block wall footing depth


Old 03-08-00, 10:06 AM
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Me again - I'm about to pour a footing which I need to be approx. 10" deep. The ground is sorta level - maybe 6" variation up and down in various places. If I want 10" of footing how deep do I dig, and how do I know the lowest spot so I make sure that I have at least 10" of concrete footing?
Secondly, the overall height of wall will be approx 30". With standard block how do I make it exactly the height I want? Thanks for your help.
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Old 03-09-00, 10:53 PM
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The last question first: the overall height of wall will be approx 30". With standard block how do I make it exactly the height I want?

Adjust the footing depth and the number of courses. For example, if four courses of 8" concrete block existed plus a 2" wall cap, that would be 34" total. To reduce this to a 30" exposed height the footing depth would be lowered 4". Carrying this one step further: 34" total wall height plus 10" of footing depth plus 4" of adjustment equals 48" from the top of the wall to the base of the trench. BTW, "about" and "exactly" are not the same. One is relative, the other is precise.

Now your first question, and some others you might have:
I suspected that you might return, so I spent 30 minutes and dug through some of the more well known Home Improvement Web Sites, attempting to find information with graphics included. In a momement or two, click on the link below.

You need to establish Zero "0.0" or B.M. (bench mark). Zero is an assignment. Got that? Zero can be assigned to the top of the wall, the bottom of the wall, the middle of the wall, or any other convenient point. Possibly, use the top of the wall, since you know exactly where it should fall. Mark the top of the wall as "0.0" B.M. on the plan you have drawn. You have drawn a plan, right?

Near to one end of the trench drive a stake of suitable length or a length of conduit into the ground (remember "plumb"), then mark the top of the wall. If the top of the wall is relative to a more distant point, pull masons line from that point and use line level or use a water level between the two points to establish the top of the wall.

Setting Batter Boards is covered on the Web Site. When the batter board cross bars are set, they are set at a convenient height, which is typically at one of the course heights. Thus measure down from "0.0" on the stake driven to represent the wall height, mark a course line; transfer this mark to one of the batter board stakes, then set the cross bar at this height.

After the batter boards are set and a level line is pulled, a Builders Square is used as a depth guage while excavating the trench. A Builders Square is simply a site built large wooden sqaure that looks like a lop-sided "T" with an angle brace extending from the short side of the cross bar to the extension of the upright that extends above the cross bar.
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The cross bar must be long enough to reach the string line from within the trench. The upright is the depth gauge protion, and must be adjusted to length (considering the wall height in realtion to the string line, and the total depth required). When the cross bar hits the string line and the upright touches the bottom of the trench at the same time, the trench is at the proper depth and it's level.

One last consideration. I assumed that you knew (maybe a poor assumption on my part), but the footing must be at least 6" below the frost line. Further details can be found here:


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