Concrete – How should I drain an area of gravel between a sidewalk and retaining wall

I wouldn't call that a typical base, but if it was properly compacted, it should work. The typical base is pretty much what you described:

• topsoil dug down to undisturbed soil and/or topsoil dug out then soil machine compacted
• 2" layer of crushed rock, machine compacted
• 2" another layer of crushed rock, machine compacted
• repeat until you get to 4" or the desired height (if above 4")
• 1" sand, leveled
• pavers

Note that in this system, you're not designing for permeability or drainage at all. If you are using pavers, this is hardscaping, and is meant to shed water, not absorb it. To properly shed, it needs to be properly sloped. This is true for the base layer as well.

The purpose of the base layer is to create a solid, stable surface. Ability to drain is desirable (to avoid frost heave) but that's secondary to providing a proper surface to shed water.

In your case, it sounds like you already have 4" of compatible material. I'd do the following:

1. remove pavers
2. rent a compactor and compact existing base
3. add additional base material and compact to get the slope you want
4. add sand
5. put pavers back
6. add locking sand to joints

the big catch here is that this will ultimately raise your patio next to your house several inches. If that's tolerable, you're good to go. If not, then you have to do the reverse...instead of adding material closer to the house, you need to take material and remove it further away from the house. This will reduce your total base at the outside edges, though. That may be acceptable in your case.

Two things about an exterior concrete slab.

1. Proper drainage. Pitch the slab slightly so the water drains away. 1/2" to 1" for 10' of run should be enough since it is covered.

2. Proper compaction. Make sure the material under the slab is properly compacted. If you believe it needs more sand or gravel for better drainage or to level the area, then that added material has to be compacted before placing the concrete. Sand or road-gravel compacts better than loose stone. Undisturbed gravel with compacted sand for extra drainage or leveling would work great. Top soil makes a poor base since living matter can be compressed, so it is removed and made up with road-gravel or sand. Since you have a base that is not top soil and has been compacted by cars and settled for years you don't have that problem.

You have a well thought out design.

Good luck!