Framing Basement Walls

DEAR TIM: I have plans to finish my basement in the near future, and I live in an area where expansive-clay-bentonite soil is very prevalent. I understand that I need to install floating walls in the basement. Do you have any suggestions or references that I could use to see how these type of walls are installed? Art Gomez, Lubbock, TX

DEAR ART: Gosh, if your basement slab is moving up and down at a greater rate than the actual foundation walls because of the expansive clay soil, you might have all kinds of serious issues down the road. But let's not concern ourselves with that right now as there are ways to stabilize expansive clay soils so the movement is eliminated or minimized.

The trick to framing basement walls that rest on slabs in contact with expansive clay soils is to ensure there is a space between the top of the wall and any floor joist above. The space can be as little as 3/4 inch or up to 1.5 inches. At first blush, you might see a problem as the top of the wall would be flimsy and tilt or tip over when something pushes against it.

A simple L-shaped steel framing clip solves the problem. This clip is used by professional builders and rough carpenters for basement walls and roof trusses. Roof trusses in cold climates also present the same problem. During cold weather, the horizontal bottom chord of a wooden roof truss can bow upwards. If the interior walls are tight to the truss and nailed to it, the walls can lift up along with the moving truss and all sorts of havoc occurs.

The L-shaped clip is nailed tightly to the top of the wall, but it has long slits in the vertical leg that rest against a floor joist, truss or solid blocking between trusses or joists. Nails are driven through the slits into the truss, joist or blocking. This connection keeps the wall from tipping over, but allows the wall to move up and down as the slab moves.

These clips are inexpensive and you must use special structural nails that are made by the same company that manufactures the clip. Do not use roofing nails as they are not as strong as the thicker, structural nails.

Expansive clay soils can be stabilized, if you maintain a steady or constant level of moisture content within the soil. These soils shrink and swell in response to the loss and gain of water. Installing irrigation piping in the soils allows you to add water to the soil during dry spells or periods of drought. Since the soil stays wet year-round, it doesn't move.

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