Concrete – How to fill in pits in a concrete floor so I can install laminate

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.

The main problem you will have is thickness, thin concrete tends to crack. Your laminate flooring is all of 1/2 inches thick most likely, so the maximum your concrete floor could be is that much, and that's not enough. Other problems are:

• unless you are pouring on slab the flexing of your floor will cause the concrete to crack
• unless you are pouring on slab the concrete will drop between the floorboards
• unless you are pouring on slab the floor will probably not be designed to take the weight and could lead to structural problems
• Creating polished concrete requires special materials and repeated sanding with ever finer diamond wheels, equipment which would be very expensive even to rent, presuming you can rent it

I'm not saying it's impossible, if you're on slab you might be ok, but if it were me I'd be considering a cool looking tile rather than polished concrete.