Substrate under stall mats?

I can totally sympathize with your situation. Our barn sits at the bottom of a hill, and every time we get a heavy rain our barn floods. The horses had also caused some major dips in their stalls over the years.  When we built our barn we just had some fill sand brought in and then we placed mats over it.  That obviously didn't work out long term, so last month my hubby tore the stalls apart, built frames inside each stall to raise the level of the stall floors, and then he filled them with crushed lime, rented a compactor to pack it all down tight, and then put the mats back down.  We've had several pretty good rains since then and so far the stalls are staying dry!  Hopefully they will continue to stay dry now, and hopefully the crushed lime won't shift and create dips again.  Fingers crossed!

As for mats, I guess someone else will have to give you some advice on that.  Our mats and stall walls are rubber, but they aren't store-bought mats.  The BO where we used to board had bought some huge rolls of tire rubber which he was using in the new stalls he was building, so we bought some from him to do our stalls with.  Our rubber is pretty thick and VERY heavy (and hubby had a heck of a time cutting it to fit), but it doesn't shift, and we have very few seams.

Here is a picture of one of our stalls after hubby tore it all apart, built a frame, and filled it with crushed, compacted lime.

This picture is of the other stall after the sub-floor was finished and hubby had laid the rubber back down.

Here is one of the stalls after hubby had completely finished redoing it and had it all put back together again.

We had initially planned to do concrete when we redid our stalls, but my farrier STRONGLY advised against it. He is the one who recommended the crushed lime. We had already gotten a rough estimate for the concrete work (well over a thousand dollars).  Hubby picked up 2 tons of lime in his truck for $40.00. The job took us 4 tons of lime total, so $80.00 for the lime as opposed to thousands for concrete.

He rented a power tamper/compactor (whatever it's called ).

dbl post

The 4 tons of crushed limestone filled two 12' x 12' stalls, plus we had some left over that we used to fill in low spots in other areas of the barn.

I think that, if hubby hadn't built a frame inside each stall and built up the level of the stall floors with crushed limestone to the tops of the frames, we might still have problems, but raising the height of the floor (with the treated wood frame around the perimeter to help hold everything in place) has kept my barn completely dry so far, and we've had quite a few heavy downpours since he finished the project. In the past, any one of those downpours would have left us with flooded stalls.

I just remembered a couple more things he did... since the frames for the new stall floors don't go all the way to the exterior barn walls, it leaves about a 4" to 6" gap between our stall walls and our barn walls, and he did put concrete down in that gap on the side where most of the water usually comes in.  He made sure when he was finished that the level of the concrete was higher than the bottom of the barn wall by about 6 inches, effectively closing the gap between the barn wall and the ground.

In Zena's stall (which he did 2nd), he was getting tired of shoveling and compacting crushed lime, so he put a base layer of CA6 rock down and then put the crushed limestone over that.  Like I said, both of our stalls have stayed completely dry.