Hi,

We have minimal basement moisture issues, but before finishing off about 600 sq. feet, want to put in an under slab french drain ourselves.

I've been doing some research and have settled on not using the corrugated black pipe, not using a sock on the pipe, and not using landscape fabric around it. It seems the risk of those things clogging is higher than using perforated schedule 20 white pipe embedded in enough washed gravel and having cleanouts.

The parts I'm unclear about are:

What is the best membrane that goes up the wall to use?

We have to go about 90 feet or so around corners too. What is the minimum pitch needed for the pipes?

Should the top of the drain pipe be at the bottom of the footing or somewhere near the top of the footing?

We have a waste pipe we have to cross by going under it with elbows. Is there an issue since that will form a P-trap kind of arrangement?

Thanks, guys!

 

It is common for foundation drains to not have fall. Personally I like to have fall but how much depends on your installation as 1/4" per foot really adds up when you are doing 90 feet.

The holes in your pipe should face down and the bottom edge of the pipe is the beginning of the water protection. The top of the pipe doesn't matter so much. The key is the bottom of the pipe. There will be NO water control below the bottom of the pipe. So, getting the pipe lower is better and you absolutely want the bottom of the pipe lower than the top of your slab in the basement.

I would not create a P trap in the French drain line. It's going to be flat or have minimal pitch at best so there isn't a lot of energy to keep the water and debris moving. A trap will really slow it down and create a perfect debris trap. If you absolutely have to then I'd install clean-outs on either side so it's easy to unclog.

 

Thanks Pilot Dane.

"...bottom of the pipe lower than the top of your slab in the basement." I have to have the top of the pipe lower than the slab to pour a slab back over it.

"I would not create a P trap in the French drain line.' I have a 3" cast iron pipe just below the slab in a corner that we have to get by. How else would you do it?

 

If you are putting your drain inside the basement's perimeter then that will leave the area outside somewhat unprotected. You could have water leaking into the basement where the walls meet the floor. With the drain tile inside the basement the water must get inside the perimeter before it ever reaches your drain pipe. How do you plan for water to easily get to your drainage system? There is likely hard, virgin soil underneath your footers which will act as a barrier holding the water outside.

You said you would be going underneath the drain pipe with elbows creating a trap. Have you planned the slope of your drain pipe? Maybe it will be deeper than the 3" drain and not an issue.

 

It's a block wall. Every block void needs to be drilled. That's the point of the membrane I asked about - it provides a path from wall to drain field.

The waste pipe will cross the drain at about 1/3 of the way the french drain has to go. To get enough pitch in that distance to clear the pipe would put me a mile deep at the sump pump. I can only see elbowing under it as the only option unless someone knows an alternative.

 

Is there a reason not to put it outside of the house? That's where they're typically installed. Perhaps a water-guard system would be better for your application:

https://www.basementsystems.com/base...nch-drain.html

Using fittings to go under another drain is inadvisable.

 

From a perspective of installation effort, probably 3-4x times the work for an external french drain.

Typically here in NJ, it seems that retrofit french drains are interior drains.

Somewhere I've heard of issues with waterguard but that still doesn't resolve crossing the waste pipe below the slab.

What is the alternative?

 

You are between a rock and a hard place. Best is to install the drain outside. By going inside you are starting with a compromise. Throw in the low spot (trap) and it's just another degree of compromise.

At some point cutting up the concrete and adding clean outs for the low spot has to be balanced against digging outside and having a much better drainage system. I am very aware that not everything can be done "perfect" but there are some strong points favoring digging up the yard. I would be giving both options a serious looking over. By the time you factor in all the concrete cutting, hand digging and pouring new concrete for the inside work a day for a pro on a excavator outside might be in the same price range.

 

Before starting, observe if water collects against the foundation anywhere. This must be prevented to assure that a French drain will do a good job protecting the basement from flooding. Do not have a depression at any point next to the foundation in order to hold a layer of mulch or gravel.

Normally the bottom of the collector pipe will be several inches below the basement floor surface because, among other reasons the pipe sitting below the floor should not detract from the thickness of the floor slab re-poured after the pipe is installed.

But do not carve the ground under the basement slab below the level of the bottoms of the foundation footers. So the layer of gravel under the collector pipe, the collector pipe itself, and any gravel above it have to build up from the level of the bottoms of the footings. The bottom of the drain collector pipe will be an inch or two above the bottoms of the footings..

It should be rare if ever that water cascades through the collector pipe on the way to the sump pump pit. Rather the collector pipe will most of the time act as a continuous air pocket below basement floor level and going all around the perimeter of the foundation.

If some other pipe exits the house undeground and interferes with the French drain collector pipe, it is okay to have the colelctor pipe do a U under the other pipe amidst a good bed of gravel at the crossing point. Yes it would help to have a provision to clean out dirt or debris that may collect in the U bend. Drill a few extra holes in the drain pipe in the vicinity of the crossing.

Most of the water below the collector pipe will stay behind. Some may slowly make its way through the gravel and the soil underneath over to the sump pump pit if there are holes (recommended) in the latter below where the collector pipe(s) dump in.

Another advantage of putting the French drain collector pipe on the outside is that the outside surface of the foundation will be exposed to allow coating with a waterproofing compound or membrane. Construction of the drain pipe and trench would be the same for either outside or inside laying.

 

@ Pilot Dane - external is just not possible. One basement wall adjoins the garage so there is no way to get to the outside of it, then aside from wiping out thousands of dollars of mature plantings, the patio, steps, and front porch would be destroyed, fences align to the house, gas pipe, water pipe, chimney, etc.

 

@ Allanj - some excellent points. I've been concerned about having room for the pipe and gravel while maintaining a full floor thickness. I've seen "pros" that put an inch or so of concrete over the pipe, get paid, and disappear. Right now I don't know how deep the footer is to know what I can fit.

Yeah, I've wondered about digging deeper than it and creating a weakness where the foundation will want to move inwards.

 

Your footer is probably about 12" thick. You can usually dig down to the bottom of the footer without issue. If you want to dig deeper than the bottom of the footer my building inspectors say go from the bottom corner of the footer and draw a line down and out at a 45 degree angle. Do not disturb the soil inside that 45 degree line and under the footer. Outside that 45 degree line and you are generally safe to dig.