DEAR TIM: How thick should a poured concrete foundation wall be? Is it a function of the size of the house? Bob McKnight, Hagerstown, MD

DEAR BOB: The answer, although fairly intuitive, is not as crystal clear as you might think. As a poured concrete wall gets taller, it needs to be thicker. But there are many other variables that control the thickness of the wall. A structural engineer would consider lateral loads as well as loads from the structure above. Even concentrated loads from columns and beam pockets within a wall must be considered.

The thickness of the foundation walls you are looking at are not the same. The wall below and the one to the left of the hose bib faucet are only 8 inches thick. The other walls are 10 inches thick even though there is less soil pressing against them. Why? The 8 inch thick walls are very short and they form part of half an octagon that creates a vast amount of strength. The thicker wall is straight and runs nearly 40 feet from the corner where you see the downspout.

To add further confusion, you also have to factor in reinforcing steel. Steel bars that are incorporated into poured concrete add enormous strength to the wall system. The placement of the steel is critical depending upon how you are trying to strengthen the wall.

For example, if soil loads are significant and a house is built into a hillside, vertical reinforcing steel is a must. The pressure of the soil creeping down the hill can cause a foundation wall to develop a horizontal crack much like when you snap a saltine cracker in half with your fingers. Vertical steel bars of a given thickness spaced a distinct distance apart and placed at a precise place within the wall can help ensure the wall will not fail. Structural engineers know exactly where the steel needs to be and how much to include.

There are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes. Poured concrete foundation walls that are less than 8 feet tall and have soil outside that is 6 or 7 feet deep against the wall can often be 8 inches thick and function quite well. As soon as you go higher or have greater depths of soil pushing against the wall, you need to increase the thickness to 10 inches.

Don't forget that the concrete comes in different strengths depending upon how much cement is added at the ready-mix plant. I would use a minimum mix of 3,500 pounds per square inch(psi) mix. You can upgrade to 4,000 psi concrete if you desire, but I would only do this if it was specified by the structural engineer.

One last point: Remember that the specifications in the building code are minimum standards. You can always improve upon the guidelines you see in the code.