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sloojoe 05-03-2010 05:15 PM

French Drain Help
I want to install an exterior french drain, my footer is 4' deep from the top of my lawn. How far from the top would I fill the trench in with gravel?



Daniel Holzman 05-03-2010 06:56 PM

Typically the perforated pipe for a perimeter drain is set at least six inches lower than the basement floor. The pipe is placed in a gravel bed approximately a foot wide, and the gravel is brought to the surface, allowing surface water to be carried down to the pipe, where it drains either to a low point on your property, the storm drain system (if allowed), or into a sump pump pit where it is pumped out.

sloojoe 05-04-2010 04:49 PM

thanks, I wasn't sure how high to fill with gravel.

tpolk 05-04-2010 06:43 PM

are you doing a french drain or a foundation drainage system?

sloojoe 05-05-2010 10:05 AM

French Drain
that was my next question. my home is built 1920's, I had a foundation is poured concrete. Should I place the 4" perferated pipe next to my footer or above my footer?

If I dig 6" below the footer, will it shift the soil around the house and cause more problems?

Finally, with some help I will take photos and thoroughly detail every step and post it to forum.



Scuba_Dave 05-05-2010 10:44 AM

Never dig below the footer

AllanJ 05-05-2010 12:08 PM

The drain pipe can sit next to the footer without digging below the level of the bottom surface of the footer. Depending on the exact level of the basement floor inside, you should be able to get at least an inch preferably two of gravel down before you lay the pipe.

For maximum performance, especially if the basement floor level is close to the bottom of the footer, do not let the drain pipes fill up. This is accomplished by having the sump pump come on when the water level in the pit just goes over the bottom edge of any of the drain pipes entering there.

Even without great precision, you can probably get the drain pipes level enough that there are no pockets where water collects more an one inch deep inside after the sump pump has shut off. The remaining three inches of pipe diameter (4" pipes recommended), being empty most of the time, can count towards the ideal six inches below floor level for the drainage system.

When installing the drain pipes, turn them so one row of holes is at the bottom. Wrap them in a layer of weed control fabric or fabric meant for these pipes to keep dirt from getting inside.

sloojoe 05-05-2010 02:02 PM

French Drain Plan:
French Drain Plan:

1. Dig 4’deep by 1’ width trench around exterior perimeter of house down to the footer.
2. Roll liquid tar on poured concrete foundation and cover with 3 mil plastic.
3. Line trench with weed block landscape cloth.
4. Fill trench 2-3 inches with washed gravel.
5. Place 4” perforated pipe with holes down next to footer and wrap with drain sleeve.
6. Adjust pipe slope to 1/8” for every foot.
7. Fill rest of trench with gravel.
8. Fold the landscape cloth over top of gravel.

Does my plan make sense?
Do you think I should install a buried sump pump in my front yard?

AllanJ 05-05-2010 04:28 PM

It is possible that you cannot have all of: a three inch layer of gravel under the perimeter drain pipe and a 1/8" per foot slope and not dig below footing level at the low point and keep the pipe low enough to divert water away from the basement floor at the opposite corner of the house. If I had this problem I would compromise on the layer of gravel under the pipe and compromise on the pipe slope. Another solution is to have two low points at opposite corners of the house.

You can if you want to have an outdoor sump pump pit at the chosen low point(s) for your drain pipe system. This would probably be about 18 inches across. An opening must extend all the way to the surface so the pump can be extracted and replaced if necessary.

If you prefer an indoor sump pump, the connection from the outside drain pipe system to the inside pit must go through or under the footing, not over. Digging a six inch channel crossing under the footing at one spot should not cause any structural problem.

Better than a sump pump is a lateral (and unperforated) drain pipe away from the house to the outside. Needless to say, the ground surface must have a substantial slope down away from the house because the lateral drain pipe starts at foundation footing level and must also slope downward away from the house and still reach the surface "sooner or later". If you have such a suitable terrain slope, choose the low point of your (perforated) drain system to be on that side or corner of the house. It is rare to have such a terrain slope on both sides of the house if your pipe slope required two low points and two such lateral pipes.

sloojoe 05-05-2010 05:05 PM

Thanks allan, do you think I should just settle for a internal drain in my basement with a sump pump?

AllanJ 05-05-2010 08:14 PM

An internal French drain is installed using the same rules as an external system (don't dig below footer level, etc.). An internal system is usually easier to install even though you have to jackhammer the basement floor all around the foundation perimeter. The performance is not quite as good but usually suffices in keeping the basement dry.

Both the internal and external systems require a way of expelling the water, either a sump pump or a gravity drainpipe out to the ground surface.

Sometimes a dry well is dug to accept the water from the French drain. It works decently until such a big rainstorm comes that the dry well fills up.

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