumb the corner with a 4′ level. Have some 2×4’s and form stakes nearby. When you determine where the wall needs to go place your 2×4 bracing accordingly. Nail it to the top whaler and place a form stake a distance away that will allow you to nail the stake to the 2×4 bracing. Push and pull the wall until it is plumb and square.
You are now ready to install your screed strip. A screed strip is a reference line that you can’t miss when pouring concrete made of 1/4″ x 3/4″ pine or something similar. Use a builders level or a laser level to put a line around the entire inside form at finished concrete height. Nail the top of the screed strip at this line. I suggest putting it to the inside form so that it won’t show after stripping and backfilling. Now when you pour the concrete you will have a good reference of exactly where to finish to.
Pour the concrete and finish the top level with the screed strip. After finishing the top of the wall place your anchor bolts 6″ from each corner and 3′ on center around the wall. Some say that 4′ is good enough and it probably is, but I usually go with 3′ or sometimes less. The anchor bolts will be used later to hole your sill plate down.
Stripping your forms is a simple task that involves popping the cams off and stripping the whalers off. Then strip the plywood or prebuilt forms off and break your ties off. If the wall will be exposed you can patch the tie holes back in later. Now strip your footing formwork.
The last thing you will need to do is waterproof the wall. This can be a pain, but is a necessary step. There are many different types of waterproofing materials; ask your local home building center what they recommend.