Q&A for How to Build a French Drain

No. The water travels to the lowest point in the gravel, which is beneath your pipe. If you put the holes on top, they would probably catch some of the draining water, but most of the water (considering the width of the trench vs. the holes) would actually begin pooling below the pipe. With the holes on the bottom, they are closer to the lowest point and therefore will catch more water because gravity will compel it toward the path of least resistance (in the pipe).

Washed gravel. The fine sand/clay can clog up your freshly installed French drain.

You'll need to find access points to the drain. Ideall,y both through the drain in your garage floor and wherever it drains to: the street, sewer, or drywell. You'll probably want to rent the smallest motorized snake (a roto-rooter type machine that has a couple small bits: paddle and screw) you can find to avoid damaging the pipes/drain tiles. Use the snake to go into the drain-line from every entrance you can. The more junctions, the less likely you'll clean it completely. Older homes will often have broken french drain pipes, either from age (with terra cotta or iron), or mechanical breakage (roots, digging, etc.).

You can, but French drains only last so long and one day you will have to replace it.

Yes, you can. What you place for top lay is up to you. Rock works great and allows you to use your garden hose to keep the rock, and thus the drain, clean.

Always best to minimize joints. Use a roll of perforated corrugated black plastic if you're doing it on the cheap. But use 4' or 6' diameter PVC with holes if you can afford it -- it will last much longer and is easier to use. Straight pipe (like the PVC) will have more junctions than the black plastic, but if you join them correctly, which isn't hard, it's still a much better quality option overall.

Best not to - the water will filter through the cloth easily enough. The last thing you want is for small particles to filter through and into the pipe and block it up or interfere with the flow.

It will work in clay soil, but less water will be absorbed by the earth around it. Make sure you get the stone portion right for your sized drainage problem.

Yes, but the drainage area should be covered with gravel to prevent large puddles from forming.

I haven't tried yet but I think traditional wisdom is AT LEAST three inches. However, I think if you don't have more like six inches, you're unlikely to keep grass through the summer drought without watering frequently. Grass roots should be deep and only giving them three inches could be asking for summer trouble.

As long as you can place the drain extension on a route that isn't going to be a tripping hazard, then you can make it as long as needed. The goal of the gutters and downspout is to move water away from the foundation and prevent foundation damage. So if an extension to your downspout, if it is not properly moving the water to a desired distance from the foundation, is perfectly fine to add.

NO! The point of everything you are doing is to let water easily flow towards your pipe and away. You do not want water stuck trying to push/rise against plastic and never making it to the pipe.

The bigger question is where to end the french drain so as to not be liable for damages to your neighbor's property. I had to construct my french drain in such a way that I drew the water away from MY foundation, but still allowed the water that the french drain "caught" to not go like a freight train into my neighbor's yard. So, divert the water away from your own foundation, but stop the french drain immediately past the problem area so the water will absorb into the ground. Don't terminate your french drain 6 inches from your neighbor's yard.

Anywhere that works for your yard: a rock bed, a better sloped area that doesn't collect water, the city's drainage system, or a prepared rock-based deposit area. The idea for this system is to get water underground and to "spit out" along its run. The top layer just gets saturated quickly. You're spreading the volume of water along areas that haven't gotten saturated and you're putting it 18inches+ underground (again, not saturated) and letting the water drain back to the aquifer/water table.

You will have to create the slope yourself. Otherwise the French drain will not work properly.

A 2cm/m (or 1 inch/yard) slope is sufficient for gravity to overcome friction and keep the water moving instead of pooling. You could make a greater slope than that, but it would only mean more digging, and the increased flow of water wouldn't be helpful due to the limited absorption capacity of the ground.

You only need holes in the area you want to drain.

No. This would almost certainly be against code and also a bad idea. Your septic system is designed to handle the average flow rate from your house, and adding the amount of water that would be delivered by the drain would cause it to not work as designed.

It can be dug next to the foundation or a few feet away. In order to work well, your French drain needs to be constructed on a slight downhill grade. Also, be sure to plan your drainage route so it runs at least a meter away from any walls or fencing, and try to avoid any posts, shrubs or tree roots.

They don't mean to START the drain that far from the house. However, it is suggested that anywhere water will exit the system (especially the terminal point) should be at least 10 ft away. I actually think 3-5 feet from the foundation is okay for the corrugated pipe to run, but I'd like to see 10-15 feet away from the foundation for the terminal point.

Typically, make sure that the drainage sections are 10 feet or more from any building foundation. You can use PVC to move water away from the house to a French drain and back into the yard.

As long as you have a positive slope to your drainage pipe, it won't matter how far away it is from your foundation.

I have a cap on the end of mine that has slits in it, which allows the water to be freely expelled while keeping larger debris from entering. It is level with the ground so the mower doesn't hit it. But I have a natural pitch to my yard, so even having it show, it's still 9 inches below starting grade.

Fully wrap the perforated drain and the large stones acting as drainage paths in geotextile membrane and be sure to have at least 400 mm laps when joining sheets. Ensure the membrane is under the drain, also. It would also be good to add one or two flush points accessed at grid level. You could use these to insert a CCTV camera via USB to your laptop to visually check, but mainly to flush water through and clear any sediment build-up.

Yes, but a sump and pump would be needed. If you're asking this question, then I would recommend hiring a contractor for the job as it might be above your skill level.

Probably PVC, diameter dependent upon need.

The water travels to the lowest point in the gravel, which is beneath your pipe. Most of the water (considering the width of the trench vs. the holes) would actually begin pooling below the pipe. With the holes on the bottom, they are closer to the lowest point and therefore will catch more water because gravity will compel the water toward the path of least resistance (in the pipe).

Stuff it with scrunched-up chicken wire and some landscape cloth. You want to prevent rodents or small creatures entering the pipe and blocking it with nests etc.

Yes, you can. Start by inserting 1-3 feet of smaller ID pipe into the tile. (Use a reducer if necessary.) Then, seal it with a PVC rubber seal and/or very thick sand mix sakrete. Don't use anything that could later decay or wash into the drain pipe.

No. The water should drain freely downhill and away from your house. Catch basins retain water to buffer the outflow so downstream pipe systems don't get overwhelmed by heavy rains. With French drains, there is no downstream pipe system; the downhill end of the pipe shouldn't have anything deterring free flow. If your property is so flat that you have no downhill options, you could build a "dry well" (a barrel-sized gravel-filled pit).