Q: I have a client who needs more basement headroom in a house that I’ve already framed, so the slab would need to drop below the edge of the footing. What’s the best way to detail the edges so that the slab doesn’t crack?
A: Rick Arnold, author of Working With Concrete and a frequent presenter at JLC Live, responds: Dropping the level of a basement slab to gain headroom isn’t done very often—I’ve been asked to do it only a few times. If you catch the problem before the first-floor deck is framed, it’s much easier just to add a layer or two of mudsills. That said, dropping the level of the slab can be done after the fact. The key is to isolate the slab so that it moves independently from the footing and foundation or stem walls. If the slab binds to the footing and wall, the odds are much greater that defects (cracks) will occur as it tries to move.
The first way to allow the slab to move independently is to install a bond breaker (expansion joint) around the perimeter of the slab. Products such as Reflectix Expansion Joint are readily available at most building supply stores. This foam-like material is 4 inches wide and ½ inch thick and comes in a roll.
Next, I would install an isolation membrane or slip sheet between the underside of the slab and the footing. This could be something as simple as a strip of 6-mil polyethylene that covers the top and side of the footing and that you put on just before you pour the slab. If you’re installing a poly vapor barrier over the entire basement slab area anyway, just continue it up and over the footing.
The good news is that basement slabs are not exposed to the elements or wide variations in temperature, so they are much less prone to movement than exterior slabs, such as one in an unheated garage. This means that you might get away with reducing the thickness of the slab on top of the footing to as little as 2 inches, a situation that would normally cause defects in an exterior slab.
The full-thickness slab does move at a different rate than the thinner part that laps over the footing, which makes the transition point between the two thicknesses particularly vulnerable to cracking. So one thing you could try is creating a beveled edge on the top inside corner of the footing. Just shave or break away about 2 inches of the footing concrete at roughly a 45-degree angle with a flat-bladed shovel while the concrete is still green. (If the foundation is in and the floor is already framed, chances are that the footing concrete would be too hard to “shave” very easily). It may not look pretty, but the angled edge of the footing makes the transition more gradual and reduces the potential of a cracked slab where it transitions to full thickness. But even with this added precaution, I would never guarantee that the slab would not crack.
I normally don’t use wire-mesh reinforcement in a basement slab. If the base is prepped correctly, wire mesh shouldn’t be necessary. A basement slab won’t move up or down, and with it closed in on all sides, it has no lateral forces acting against it, unlike an exterior slab. For those folks who do use wire mesh, placing it correctly in the center of a 2-inch slab would be nearly impossible. It’s hard enough placing it in the center of a 4-inch slab.
I don’t believe that fiber-reinforced concrete would be much help, either. The fibers are meant to help prevent microscopic cracking that occurs during the curing process, but they don’t generally increase the flexural strength of the concrete, so it’s doubtful that the fibers would prevent cracking where the slab thickness transitions. Also, fiber-reinforced concrete can be a real pain to work with during the troweling, and it adds significantly to the cost of the concrete. I would just go with the standard 3,000-pound pea-stone mix.
Another thing that could be done if you think you might be asked to provide more headroom in the basement is to take a little extra time after pouring the footings to trowel a decent finish on top. Then I would prep the bottom of the hole so that the top of the 4-inch slab is even with the top of the footing. This strategy provides a full 4 inches of extra headroom and keeps the thickness of the slab consistent throughout the entire basement, minimizing the potential for cracks.