Plumbing – How to keep in mind when altering the drainage around the basement

You can install an interior weeping system similar to what you would normally do on the outside, but without the waterproofing.

Start by jackhammering out about two feet of floor around the edges of your garage, as close to the walls as you can get. Dig down until you reach dirt, and if that's not at least two feet down, keep going. Now, put down a couple inches of 3/4" crushed gravel, and then install weeping tile (it's not tile anymore; it's 4-6" corrugated plastic drain pipe with a nylon mesh sock over it). Make sure the sock is continuous across any joins you make in the weeping tile for corners etc. This is a good time to direct those drain holes down into this trench, where the water they drain will flow into the weeping tile. You'll also need a sump, or a connection to a downhill storm sewer; direct the weeping tile to this sump pit or the storm drain. While you're doing all this a center drain can't hurt too much; you can tie it into the weeper wherever it's convenient, just make sure the garage floor and the drain flange are level, or that the garage floor slopes slightly to the drain.

Now, you can install waterproof sheathing on the interior walls contacting the concrete. Normally this product is designed to go outside the foundation walls, but in cases where that's infeasible it can work this way too. The idea is to trap moisture that weeps through the wall behind the sheathing, where it will then be directed down the wall and under the slab to the weeper. So, you'll install the sheathing, which should have an air space between wall and most of the layer, and make sure the end is down in the trench by the weeper.

Backfill the rest of the trench with 3/4" crushed gravel up to the slab footing, then patch the slab with new concrete. Your garage should stay much drier.

There are too many questions in this to do a good job of answering so I will tackle the most important one.

I would build a French drain from above the house along the property line: build it as high up the property as you can afford to go and at least 3' deep. The depth will help capture both surface and subsurface water movement. If you also have water coming down the slope to the top of the property, a drain 20-30' in front of the house that is higher on one side sloping down to where it can tie into the property line drain would be the best starting point to try and reduce the water issues.

If there is still standing water in the lowest corner, you may have to dig a trench under the footer and install another drain there, though not as deep as the main one along the property line going straight downhill until it is at the surface and gravity will keep the crawl space dry.

When I put drains in I use the perforated pipe that comes in a sock. It prevents the small gravel from getting in the holes and filling the pipe. pipe in sock it cost more but really helps over the years.