# Drainage / French Drain Questions



## concretemasonry (Oct 10, 2006)

French drains collect water from around them or just from the top. If you use pipe for collection, use perforated PVC with the holes at 4:00 and 8:00 have at least 4" of well drained fill under and around the pipe. You can use a "sock" around the pipe, which is the easy way, but installations that have lasted for decades used a properly grades mixture of clean granular materials definitely just "rock" which can promote clogging and reduce the collection ability. Landscaping fabric (not available decades ago) between the natural soil new graded granular soil around the pipe is always a plus.

Dick


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## AllanJ (Nov 24, 2007)

A French drain can also be on the surface as a hard surfaced channel or ditch(no pipe) that carries the water away (by gravity) before much can soak into the immediate ground.

This can also tie into dry wells or sumps or underground French drains.

For an underground French drain, a perforated pipe will collect only the water even with it or above it. Some water will flow through the gravel under the perforated pipe to get to the sump (pit). Most of the water below the layer of gravel in the trench will stay behind.

So an underground French drain outside used to collect rain water around the house but which is above foundation footing level will not collect water already down at foundation footing level.


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## concretemasonry (Oct 10, 2006)

The depth of the pipe is precisely why the "french" drain around the home is placed with the bottom of the footing so it prevents excess water against the wall and reduces the pressure that can cause leakage. On the interior, it cuts off the water flow from ouside the footing perimeter and reduces the hydrostaic pressure under the slab that can cause leaks or crack, depending on the soil. The level of the bottom of the pipe or gravel controls the bottom level of where they are effective.

French drains do not drain water away quickly like a curb or formed drain, they do increase the amount of moisture in the soil that allows the soil to take onextra water for storage before draining more actively. Some french drains may not drain water into a sump or to daylight immediately after a storm, but only when the soil at that level becomes saturated.

The open "french" drains with or without a perforated pipe have been used for decades on golf courses to minimize the wet areas since they are hidden and buried in a trench that does not alter the contours of the golf course. The course I played on from 1970 to 1990 had active subsurface drainage that was installed before, but still worked well even without perforated pipes. "If it ain't broken, don't try to fix it".

There are many thousands of acres of fields in Iowa that are drained for agriculture using loosely jointed clay and concrete pipe ever row of three. This allows earlier planting and a longer growing season.

French drains are everywhere, but you never see them.

Dick


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## bsmith95610 (Feb 14, 2011)

*Drain Pipe Question*

So when I install the french drain should I dig it down so the pipe is a few inches below the bottom of the concrete patio and one foot or so to the side of the patio?


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## AllanJ (Nov 24, 2007)

By the way, the surface French drain is named after the country. The underground French drain is named after someone.

For draining water running off the patio/driveway only, an underground French drain need only be a few inches below the surface but can freeze up in winter. The desirability of a surface French drain also depends on how good it looks after figuring in the needed slope. 

To avoid undermining and possible cracking of the slab, it's a choice of whether to dig the trench almost against the patio/driveway at a depth equal to the patio/driveway slab bottom, or a foot away and a few inches lower.


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## handy man88 (Jan 23, 2007)

I think what you're referring to are "catch basins," not "french drains."


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## AllanJ (Nov 24, 2007)

A catch basin is an underground tank or vault with a surface inlet where water flowing from elsewhere (perhaps via a French drain) is dumped in. It must have some outlet down below (perhaps to a storm sewer) or be equipped with a sump pump.

A "French drain" that is supposed to be the end destination of the water which soaks into the surrounding soil is a leach field.


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## handy man88 (Jan 23, 2007)

bsmith95610 said:


> Hello,
> 
> I recently had a new concrete patio poured and the water is draining away from my house which is good. But I'm worried that the water will pool up on the dirt right off of the patio. I was thinking of installing a French Drain running parallel with my patio and another running parallel with my driveway to collect water. Then connecting those french drains into a drainage box with surface drain then from the drainage box it would connect to the 4 inch non-perforated PVC pipe that is running under the patio which would take it out to the street.
> 
> ...


You can "catch the runoff from the patio" via catch basins and connect to your non-perforated pipe that is already underneath your patio. 

Hopefully, this pipe outlets into the street so that it doesn't need to be pumped.


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## bsmith95610 (Feb 14, 2011)

So would it be better to use two or three catch basins instead of a french drain? I was going to install one catch basin in the corner then do two french drains as well. But if it is better to do multiple catch basins I will try that instead. 

Wouldn't a lot of water get missed with the catch basins since in between two catch basins there could be a lot of water coming into the yard? I imagine the water will come down pretty much evenly across the patio.

Thanks


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## handy man88 (Jan 23, 2007)

bsmith95610 said:


> So would it be better to use two or three catch basins instead of a french drain? I was going to install one catch basin in the corner then do two french drains as well. But if it is better to do multiple catch basins I will try that instead.
> 
> Wouldn't a lot of water get missed with the catch basins since in between two catch basins there could be a lot of water coming into the yard? I imagine the water will come down pretty much evenly across the patio.
> 
> Thanks


I would recommend using catch basins and do whatever you can do to get the grading to drain towards the catch basins.

If water is flowing at a fast rate, I seriously doubt if french drains (trench filled with gravel, then with perforated pipe, and then with more gravel) will allow water to drain fast enough.

For long narrow lengths, you should consider storm drain/trench channels:

http://www.google.com/imgres?q=drai...tart=0&ndsp=25&ved=1t:429,r:1,s:0&tx=96&ty=86

http://www.google.com/imgres?q=drai...t=25&ndsp=25&ved=1t:429,r:23,s:25&tx=98&ty=91


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## runoff (Sep 20, 2011)

*Drainage/French Drain Questions*

If those circles are representing inlet basins, I don't think they will catch much water if I'm seeing the pictures right. You could just tie your pipe straight into the pipe going under the patio. The depth of the french drain pipe can only be as deep as the pipe going under the patio. The pipe should have a layer of stone under it then position the holes down. Otherwise the pipe will collect silt and plug up. Then envelope the entire pipe with stone. Ideally , you can put a sock over the pipe and wrap the stone and pipe in fabric. The stone should be what is called "poorly graded" stone. Meaning the stone is close to the same size with no fines. Number 57 size stone is a good size. The only problem with this is you will be constantly maintaining the surface so that silt, leaves and such won't be resting over the trench blocking the drainage.


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## bsmith95610 (Feb 14, 2011)

If I try to use the storm drain/trench channels can I install those at the end of the concrete where the grass will start? I don't want to have to pour more concrete or chip away existing concrete to fit a drain/trench channel.


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## runoff (Sep 20, 2011)

*Drainage/French Drain Questions*

No, just install them in the ground right against the concrete at ground level.


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## bsmith95610 (Feb 14, 2011)

Runoff - So are you saying I can use any product similar to the one listed below and just install it right up against the concrete then put in either at the same level or an inch below the level of the concrete?

So will the trench drain eventually shift down since it will actually be sitting in the soil? If so should I dig the ditch tamp the bottom of it then put an inch or two of rock below it so it doesn't shift as much?

I appreciate the input.

http://www.google.com/products/cata...a=X&ei=5g96Tv6ROKXliAKfvIirDw&ved=0CHsQ8wIwAw


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## runoff (Sep 20, 2011)

*Drainage/French Drain Questions*

Yes, to the positioning of it. Yes, to the tamping. No, to the the stone under it. That will only allow water to seep down around it and sit underneath it. Not sure where you live but if you have freezing weather, the water sitting under the channel will freeze and heave the channel up out of the ground and if the ice is close enough to the edge of your concrete, it will crack and heave it also. 
Secondly, the channel that you showed a link to is way over kill. You don't need a steel channel. They make the same thing in vinyl that I'm sure is much cheaper. You'll have to shop around to find it.


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## bsmith95610 (Feb 14, 2011)

Sounds good I will try the trench drain then and just put it up against the concrete and tamp the dirt below it then pack in the dirt tightly on the other side. Thanks for the help with this I appreciate it.


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## handy man88 (Jan 23, 2007)

bsmith95610 said:


> Sounds good I will try the trench drain then and just put it up against the concrete and tamp the dirt below it then pack in the dirt tightly on the other side. Thanks for the help with this I appreciate it.


I would recommend small rocks vice dirt.


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## runoff (Sep 20, 2011)

*Drainage/French Drain Questions*

Then you have a place for water to seep down and under channel. Trapping it instead of sealing it off so it all runs into channel. MHO


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## handy man88 (Jan 23, 2007)

runoff said:


> Then you have a place for water to seep down and under channel. Trapping it instead of sealing it off so it all runs into channel. MHO


To a certain degree, but dirt will wash away and could render the channel unstable, or clog it.


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## bsmith95610 (Feb 14, 2011)

So this is what I'm thinking of doing let me know if you guys think this is a good idea. I'll dig everything out then level out the ditch tamp the dirt then mix up a few bags of concrete then pour a four inch slab that the trench drain will sit on then possibly put some concrete on the side of the drain that will be facing the lawn but leave it so the concrete on the side stops a few inches below where the grass will be so I can lay sod over it. Then the other side of the drain will be right up against the patio. If I do it this way it would seem the drain should hopefully never move. Let me know if you guys think this should work.


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## runoff (Sep 20, 2011)

*Drainage/French Drain Questions*

Well, first, besides it being a lot of extra work, small patches of concrete tend not to last very long. They tend to get moved around a lot from freeze and thaw then cracks and eventually crumbles. Only large pours of concrete can withstand freeze and thaw for any length of time and even they eventually give in to nature. Public sidewalks move all over the place from freeze and thaw and are constantly being maintained. So personally, I wouldn't use the concrete because I feel the drainage channels won't stay in place for long. Also, if you place sod over the concrete, the sun will burn it up because it will heat up the concrete. Also, the sod has no place to draw moisture from. I think if you just use soil around the channels , pack it in nice and tight then plant grass seed, it should hold everything in place. And when you do have to re-arange things in time, you won't have to break up a bunch of concrete to repair things. That's just my opinion.


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