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bsmith95610 09-17-2011 03:22 PM

Drainage / French Drain Questions
 
2 Attachment(s)
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.

After doing some more reading I'm not sure that this will work since french drains normally collect water from underneath them not water falling on top of them. Is that correct?

If anyone has any ideas on how I can catch the runoff from the patio and get it to my non-perforated pipe that is underneath the patio please let me know. I would appreciate any help I could get.

I have attached two pictures of the patio / driveway and marked where the french drains would be installed.

Thanks

concretemasonry 09-17-2011 05:24 PM

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

AllanJ 09-17-2011 06:52 PM

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.

concretemasonry 09-17-2011 09:01 PM

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

bsmith95610 09-18-2011 01:17 AM

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?

AllanJ 09-18-2011 08:05 AM

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.

handy man88 09-18-2011 09:00 PM

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

AllanJ 09-18-2011 10:00 PM

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.

handy man88 09-19-2011 03:55 PM

Quote:

Originally Posted by bsmith95610 (Post 730175)
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.

After doing some more reading I'm not sure that this will work since french drains normally collect water from underneath them not water falling on top of them. Is that correct?

If anyone has any ideas on how I can catch the runoff from the patio and get it to my non-perforated pipe that is underneath the patio please let me know. I would appreciate any help I could get.

I have attached two pictures of the patio / driveway and marked where the french drains would be installed.

Thanks

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.

bsmith95610 09-19-2011 09:53 PM

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

handy man88 09-20-2011 04:07 PM

Quote:

Originally Posted by bsmith95610 (Post 731950)
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=drain...:0&tx=96&ty=86

http://www.google.com/imgres?q=drain...25&tx=98&ty=91

runoff 09-20-2011 07:03 PM

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.

bsmith95610 09-21-2011 10:40 AM

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.

runoff 09-21-2011 10:45 AM

Drainage/French Drain Questions
 
No, just install them in the ground right against the concrete at ground level.

bsmith95610 09-21-2011 11:27 AM

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/catal...ed=0CHsQ8wIwAw


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