You need a base as dreamcatcher mentioned of something to "cushion" their legs.
That said... I did stalls in a show barn where the entire stall was turned daily to fully clean and fluff. Our stalls were bedded deep in pine shavings, 10" on the floor and deeper sides...stalls were 14'x14' boxes when bedded down probably made a 12'x12' box of usable space.
had issues of leg problems or horses sliding getting up. No soreness, pain or anything else. These were top competition show horses.
I think the secret was stalls large enough for the animal to freely move around and more
than enough bedding to give cushioning to the animal.
I never had a horse "walk" a bare spot to the floor either....
Drives me nuts to see matted stalls with a 1"-2" of bedding in them...why bother.
Doesn't truly absorb the pee so the horse if they lie down is lying in sopping shavings, rub marks on the animals coat from bones/fur making hard contact with a hard surface...and no soft place to lay the weary head...
Ever sit on a stall mat...hard as can be and can be slick underfoot too.
With all of that they then can also smell really foul because of the urine seepage under the mats left to fester...gross.
Give me thick bedded stalls of shavings on top of any surface, ....mats, concrete, dirt...whatever just give me the shavings to use.
Once established those bedded stalls also use less than the thin covered stall needing a near total stripping everyday.
jmo, sorry...off the soapbox.
In answer to your question though...over concrete nothing less than 5" thick across the entire stall floor...