What is a “concrete free slab” and why should I consider one?
Somebody really broke the traditional building box with this one! If you read our article about toxins in concrete, this article builds on that…to offer a slab solution that doesn’t even use concrete. Concrete slabs have been preferred for building “on grade” (on top of the ground), and if a building floor is “below grade”, it’s still the standard for making a stable floor. However, mixing and installing a concrete slab, or wall, or ceiling, is really an art because the composition of the concrete, steps and technique in the installation all MATTER. There’s a reason that people say it’s “set in stone”...because if mistakes are made, it’s difficult to change or replace it! There’s a new type of “slab” out there, though, with more forgiving, health-conscious and eco-friendly materials, that makes a great case for abandoning concrete to make a slab.
First off, we need to differentiate between a building’s foundation and the slab. The foundation supports the weight of the building (the walls and roof and everything placed on the upper floors), while a slab may only support and carry the weight of the flooring and items that are placed on the floor. The foundation must extend down into a stable soil in order to support the weight of the building; for this reason, in unstable soils, pilings are driven down into the earth, and the foundation is set/built on top of them. It’s true, some slabs are integrated with the foundation by being poured together in concrete; this is called a monolithic slab or slab-on-grade foundation. Note that the word “slab” is used to describe the foundation. Otherwise, if the slab is not structurally connected to the foundation, the two are separated and it’s a “floating slab”. In this article, we are addressing a separation of the foundation and the slab. The alternative materials only create a stable surface for the flooring and items placed on the bottom floor, while the foundation (which still may require concrete) is separate and supports the building.
Finehomebuilding.com and Greenbuildingadvisor.com examine practices in the construction industry by analyzing building science and documenting experience of the best builders and builds, so that home construction is continuously improved, and even non-professionals (DIYers) can benefit. One such article is on the how-to’s of Assembling a Concrete-Free Slab. It was published in 2022 but the author admits that he didn’t invent it and wasn’t the first to use it. I can see, however, from this 2016 Matt Risinger video which says “Steel reinforced slab on grade is the best type of foundation” that the concrete-free slab was probably introduced in the late 2010’s. Early in the article the reasons for using alternate slab materials: reduced embodied carbon, fewer delays and moisture reduction, and hassle-free flooring.
When skimming down through the article, I saw the over-riding principle for making alternative materials acceptable for a slab: disconnect/isolate/insulate the slab from the moisture in the soil. As with all good building practice, several methods of isolation and insulation are utilized, so that if one fails, you don’t get failure of the material from mold or rot! Here’s a list of the layers used for a floating “slab” that is not mechanically connected to the insulated concrete form (ICF) foundation, which supports the walls and rest of the building:
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Stable soil (if necessary, test it)
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Crushed stone: it’s used as a “capillary break” to prevent water from wicking up from the soil to the layers above.
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Gravel: due to its small size, gravel compacts better than larger stone, and continues the capillary break begun with the crushed stone.
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Rigid Foam: This layer provides a thermal break to prevent heat from transferring down into the soil. This builder has chosen two layers of 2-in.-thick graphite polystyrene (GPS) foamboard, oriented perpendicular to one another with the seams offset to ensure minimal air gaps.
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Polyethylene sheeting: This layer is the final capillary break; it’s a class 1 vapor barrier which also doubles as an air barrier. This builder has chosen 10 mil poly called “W.R. Meadows SealTight Perminator”, which , also retards intrusion of radon, which, if a radon evacuation system is also installed in the stone layers below, virtually eliminates any radon getting into the home. All plumbing and other penetrations must be carefully taped and sealed to the poly with compatible tape.
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Two layers of ¾” tongue and groove (untreated) plywood: These layers are installed carefully so as to provide the final stable subfloor. The bottom layer is given ½” space from the foundation to expand, and the top layer is glued and screwed onto it with screws that do not penetrate the poly sheet below.
Source: Assembling a Concrete-Free Slab
The beauty of this “disconnected” slab/foundation design is that construction of the walls/roof is not delayed by waiting for the slab to “cure”, and if this outdoor “shell” is completed during dry weather, then no framing has to get soaked by rain. Additionally, grading of the stone layers and plumbing work can be done simultaneously with building the exterior, if the trades can work considerately of one another. Best of all, this system seems so much more comfortable to human feet than concrete with a miniscule layer of padding that most modern flooring provides.
What could go wrong? Obviously, if the building floods by internal (plumbing-related) or external (flood-related) means, the plywood flooring is susceptible to water intrusion because pockets of water could be impossible to remove. This could force the homeowner to demolish and replace the plywood (and probably, the vapor barrier as well), which would be a big job. The poly sheeting could also be punctured by anyone (framers, flooring installers) who are not careful to maintain it. Cement slabs also typically have poly sheeting as a vapor barrier under them, but unlike plywood, if cement gets wet, it will dry to the air (fans and heat help) and doesn’t require removal. Finally, some very chemically-sensitive people may be reactive to the adhesives used in the plywood; they should test them before deciding to go this route.
“Cement free slabs” are becoming a popular subfloor in accessory dwelling units (ADU’s) and other smaller residences. As with all new types of construction, having a builder (and plumber) familiar and experienced with the technique makes all the difference in doing it properly and efficiently, as attention to detail makes all the difference in ensuring it to be a comfortable, durable subfloor. It’s best suited to homes that also have:
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Continuous air and vapor barriers on the other sides of the home, walls and roof, because without attention to sealing these, the possibility of a leak coming in from the walls or roof could be disastrous for the floor.
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A fresh air ventilation system, since airtight homes need a way to evacuate humid, stale air
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Active, continuous leak detection on water supply lines, like flow-sensing systems we mention in this article.
If you’re building a new home and are already including these features, consider going “concrete-free” in your slab to make a comfortable, environmentally friendly flooring.
Photo by Etienne Girardet on Unsplash
