How thick should an RV garage slab be?

Weight is the reason the standard 4-inch car-garage slab will not hold up under a coach. A loaded Class A motorhome can hit 30,000 pounds, and it lands almost all of that on a handful of tires, so the slab under an RV bay needs to go to 6 to 8 inches of reinforced concrete. A thin slab that is fine under a 4,000 pound car will crack and settle under that load. On the expansive clay soil common across the Phoenix and Scottsdale metro, the right answer is an engineered slab, either thicker reinforced concrete or a post-tension design, poured over a well compacted base. The exact thickness should come from an engineer who knows your soil and your coach.

The code minimum and the right design are two different numbers here. Below is what the code requires, what the weight of an RV actually demands, and how Arizona soil pushes you toward an engineered slab.

What the code requires versus what an RV needs

The building code sets a floor of 3.5 inches for a residential slab, but an RV garage needs more because the code minimum was never meant for a 30,000 pound vehicle. Under the 2021 International Residential Code, which most Valley cities build from, Section R506 requires a slab-on-ground floor to be at least 3.5 inches thick. That number assumes ordinary residential loads. A standard attached car garage is poured at about 4 inches and carries passenger cars and light trucks without trouble.

An RV garage is a different problem. A Class A coach is the large bus-style motorhome, and a fully loaded one routinely weighs 30,000 pounds, with big diesel pushers reaching 40,000 pounds or more. That load does not spread out evenly like a parked car. It concentrates on six tires, so the pressure under each tire is far higher than anything a car puts down. A 4-inch slab over a soft base flexes under that point load, and concrete is weak in bending, so it cracks. That is why builders pour an RV bay at 6 to 8 inches with steel, not 4 inches.

The American Concrete Institute publishes the engineering reference for exactly this situation. ACI 360R, Guide to Design of Slabs-on-Ground, covers slabs that carry vehicle and concentrated loads, and it walks through plain, reinforced, and post-tensioned designs. The takeaway from that guide is simple: a slab holding heavy or concentrated loads is an engineered element, sized to the load and the soil under it, not a one-size number.

Why a Class A coach needs an engineered slab

A 30,000 pound coach needs an engineered slab because its weight rides on small tire contact patches, which drive high stress into both the concrete and the ground below it. Think of the difference between standing flat-footed and standing on a stiletto heel. The same weight through a smaller area punches harder. A heavy motorhome is closer to the stiletto, so the slab and the soil both have to be built to take a sharp, repeated point load every time you drive the coach in and out.

Two things carry that load. First is the concrete itself, made stiffer by thickness and by steel reinforcement. A 6 to 8 inch slab with #4 rebar on a grid, or a post-tension cable layout, resists the bending that a thin slab cannot. Second, and just as important, is the base under the slab. Engineers call it the subgrade and subbase. A compacted aggregate base, usually 4 inches or more of crushed rock packed to roughly 90 percent density, spreads the load and gives the slab uniform support. A thick slab over loose dirt still fails, because the soft spot lets the slab bend until it breaks.

ACI 360R is blunt about expectations even on a good slab: "it is normal to expect some cracking and curling on every project." The goal of the right thickness, steel, and base is not a slab that never shows a hairline. It is a slab that carries the coach for decades without structural cracking, settlement, or a tire-rutted floor. Skimping on thickness or base to save a few yards of concrete is the classic false economy, because tearing out and repouring a cracked RV slab costs far more than building it right once.

How Arizona's expansive soil changes the answer

On the expansive clay common across Maricopa and Pinal County, the slab has to resist the soil pushing up, not just the coach pushing down, which is why post-tension is so common here. The Arizona Geological Survey notes that expansive smectite clay "swells to many times its original volume" when wet, then shrinks as it dries. That cycle heaves a slab up in wet areas and drops it in dry areas. A heavy point load from an RV sitting on a slab that the soil is also trying to bend is a double problem, and a plain thick slab does not solve it on its own.

A post-tension slab answers both forces at once. Crews run steel cables through the slab and pull them tight after the concrete cures, which squeezes the whole slab into compression. The Post-Tensioning Institute explains that this prestress works because the "compressive stresses resist the anticipated tension stresses induced by the soil movements," and that post-tensioning will "reduce cracking and keep any cracks that might form tight." For a slab that has to hold a 30,000 pound coach over moving clay, that pre-squeeze is exactly what keeps it flat and crack-free. PTI lists typical post-tension slab-on-ground thicknesses of about 7.5 to 12 inches, designed to the engineering standard PTI DC10.5 for slabs on expansive or stable soils. To compare the two systems in depth, see post-tension slab vs rebar slab and why post-tension slabs are common in Arizona.

The practical path on a Valley lot is to start with a soil report, also called a geotechnical report, that measures how much your clay moves. That number drives the slab design. On high-movement clay, expect a post-tension or heavily reinforced engineered slab at the thicker end of the 6 to 8 inch range, with thickened edges. On stable, low-swell ground, a reinforced 6 inch slab over a good base may be plenty for a coach.

Getting the slab right on your build

Match the slab to your actual RV and your actual soil, then pour it during the main build, because a heavy engineered slab is far cheaper to do once than to redo. Start by knowing your coach. Pull the gross vehicle weight rating off the door placard or the manual, since that is the loaded weight the slab has to carry, not the dry weight. A 26-foot gas Class A is lighter than a 45-foot diesel pusher, and the heavier the coach, the thicker the slab and the stronger the base.

A few rules of thumb apply on most Arizona RV garage builds:

• Thickness: plan on 6 to 8 inches of reinforced or post-tension concrete, set by the engineer to your soil and coach weight, never the 4-inch car-garage default.
• Reinforcement: rebar on a grid or post-tension cables, not bare concrete, and not just light wire mesh for a coach this heavy.
• Base: a compacted aggregate base, commonly 4 inches or more, packed firm before the pour. The base matters as much as the slab.
• Concrete strength: a mix in the 3,000 to 4,000 psi range is typical for a load-bearing slab, confirmed by the engineer.
• Edges and thickened sections: thicker turned-down edges where the coach drives on and off carry the wheel load at the slab's weakest point.

Building the RV bay with the house lets the concrete crew pour the heavy slab while they are already on site, sharing the grading, the base prep, and the mobilization with the rest of the foundation. That is the same logic that makes building an RV garage into the original plan cheaper than adding it later, and it ties into the broader cost to build an RV garage in Arizona. Because the exact thickness, steel, and base depend on your measured soil and your coach's loaded weight, have the foundation engineered to those numbers rather than poured to a generic spec.

The Jematell Homes approach

Clear allowances and an honest cost breakdown are how we start every custom home. We would rather answer your questions before you build than after, so get in touch any time.

Sources

• ACI 360R-10: Guide to Design of Slabs-on-Ground (American Concrete Institute)
• Post-Tensioning Institute: Post-Tensioned Slab-on-Ground Foundations
• 2021 IRC Section R506.2.3 Concrete Floors (On Ground) (ICC Digital Codes)
• Arizona Geological Survey: Problem Soils (expansive and collapsing soils)