For a garage floor you want at least six inches of compacted ballast as a minimum but eight inches is better especially if you are parking vehicles or storing anything heavy. The ballast layer spreads the weight of the slab and stops it from sinking or cracking over time. If the ground underneath is soft or has poor drainage you might need to dig deeper and build it up even more. It is better to spend a bit more time on the base than deal with concrete problems later. A deep solid base gives your slab the best chance of lasting for years without any issues.
Yes if vehicles are going in and out the base needs to be strong and properly compacted. A light base might work for a shed or walkway but once you start adding vehicle weight things change quickly. Use a well graded sub base like MOT Type 1 or solid ballast and make sure it is laid in layers and compacted properly throughout. The edges need just as much attention as the middle or you will start seeing dips where the tyres roll in. A proper base gives the concrete support from underneath so it does not start cracking or moving under pressure.
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Do I need a sand layer over the ballast before pouring concrete in my garage?
A thin sand blinding layer is often used over the ballast to fill in any small gaps and create a smoother surface for the concrete to sit on. It helps prevent sharp stones in the base from pushing up into the concrete while it is curing and gives a cleaner pour overall. You do not want it thick—just enough to cover the top of the ballast and stop the concrete from settling unevenly. If your base is already smooth and compacted some might skip it but in most cases a light blinding layer helps tidy things up before the slab goes in.
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Can I pour a garage floor directly onto soil if it feels firm?
Even if the soil feels firm when you walk on it, that does not mean it will stay that way. Soil holds moisture and expands and contracts with the weather and is likely to settle over time. Pouring straight onto it is risky and often leads to cracking or sinking down the line. A proper ballast or sub base gives the concrete something stable to rest on and stops the ground underneath from causing trouble later. If you are putting in the time and money for a garage floor it is worth doing the base properly instead of cutting corners and hoping for the best.
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Can I reuse an old base if I am replacing my garage floor?
You can reuse an old base if it is still solid level and well compacted but you need to check it properly. Dig down in a few spots and see if the material underneath is firm and clean and not full of soft spots or loose rubble. If it looks good you can top it up with fresh material, recompact it and get it ready for the new slab. But if the old base is uneven holding water or showing signs of movement it is better to take it out and start fresh. Reusing a weak or damaged base will only carry those same issues into your new floor and you will be back fixing it sooner than you should.