How much basement moisture is tolerable/ normal and how to tell if a french drain or other precautions are necessary?

Question

It's necessary that I replace the floor in the basement of my duplex. I'm in the process of breaking out the old concrete and the earth under is wet. I understand that there should be moisture in the dirt but how wet is too wet and how do I figure out if an interior french drain and sump pump is necessary?

A little more information

Located in Salt Lake City.

Evidence of moisture on some of the walls including under a window well.

The old floor was a really bizarre construction. There was about 2 1/2" of concrete poured and 2x4s set directly in the concrete on 16" centers and used as floor joists and a plywood sub floor on top. This was not discovered by me or my home inspector while doing due diligence. When I began tearing out the old floor about half of the wood was moldy and rotted.

With this evidence of moisture I want to make sure I proceed with the rebuild correctly to avoid problems down the road.

Thoughts, advice and recommendations greatly appreciated.

Adam

Bill S. · Answer

@Adam Olsen Standard practice for grading and drainage is 6 inches of fall away from the house in 5 ft. This is all away around the house. Also gutter run-downs would be rigid and at least 5 feet long sloped at least 10%. Better practice is 1 ft of fall in 10 feet away from the house with 10ft run-downs. All around the house should have positive drainage. Meaning that water on one side has a way around the house. Grass line drainage should be 2% minimum. Offsite drainage should be conveyed around the property or through the property in properly designed conveyance. Again minimum natural slope of 2%. If you wonder what this looks like go see a new subdivision (last 5 years) and you will notice that it looks like each house is on a mini hill and front to back is also sloped pretty good.

All that said, it can be high ground water. Again the exterior drain is best because it keeps the water away from your basement walls. Concrete is actually a porous material and if water sits consistently on one side it will eventually soak through and over time cause it to fail. An interior French Drain does lower the water table and it does help but hydraulics dictate that some of your foundation will always have water on it. In all likelihood an interior drain will work for most of your lifetime but toward the end of this century the house's foundation will likely have some issues as water accelerates the decay of concrete.

Bill S. · Answer

@Adam Olsen too much moisture, IMO is anything more than atmosphere. Where you are that means it should feel dry. If not, you have water coming into your structure which is bad. Lots of times wet basements are the result of poor exterior drainage. Poor exterior drainage comes from broken, missing, and bad gutters, improper grading around the exterior, and improper drainage onto the site.

Before you plan for drying out your basement you have to figure out what is the source of the water. There was one unit that I looked at that the gutters were bad and dumped all the water into a low spot which then literally flooded the cellar. Sometimes the fix is quick and surprisingly easy so your interior French drain would be a waste. That's just one example of why you must identify the source of the water.

If your problem is high ground water (rare in the West ), then really truly the correct solution is an EXTERIOR French Drain. Anything else (ie, interior French Drain) leaves the water against the concrete on the exterior which is bad for the concrete long term. I know people get by with less and I know companies that go around selling less but I don't consider them to be long term solutions even if they are cheaper and easier.

Adam Olsen · Answer

@Bill S. 2 years ago I replaced and repaired all gutters that leaked as far as I can tell they are working properly. At the same time I added flexy pipe to bottom of all downspouts and routed them at least 4 ft from the foundation. The grade seems to have sufficient slope. I thought I had taken care of any drainage problems which is why it was such a surprise to see moisture in the dirt under the old concrete. I hope I don't have a leaking water main. I was thinking of the interior french drain since the floor would be out it seemed an easy route to go.

Rick Baggenstoss · Answer

As dry as it is there, you should be able to put a 6 mil plastic moisture barrier down, overlap and tape the seams,  This will block ground moisture from getting to most of the wood.

A french drain and sump pump are required when water comes in and has to be channeled and pumped out on occasion.  Happens too often in Georgia.

Rick

How much basement moisture is tolerable/ normal and how to tell if a french drain or other precautions are necessary?

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