Building Concrete Foundations--Footings

A footing ditch is required under slab and perimeter foundations. Construction manuals often explain how to lay out the site with batter boards, briefly mention that you should dig the footing ditch, then move on to the next step.

Footing ditches deserve more care. How deep should they be? How wide? What if your ground isn’t very level? The answers to these and other questions given here are based on the Uniform Building Code, but local codes may vary, so be sure to check with your local inspector before starting a project. Additionally, if an architect drew up your plans, make the ditch according to the plan’s specifications, which should meet or exceed local code requirements.

A footing ditch must be dug in undisturbed soil, not fill dirt. It must be deep enough to reach below the frost line. If you live in an area where the ground does not freeze, the required footing depth is 12 inches for a one-story house and 18 inches for a two-story house.

Perimeter foundation footings must be 12 inches wide for a one-story house and 15 inches wide for a two story house. One-story houses re quire a 6”-wide foundation wall; two-story houses require 8”-wide walls. The footing must be 6 inches thick for a one-story house and 7 inches thick for a two-story house.

The footing ditch around the perimeter of a slab must be 12 inches deep or below the frost line, and 6 inches wide.

Into all footing ditches must be placed 1/2” reinforcing rods, known in the trade as re-rod, rebar, or steel.

82 Marking the footing trench: Hold plumb-bob string against the footing-marker-string to see where the edge of the footing is on the ground. Flour or sand marks edge of the footing and shows your helpers (or backhoe) where to dig.

Digging the Footing Ditch

For all the care you have put into getting the strings placed accurately, the footing ditches are not dug with great accuracy. Work carefully, but the footing may still end up a few inches out of line. However, the foundation wall itself must be placed with great accuracy. With the perimeter strings in place, mark where the footing ditches will be dug.

If a backhoe is digging the footing ditches, you need only mark the out side edge of the footing ditch. The line is marked by dribbling lime or flour from a coffee can. The operator will follow that line with the backhoe bucket. After marking the ditch, re move all the strings so the backhoe can get in there. You can replace the perimeter strings exactly as they were because the saw kerfs still mark the string locations on the batter-board crosspieces.

If you are digging the footing ditch by hand, stretch temporary strings on the batter boards, marking the inside and outside of the footing ditch. For a 6”-wide foundation wall, the out side of the footing will be 3 inches outside the perimeter string and the inside of the footing will be 9 inches inside the perimeter string. Use a plumb bob to transfer the string lines accurately to the ground, then mark the ditch outline.

Uneven Ground

Constructing footings for houses on steep slopes is beyond the scope of this guide, but it's not uncommon to build a house or cabin on uneven ground. This may mean, for example, that the foundation wall at the high point of ground will be the minimum 16 1/2 inches above grade, but the other end will be 3 feet high. That means pouring a lot of concrete. If practical, the best solution is to have a bulldozer level the ground for you. The alternative is to step the foundation down. Short stud walls, called cripple walls, are then built on each foundation step to bring everything up to level.

83 Digging the footing ditch: A backhoe can dig a trench considerably faster than you can with a shovel! Trench outline marked with flour or sand. Foundation on uneven ground: Foundation wall, Mudsills, Cripple wall: 2-by-6 studs and top plate. To save concrete on sloping ground, step the foundation and fill in with a cripple wall.