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Old 03-01-2012, 01:05 PM   #1
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French Drain Procedure


Hi All,

I live in a middle townhouse so only my back and front wall are exposed to outside water. I'm looking for some experienced folks to comment on the following steps I plan to take, if possible. The house already had a sump pump installed, but it only had 1 drain coming from the hot water tank.

1. Dig out 12in x 12in trench from the back wall and clean out debris.
2. Drill 4-5 weeping holes per cement block the entire length.
3. Add a layer of rock to increase pitch toward the sump pump.
4. Insert corrugated pipe that includes the rock and sock (from HD and Lowes for 47.00)
5. Cover pipe and fill trench with rock.
6. Top off trench with concrete.

One question I have is how much concrete to I use to top off the trench? 1in, 5 inches, etc?

Thanks for reading.

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Old 03-01-2012, 02:58 PM   #2
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French Drain Procedure


4-5 holes per cement block? Sounds like trouble...

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Old 03-01-2012, 02:59 PM   #3
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French Drain Procedure


After reading and watching videos, this is how you release the hydro static pressure, correct?
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Old 03-01-2012, 04:00 PM   #4
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French Drain Procedure


I don't really follow what you are planning to do. You need to decide if you are planning an exterior or interior drainage system. An exterior system is located outside the foundation, an interior system is located under the floor slab of the basement. It is usually easier to install an interior perimeter drain system in an existing structure.

The invert (bottom) of the drain is typically set at least one foot below the floor slab, better is to be 18 inches to 24 inches below the slab. The drain is typically about 12 inches wide, with a perforated pipe set on the bottom of the drain, and the pipe is surrounded by about 12 inches of crushed stone. The crushed stone is typically wrapped in filter fabric.

The pipe runs around the entire slab, and empties into the sump pit, where the collected water is pumped out, typically to the street drain system where permitted by the municipality. The pipe can be laid flat, as long as the sump pit is lower than the pipe, water will drain to the sump pit regardless of pipe pitch.

I am not clear what the purpose of the drilled holes in the block is, the groundwater will enter the perforated pipe from underneath (the holes are typically pointed downward). You don't need holes in the block foundation. The purpose of the drain is to remove groundwater, lowering the groundwater level to below the slab level. You are protecting the interior of the house from groundwater flooding, you are not protecting the foundation, as long as you have a concrete foundation it could care less if it is wet.
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Old 03-01-2012, 05:19 PM   #5
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French Drain Procedure


I can understand the drilling of the holes in the walls for flexible tubing in case of failure of the external water and sealing measures. You only need 2 (newer block) or 3 holes (old fashioned block) into the cores of the block, depending on the number of cores.

I had a friend that built over 2000 homes and never wanted a call back for water in the basement and he never had one. Both internal and exterior pvc with perforations laid in the bottom half with the pipe laid horizontal and the excavated areas were line with fabric and he used a mix of 3/4" rock and sand inside of that, 4" under and around the pvc. Because it was standard on every home with no extra charge, he had pre cut 3/4" flexible pipe inserted in holes in the interior side of the block, leading into the rock/sand mixture to provide absolute insurance. Since it was new construction, the cost was minimal and cheaper since he was selling reputation and not just advertising. His construction techniques were a little different and he even had special block made for the frost course that other contractors copied.

Trucon -

You have what you have and what was built. I would never use a flexible corrugated, slotted pipe to collect water even though is it easy to sell and is profitable for a big box. Pvc should be laid horizontally as Daniel said, since it is easier (less excavation) to do a good job and water always seeks its own level. The sock can be easily plugged because the high velocity of the limited pipe intake draws more fines in the can plug the sock or settle in the corrugations and dips in a flexible pipe.

You want about 4" of rock/sand under and around the pipe to gradually collect the water. That is the way it has been done for centuries.

Beware that a good interior drain tile system will collect water slowly continually and reduce the pressure under your slab and outside the exterior walls, so will be helping others without them knowing or thanking you.

Dick
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Old 03-01-2012, 06:42 PM   #6
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French Drain Procedure


Apparently, everyone has a different idea on what's "typical", but this is how we do it here in "Cheeseland":

- Remove floor & soil next to footing for at least 12" wide, only to bottom of footing.

- Drill one hole per core in every block. Drilling a hole halfway up the wall (typically in a bed joint) and flushing with water to ensure the cavities are clear is also a good idea.

- Install "toe-drain" mat like "Mira-drain" or MTI's system to allow water to flow from block cores to new draintile.

- Install ~4" of washed stone (1") next to footing.

- Ensure that any cross-bleaders through the footing are clean and will allow water flow.

- Install 3" draintile no higher than top of footing, keep as level as possible. (For this, I just use cheap corrugated, perforated black ABS that comes in rolls, as I've seen through my experience that teh vast majority of water runs in the stone below the tile)

- Backfill with same washed stone.

- Cover pipe & stone with plastic to ensure concrete doesn't plug stone or pipe.

- Re-pour concrete to within 1" +/- of existing floor.
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Old 03-01-2012, 07:50 PM   #7
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French Drain Procedure


Up until my most recent French drain project, they have clogged up and eventually stopped working. My friends tell me about similar experiences with their French drains.

I finally built one that won't clog up. And if it does, it is cleanable without a lot of effort.

The description and photos are here: http://housengineer.com/825/
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Old 03-01-2012, 09:50 PM   #8
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French Drain Procedure


jomama -

Where does the water in the stone go or does it just stay under the footings, so you just have a water drainage system?

I have seen many with acess for flushing out the silt and fine sand that comes through the stone voids.

Dick
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Old 03-01-2012, 10:57 PM   #9
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French Drain Procedure


I'm finishing a basement that has had the B-dry french drain system in since 98. They had to open the system to install the plumbing and it has a very thin layer of concrete over it.

FWIW, I had the B-dry guy out (to keep the warranty since we opened the system) and talked to him about some other drainage issues. I thought it was interesting that he prefers the smooth pvc type pipes for drainage rather than the corrugated since they are less likely to trap debris and plug up. Makes sense, but everyone else around uses the corrugated.
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Old 03-01-2012, 11:05 PM   #10
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French Drain Procedure


canoes - regarding the corrugated pipe -

That is because they are easier to handle and sell, cheaper and have the gimmick mesh direction around it so the retailers can handle and sell them easily. The B-Dry system is just a water collection system after the water has already leaked into the interior and does nothing structurally for the wall or slab.

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Old 03-02-2012, 09:33 AM   #11
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Quote:
Originally Posted by concretemasonry View Post
jomama -

Where does the water in the stone go or does it just stay under the footings, so you just have a water drainage system?

I have seen many with acess for flushing out the silt and fine sand that comes through the stone voids.

Dick
Forgot to mention that the pipes need to be connected to the sump crock.

Most commonly here, the hole(s) in the sump crock aren't a perfect fit around the pipe. The water clearly favors coming in through the gap under the pipe rather than through the pipe itself. If you want to lower the water level even more, you can always drill a small hole lower in the crock.

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