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Foundation details

brentwilson | Posted in General Questions on

The plan is to start excavation on the foundation area for my parent’s house tomorrow. It will be a conditioned crawlspace. My thinking is to over-dig a bit and add about 4″ of crushed/washed 3/4″ gravel under the footings. It would then be compacted with a walk-behind vibrating compactor. Does this sound acceptable? Or would I be better off just having the footings bear on the natural soil?

In addition, my preference would be to have the top of the foundation stem walls all at one level for both the house and the attached garage. But the high corner of the existing grade is one of the front corners of the garage. The low corner (one of the back corners of the house) existing grade is about 2’2″ lower than the high corner. What should I do? The stem walls will be 40″ tall, on top of 10″ tall footings. So if the high corner stem wall extends to 1′ above grade, the bottom of the footing at the low corner would basically be sitting at grade. Of course, over-digging a bit and adding gravel could potentially get rid of some of the topsoil from under the footing location. If I lower the foundation, then I end up with needing to do some grading in front of the garage to get my slope away from the garage – the ground in front of the garage location continues to slope up slightly.

Another option could be to have the garage foundation positioned higher than the house foundation. This might mean that the house-to-garage doorway wouldn’t need as many stair steps as the garage floor would potentially be higher. But is that going to look weird on the outside to have the bottom of the siding stepped up for the garage?

Any thoughts or advice?

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  1. onslow | | #1


    Don't want to sound harsh, but these questions should have been asked months ago. Do not plan on putting wash rock under a footing. Without a soil engineers approval it will most likely be a disaster that happens slowly over time. Imaging pressing crushed walnuts into brownie very, very, slowly. I do believe that most codes call for footings to be placed on undisturbed native soils with proper bearing values.

    I do not understand why you would not simply use stepped down footings to accommodate the grade. Are you in an area that does not have a frost depth you must excavate below? In regard to raising the garage foundation, so the slab matches the house deck, well that is detail that I can't address without a plan view. There are tricks to matching the siding bottoms, but many homes have steps in the sight line as you go around the house. I wouldn't be too rigid about that feature.

    I would worry more about the grade continuing uphill from the garage apron. Do you have plans for guiding surface run-off or melted snows around the house.? There are many things going on here that suggest not digging right now and getting your ducks in a row first. The low corner of the lot you mention as having top soil means it is likely not suitable for a footing directly. Stepping down the stem wall and footings is the way to go.

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