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PSA: I used to think a 4-inch concrete slab was fine for a garage floor, but a job in Tucson changed my mind.
I was helping a friend build a detached garage there last spring. We poured a standard 4-inch slab with wire mesh, just like I'd done a dozen times before. About eight months later, he called me back because a long, straight crack had appeared right where he parks his truck. The local supplier who delivered the mix asked what we'd used for a base, and when I said compacted road base, he just shook his head. He told me, 'Out here, with our soil and heat, you need a full 6 inches over a proper gravel sub-base, or it will move on you every time.' Seeing that single, perfect crack from the weight of one vehicle was all the proof I needed. Now I always spec a thicker slab and spend more time on the base, especially in areas with tricky ground. Has anyone else had a similar experience with slab thickness not being a one-size-fits-all spec?
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benc531mo ago
That Tucson ground is no joke, the clay content out there is wild. I'd argue the bigger issue than the slab thickness was skipping the gravel sub-base. The road base just holds moisture against the slab. A 4-inch pad with a proper 4-inch compacted gravel layer probably would have been fine, the gravel lets water drain away so the ground doesn't swell.
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julia_patel1mo ago
Ever see a slab heave from tree roots? We had to go 8 inches thick with rebar near a big oak. That extra depth stopped the cracking for good. The base was key too, used a solid 6 inches of crushed stone, not just road base. Thicker concrete over a solid drain layer fixes a lot of problems.
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