School me on French Drains

[cash68]

Hey All,

So I've done a LOT of reading on french drains, but there's still some stuff I don't understand and am not clear on.

So first off, here is what I am thinking about doing:

Note the addition of some 3 mil plastic sheet angled away from the block wall. I was hoping this would keep the soil next to the foundation very, very dry. If you think the moisture from underneath would try to rise, and the plastic would prevent this from happening or something, let me know. It seems like a good idea?

So now questions:

1. I bought PVC pipe with two sets of holes drilled at 5 and 8 o clock or so. Would it be beneficial to add more holes on top? I see some pipe with slits on the top. Others with more holes. I understand that the idea is the holes face DOWNWARDS, so when the trench fills up with water, it comes up through the holes, into the pipe, and out. I get it. But would more holes help?

2. Gravel under the pipe. Why? Water doesn't drain away when it's on gravel; doesn't it make sense to put the pipe as low as possible? What purpose to the gravel serve UNDER the pipe? It seems like in cross sections like this:

3. Wrapping the pipe with lawnscape cloth? Is this necessary? I understand the landscape cloth around the gravel/pipe/entire thing,but am I supposed to also wrap the pipe itself?

4. Cleanouts; any tricks here? Just an angle towards the surface with a cap?

Edit: Here is the total gameplan:

 

[spudley]

Here's what I did to avoid rain gutters. Dug a trench directly under my eaves with a slight slope in the direction I wanted water to drain. Put down 6ml (could be thicker but that's what I had) black plastic from the edge of the foundation out about 3' (I have 2' overhangs) into and back out of the trench depression. Put in 4" corrugated pipe with slits and in filter sock (keeps dirt out). Covered it all with stone.
Basement is bone dry, and the house stays clean when it rains.

 

[niget2002]

Just my understanding, but...

The french drain is supposed to carry away water when there's more rain in a period of time that the ground can naturally absorb.

I'll try to answer your questions.

1) no. more holes won't necessarily help.th water is going to enter the pipe as the water builds up from the bottom. more holes on top won't really do anything.

2) Yes. Gravel under the pipe. Dirt tends to shift. Gravel, once set, doesn't. Also, water moves more easily around the rocks. Having the gravel under the pipe lets the water flow easier to the pipe. It's only recommending an inch or so under the pipe. Just enough so that you're laying the pipe on the gravel and not directly on the landscape fabric.

3) I would. The landscape fabric around the pipe is to help keep critters from burrowing into it and keep roots from growing into the pipe. Roots love water and you can easily choke up a french drain with a few large roots growing through the middle of it.

4) No idea on the clean outs.

Mind you... my answers are from research I did when we were looking at putting a drain in at the last house. We ended up moving before I had to put that knowledge into practice.

 

[NitroGarage]

I just put a 80 foot french drain in along my driveway. The total drop of the trench over 80 feet is only 4 inches. Then the water has to go to the top of the pipe before it gets to the bottom level of an existing culvert that runs under my driveway to get to the other side. I didn't think it would work but I was committed to trying something to keep surface water from covering my entire driveway and parking area by the house.

It works amazingly well. The mere fact that there is 80 feet of 4" pipe for water to go plus the gravel around it gives me a storm surge buffer. I don't have geofabric around the gravel but I do have the silt sock on the pipe. Bought it in a roll already installed.

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[Kaizen]

The gravel under the pipe is to give the water a reservoir of sorts. Your not making a sealed gutter system. There will be water under the pipe and in the soil. Get rid of that plastic. Not going to do anything besides trap moisture under it next to the block. Also the gravel will fill with some silt raising the water level. Most important thing is sealing it. I’d buy the socks you pull over it. Wrapping landscape fabric did not work for me. Because it’s corrugated you can’t clean it out. So make sure it’s sealed. Most important is good pitch


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[cash68]

I'm using PVC, not that corrugated ********.

 

[theoldwizard1]

Read this ! My French Drain and Dry Well Project

Forget the plastic. It won't help.

You need to be below the level of the floor for it to work correctly.

The gravel under the pipe is to give the water a reservoir of sorts. Your not making a sealed gutter system. There will be water under the pipe and in the soil.

100% correct !

I used smooth pipe with a sock over it. I placed landscape cloth in the bottom, a couple of inches of 3/4" crushed limestone (mostly for leveling and adjust the pitch), more 3/4 rock, then fold in the landscape cloth. The top 6"-12" can be soil but I used "slag"/stone dust, because it drain faster than soil.

 

[theoldwizard1]

I'm using PVC, not that corrugated ********.

I assume you are using the type for this application with holes in it.

 

[PWC Repair]

I have 50ft of 4ft tall wall below grade. I dug into the hillside to start my build. I coated the wall with foundation sealant. Then creek gravel, then the landscaping cloth, then more gravel, then the perforated drain pipe, then more gravel, then cloth wrapped up like a burrito as in your drawing, then I used a piece of plywood as a divider to get gravel up the retaining wall and backfill with dirt at the same time. Then used the front end loader to pull plywood up. I brought the gravel to within about a foot of the finished grade. This spring we had flooding rain after rain after rain. My backyard turned into swamp and is literally just now drying out. I had absolutely no moisture come in at all.

 

[cash68]

It looks like the wall is already covered in foundation sealant, which seems like it didn't hold up properly. It has a black rolled on coating, and additional white stuff on cracks which looks similar to silicone.



I made progress last night, and started hitting yellow/white gravel about 1-1.5 ft down. Is it common for there to be gravel next to a below grade foundation? I suppose it's possible there is even a french drain here already, but I kind of doubt it.

Would an actual trenching shovel be any easier? With all the roots and rocks and **** in the dirt, this is just so freakin' slow. I can barely get the shovel in more than 1.5" before it hits something, then I have to chop away. This project has already claimed an older shovel and a grub hoe.

 

[Kaizen]

A serrated round point shovel would be my weapon of choice. You need this to be at least a foot wide so a trenching shovel won’t help. It’s hard work no doubt.


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[spudley]

You have a pretty good negative slope going on there. You might consider two levels of drainage...

 

[ishiboo]

It looks like the wall is already covered in foundation sealant, which seems like it didn't hold up properly. It has a black rolled on coating, and additional white stuff on cracks which looks similar to silicone.



I made progress last night, and started hitting yellow/white gravel about 1-1.5 ft down. Is it common for there to be gravel next to a below grade foundation? I suppose it's possible there is even a french drain here already, but I kind of doubt it.

Would an actual trenching shovel be any easier? With all the roots and rocks and **** in the dirt, this is just so freakin' slow. I can barely get the shovel in more than 1.5" before it hits something, then I have to chop away. This project has already claimed an older shovel and a grub hoe.

Clean gravel, drain rock, pea gravel, etc. is exactly what should be next to the foundation - something that allows water in and allows it to drain.

Hey All,

So I've done a LOT of reading on french drains, but there's still some stuff I don't understand and am not clear on.

So first off, here is what I am thinking about doing:

Note the addition of some 3 mil plastic sheet angled away from the block wall. I was hoping this would keep the soil next to the foundation very, very dry. If you think the moisture from underneath would try to rise, and the plastic would prevent this from happening or something, let me know. It seems like a good idea?

So now questions:

1. I bought PVC pipe with two sets of holes drilled at 5 and 8 o clock or so. Would it be beneficial to add more holes on top? I see some pipe with slits on the top. Others with more holes. I understand that the idea is the holes face DOWNWARDS, so when the trench fills up with water, it comes up through the holes, into the pipe, and out. I get it. But would more holes help?

2. Gravel under the pipe. Why? Water doesn't drain away when it's on gravel; doesn't it make sense to put the pipe as low as possible? What purpose to the gravel serve UNDER the pipe? It seems like in cross sections like this:

3. Wrapping the pipe with lawnscape cloth? Is this necessary? I understand the landscape cloth around the gravel/pipe/entire thing,but am I supposed to also wrap the pipe itself?

4. Cleanouts; any tricks here? Just an angle towards the surface with a cap?

Pretty much exactly what not to do. Ground water in most of WI is the biggest issue with dry foundations/basements.

Why do you put gravel under the pipe? Same reason you put the holes in perf pipe down. You are capturing water that goes down, saturates the soil and then the whole water table rises from a much larger area. Empty space is the path of least resistance for water - the area between clear stone allows water to easily flow into it and then get into your pipe. You're not likely to capture much water that percolates down and just happens to hit one of the holes. Holes on the top of the pipe are unnecessary. Imagine the small likelihood of water falling straight into one of the holes.

You do not need to wrap the pipe itself. Just the gravel around it. The gravel needs to be prevented from being filled with silt.

Excavate to the bottom of the foundation footing. Use the thickest poly you can afford, EPDM rubber roofing, spray or roll-on foundation coating, or something similar to cover the foundation. You can use the dimpled boards if you wish too... it's a small area so it wouldn't be that expensive. Throw down your landscape fabric and some CLEAR stone, and then put the perf pipe almost towards the bottom. Fill up the trench with CLEAR stone, making sure the landscape fabric totally encompasses it.

I wouldn't create a french drain there unless absolutely necessary. I would fix the grading so the water is sent away from the house, and any that gets back is captured by the footing drain. You can put plastic at or near the surface if you'd like... above the drained area. Just do not do it like you show or ground water will have no choice but to enter the foundation.

 

[ishiboo]

You have a pretty good negative slope going on there. You might consider two levels of drainage...

Totally agree!

It would be good to see an actual elevation and know where the driveway is and if there's a lower area behind the vantage point of the photo.

 

[johnyg]

where is your downspout draining to?

 

[polexican23]

The holes in the pipe should face up.

 

[ard]

It has been mentioned but I'll reiterate:

That drain pipe MUST be below the floor level inside the building.

It is easy to only think of water coming off the roof onto the ground and down to the drain. HOWEVER, water can move through the soil and up under the foundation/floor. If the drain is low, the water will never get to the slab.

 

[biggziff]

perforated pipe holes should face the bottom of the trench.

Don't use socked pipe or geotex. It's been shown to cause early failure of trench drains and is no longer recommended by anyone other than the people selling it. Look it up. NCMA says no-no since at least 2011.

Corrugated black pipe, when installed properly will last 30-40 years with no maintenance. Add a cleanout and use it periodically it will outlive all of us.

 

[lakeroadster]

... and in Wisconsin when the ground freezes the water in the pipe freezes and the french drain no longer drains.

The best plan is always grading the site such that water naturally flows away from the building.

 

[cash68]

where is your downspout draining to?

Underneath the stone stairs, right near the doorway to the garage. It comes out and drains on the main concrete pad, then runs down the driveway.

 

[cash68]

Here is the plan: French drain along side foundation, with a berm after that and the lowest point of the ground about 5' from the building. The plan is that most water will hit the berm and roll down the hill onto the driveway, and the water that soaks through the berm will hit the french drain and go down the driveway.

 

[ptgarcia]

Here is the plan: French drain along side foundation, with a berm after that and the lowest point of the ground about 5' from the building. The plan is that most water will hit the berm and roll down the hill onto the driveway, and the water that soaks through the berm will hit the french drain and go down the driveway.

That's a good plan. Grade a swale to direct surface flows away from the building.

Regarding the french drain: its functions is to eliminate, or at the least lessen, hydrostatic loads on that wall. To be most effective you need to lay that pipe as close to on top of the wall footing as possible (perforations down).

 

[lakeroadster]

All you needs is the swale. The french drain isn't needed at that point.

 

[cash68]

Once the moisture permates the swale, the hydrostatic pressure will push against the wall, no?

 

[rburke65]

Well I would think the holes should be down, and I assays thought the fabric would fill with dirt and the n fail. Glad to see a similar thought.

 

[lakeroadster]

Once the moisture permates the swale, the hydrostatic pressure will push against the wall, no?

Yes if the ground is saturated.

However, how long has the building been there, as is? Nearly all the water you are and have been dealing with will, once the swale has been finished, be re-directed away from the building.

Priority one would be the swale. Do that, and do it correctly such that it carries water away and doesn't allow the water to permeate the soil, and it is highly likely any issues you are / have been having will no longer be present.

You have the advantage of being able to direct nearly all the water that is around the building away from the building due to elevation variances around the building. Use that to your advantage.

As has been pointed out previously...

It has been mentioned but I'll reiterate:

That drain pipe MUST be below the floor level inside the building.

It is easy to only think of water coming off the roof onto the ground and down to the drain. HOWEVER, water can move through the soil and up under the foundation/floor. If the drain is low, the water will never get to the slab.

 

[cash68]

Man, idk if I can get that deep. We will see.

 

[theoldwizard1]

With all the roots and rocks and **** in the dirt, this is just so freakin' slow. I can barely get the shovel in more than 1.5" before it hits something, then I have to chop away. This project has already claimed an older shovel and a grub hoe.

Digging a ditch builds CHARACTER !

Go to HF and get a mattock. It easy specifically designed for loosening rock soil and cutting roots so that it can be shoveled.

 

[theoldwizard1]

Man, idk if I can get that deep. We will see.

If it is NOT below the floor of the building by at least 12", then you are still going to get water in the building.

My 3'x3'x20' (180 cu ft, 6.6 cu yds) ditch took me well over a week, probably 2.

 

[tjpavlov]

NDS makes a nice pre fab drain. It has about a twelve foot length of pipe wrapped in Styrofoam pellets and a sock. I used it last year for a project and it was great.

 

[Orionrising]

If it is NOT below the floor of the building by at least 12", then you are still going to get water in the building.

My 3'x3'x20' (180 cu ft, 6.6 cu yds) ditch took me well over a week, probably 2.

likely yes, but it depends on the soils.a properly graded drain to daylight should catch all the water.

 

[ard]

Man, idk if I can get that deep. We will see.

Yeah, I was getting that vibe as I read the thread. That sheet of plastic to 'protect' the wall was telling me 'he know the drain cant do it all'.....

Glad lakeroadster emphasized the point...

I had a garage that is below grade on 1.5 walls. Nice swale. Nice drainage. Gutters and downspouts feed into buried drainpipes.... Hydrostatic pressure still led to moisture in the floor. Have to get below the floor to keep water away. Just the way it is.

Screw the mattock from HF. Go to HD and rent a trencher or mini backhoe. 4 hrs, done. Heck, you'll have the drain excavated halfway don the driveway.

If you dont you will kick yourself.

 

[cash68]

Ard, I would consider it, but what would fit there? A mini backhoe seems a bit too large. A dingo seems like it might work, but I can't trench next to the foundation; the trench would be 1-2' away from the foundation. Same goes for an actual dedicated trencher, which I believe are not powered via wheels. There's no way I could muscle a 900lb trencher up there by myself. Thoughts?

 

[machsnell]

Mini excavator on tracks. Little tonka toy one. Narrow.

Digging next to foundation would even be possible.

Can a little mini get there?

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[WhoWhatNow]

Ard, I would consider it, but what would fit there? A mini backhoe seems a bit too large. A dingo seems like it might work, but I can't trench next to the foundation; the trench would be 1-2' away from the foundation. Same goes for an actual dedicated trencher, which I believe are not powered via wheels. There's no way I could muscle a 900lb trencher up there by myself. Thoughts?

Got a garden tractor or SCUT? Build something like this: You could offset it to get it closer to the building. I build one for my Deere 318 to bury some drainage pipe.

https://www.northerntool.com/shop/t...MI57uy65ic2wIVQkOGCh1Uqg-gEAQYASABEgL3-PD_BwE

 

[Trey T]

Clean gravel, drain rock, pea gravel, etc. is exactly what should be next to the foundation - something that allows water in and allows it to drain.



Pretty much exactly what not to do. Ground water in most of WI is the biggest issue with dry foundations/basements.

Why do you put gravel under the pipe? Same reason you put the holes in perf pipe down. You are capturing water that goes down, saturates the soil and then the whole water table rises from a much larger area. Empty space is the path of least resistance for water - the area between clear stone allows water to easily flow into it and then get into your pipe. You're not likely to capture much water that percolates down and just happens to hit one of the holes. Holes on the top of the pipe are unnecessary. Imagine the small likelihood of water falling straight into one of the holes.

You do not need to wrap the pipe itself. Just the gravel around it. The gravel needs to be prevented from being filled with silt.

Excavate to the bottom of the foundation footing. Use the thickest poly you can afford, EPDM rubber roofing, spray or roll-on foundation coating, or something similar to cover the foundation. You can use the dimpled boards if you wish too... it's a small area so it wouldn't be that expensive. Throw down your landscape fabric and some CLEAR stone, and then put the perf pipe almost towards the bottom. Fill up the trench with CLEAR stone, making sure the landscape fabric totally encompasses it.

I wouldn't create a french drain there unless absolutely necessary. I would fix the grading so the water is sent away from the house, and any that gets back is captured by the footing drain. You can put plastic at or near the surface if you'd like... above the drained area. Just do not do it like you show or ground water will have no choice but to enter the foundation.

Listen to this guy.

If you're going to create a drainage device for runoff from slopes, it's better to keep it open, like a ditch of some-sort. In civil engineer projects, they call it interceptor ditches. If it's a surface runoff, keep it as a surface runoff, don't let it infiltrate.

The primary function of french drain is to convey the saturated water in the soil (aka groundwater) away from the structure.

 

[ptgarcia]

If it's a surface runoff, keep it as a surface runoff, don't let it infiltrate.

How does one prevent infiltration when the surrounding land is all soft-scape? Infiltration is going to occur all around him, and once underground that water can end up anywhere/everywhere.

 

[Trey T]

Come on, man ... really? You get the point!!!

 

[ptgarcia]

Come on, man ... really? You get the point!!!

I get what you're trying to say, and its always best practice to divert water away from a building, but in this case an earthen swale does virtually nothing to prevent infiltration or ground water issues. Nothing is going to help this situation but some sort of sub-surface drainage system. When I design I do my best to never retain against a building, especially in a wet environment. The inside inevitably sees moisture.

 

[johnnyradiant]

Dig down the side of the wall to below your interior slab. Then install your drain w/ clean-out. I would sock it. The more attention paid to your sock the less likely hood of needing to crack the clean-out open. We use a dimpled membrane on the wall so that the water coming at the wall hits it and 'runs/drops' down to the drain instead of trying to push it's way through the concrete.
Here is one company's with a little pic to show the drain location below the slab:
https://d9hhrg4mnvzow.cloudfront.net/delta.dorken.com/delta-ms-webinar/4d9af900-3d-ms-mobile-update_0eo0ah0eo07i00000w.jpg

With the building at the bottom of a bowl if you find your perimeter drain (at the bottom against the wall) is doing a lot of work you could move out from the building and do a secondary drain about 1/2 as deep with gravel filling the lions share of the trench. The secondary method has been used around the Wetcoast with good success on the less than ideal building sites.