Last time we built our forms and dug our ditches. Now we are ready to set the forms to the string lines.

There are two important things to do before you set any forms. First, you should double check all your dimensions and the squareness of your layout, along with setting up your transit and making sure the top of your batter boards are still level. No matter how careful you are when you lay out your building, things have a tendency to move. If you pull strings one day and come back the next to dig and set forms, there is a possibility that some knot head may have monkeyed with your stuff.

The second thing to do is tighten the string lines because they will stretch and sag.

Rip 1/2” rigid insulation boards length wise 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 to keep the heat in that 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, and you do not want to leave wood in the ground to draw termites.

If you’ve ever tried wrecking forms without the barrier, you know how important this insulation board is. Stakes that have concrete all around them are almost impossible to get out after the concrete sets. Some people pull the stakes out while the concrete is setting, leaving the form with nothing to hold it in place. I don’t like this because people stepping on the form may push it down and create dips in the slab on the edge.

If your bottom of footing to top of slab is 20″ and your form is 12″ high, you should be able to make your insulation board about 10″ wide to give you enough to come up past your bottom 2″ x 6″ and fit between the stake and the form.

As I mentioned in part one of this article, you will need three stakes to hold a form that is 8′ long. If you do not have the form with the brick ledge, you will need four stakes because there is more give in a 2″ x 10″ than the brick ledge form.

The width of your form determines where you drive your stakes. If you have a form that is 6″ wide, add 1/2″ to that for the thickness of your insulation board and that will be the measurement from the string to the inside edge of your stake. Some people put a level on the stake to be sure that it is plumb, and this is a good idea as it helps to keep your forms straight and accurate. Tack the insulation board to the stakes after they are driven in and you’re ready to set forms

Setting forms is best done with two people, as I mentioned before, 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 1/2” below the building string. Put a nail in the top 2” x 6” and also the bottom one. The form may 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. Because you have an extra 1/2″ between the stake and the lower 2″ x 6″, you may need to put a scrap piece of the insulation board between the stake and the upper 2″ x 6″.

Another item you might use at this point is the laser level. If you have access to one, it simplifies the process of getting the top of your form to where you want it. Instead of gauging from the string, you have a more accurate leveling process with the rod and target. If you have never used the laser level, it is very simple and easy to operate after you set it up.

After you complete a wall from one corner to the other with forms that are nailed off, 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 that it doesn’t cave in when the stob is driven. The form may not be exactly plumb because stakes have a tendency to shift when you drive them into the dirt. This is not critical as long as the variance is minimal.

Set the other forms around the building in the same manner.

Next time we’ll talk about the last preparations and the pour.

  • Share/Bookmark