# French drain question



## AustinDB (Sep 11, 2006)

I was in a new construction house and read the brochure-it indicated french drains were used inside and out-this with 9' basement in Delaware, which has a high water table in places and flat terrain. How is the french drain on the outside of the footer created (i.e. 4" perf tube w/ 18" of pea gravel)? If anyone knowledgeable in foundation design can explain how a french drain is created (inside and/or outside the basement wall), I am curious  

as far as high water tables go-will a french drain on the outside divert most of the water before getting inside teh foundation wall? 
thanks!


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## PipeGuy (Oct 8, 2004)

A french drain, like a french whore, typically looks OK from a distance but often, upon closer examination, isn't worth a f*#k.
A true french drain is nothing more than a gravel filled trench that collects and stores surface water until such time as the stored water is absorbed by the surrounding soil over time. The type of soil surrounding the French drain is key to its functionality. If the soil around the drain is saturated (i.e., the drain is within the water table) the drain will not function.
Sometimes the term French drain is used to describe what is more accurately termed an underdrain. An underdrain uses a network of perforated pipes surrounded by gravel backfill to drain moisture out of the surrounding soil and discharge it, either by gravity or pump, to daylight. A properly designed and installled underdrain system can be an effectve means by which to mitigate the potential problems associated with structures built within high water table areas.
Simply putting gravel and pipes beneath or around a structure won't resolve such issues.


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## Mykey44 (Oct 10, 2006)

Hey Chevy, The way we do a basement drain systems is, after the foundation walls are up they are sprayed with a water proofing material and apply a filtered and waffled plastic sheet material acting as a drain board for the wall. Then we lay a 4" corrugated perforated pipe on the footer close to the wall all the way around and to daylight. We then cover the pipe with two feet of 3/4" gravel. On the inside we put 4" of gravel and then 4" of concrete so that makes your floor 4" above your drains. Mike


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## Tscarborough (Feb 25, 2006)

The key to a french drain is "to daylight". If that detail is left out, you have a sump, not a drain.


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## d.janvrin (Apr 2, 2006)

Hi Chevy
I guess we do them a little different 
1 )4" prefferated pvc below footing (outside) around entire footing pitched to point of entry (daylight) Gutter also drains into preffered pipe thru solid pipe usually coners of foundation and valleys(were down spouts will be).Note footing have 4' sch 80 through them evey 6-8feet (from inside footing to outside footing).
12"-18" of 3/4 crushed stone, 24" fabric above stone backfilled sand, then backfilled with dirt etc, To Determined finished Elevation.

Inside footing gets 2" 3/4 crushed stone, then 4" pref pvc around inside footing perimeter, pitched to a sumpwell (for Pump) apx 30" below finished basement floor,, (which gets pumped seprately to daylight) .8"-10" of 3/4 crushed stone,2 to 4" pea stone (3/8") .most of time pump exits thru 2" pvc to daylight...

Have done this in under high watertable many times NO Problems Yet:w00t: NH was Flooded this year Only 1 flooded basement becase of power failure...:thumbup:


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