Shim and tape a level to a long , possibly 8 ft. , straight edge to place in the trench to obtain the slope desired .

Vid at about 4 minutes explains shimming for a constant slope .

 

We used a 8' level and keep the sewer pipe a 1/4 bubble out of plumb. Takes time to get every foot of the pipe supported underneath so you do not end up with dips and humps in the line.
Cheap is not a good plan.

 

String level works fine and is cheap. Just remember to put it exactly at the center of the span.

But I am with the others on how handy a long spirit level is. I have a 78" Sola, and use it all the time - both as a level and as a straight-edge. Used it recently for setting my downspout troughs and setting the grade beyond the trough. Eight foot would be better, but very long levels can be both rare and expensive.

 

If you use a 6' level take a 3/4" square block of wood & tape it to the end of the 6' level.
When the bubble is level "between the lines" you will have 1/8" drop per foot.
For a 1/16" drop use a 3/8" block.

 

Now i think i'm going to rent a trencher. Theres no really room for a mini-excavator (part of the dig would be past a small fence gate that a excavator is not fitting through, and the whole project is cramped as is with a HVAC unit for one thing crowding a portion of it) plus that feels like over my pay grade. Trencher wont be ideal but I just want to get something in the ground to some extent and see how it goes. It wont be that hard to dig up if it somehow does nothing, it's "only" at absolute most 120 feet but probably more like 80, of which only 30-50 needs to be french drain, the rest (last 30-40 ft) will just be a buried takeaway line.

As for sloping again I'm seemingly vastly scaling back my plans here. It seems I can just lay different amounts of gravel under the pipe to get the slope needed, so I plan to just trench the trench to a constant level probably, and once again last 30-40 feet has a hefty slope as is so will take care of itself.

I tried digging a test section and did not get far. The soil is rock hard clay littered with small roots. It's basically literally not far from trying to dig rock in feel. Even with a pickaxe progress was little, i was able to get a small 6" deep hole with much effort only using a fricking pickaxe. With a shovel well haha. The shovel handle feels to break almost instantly. Theres no way I'm digging the whole line. It's trencher or bust. As I said, scaling back my ambitions at this point. Even a 6" wide trench (probably what I end up with with trencher) will have to do. I also see some YT's that claim you dont need geotextile at all, just laying a pipe and gravel over it may turn out to be necessary since i am not sure about squeezing fabric into a 6" trench. As I said, if the project gets installed and does nothing it wont turn out to be the end of the world. Also I believe in my location in Tx the water table is higher etc. I'm led to believe you do not necessarily need these massive 2+ feet deep frost line (no concern here) trenches you do in Michigan or somewhere.

 

It's usually a good idea to call these folks before digging

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Yes I have called 811 and they cleared

 

You can get away with no net slope for the perforated pipe portion of the French drain. Slight inaccuracies leaving dips with water a half inch deep in a 3 inch pipe or water 3/4 inch deep in a 4 inch pipe won't hurt the performance significatnly.

The lead out (usually unperforated) pipe needs a downslope all the way, suggest 1/4 inch in a foot of horizontal run with essentially no dips.

The perforated pipe should be positioned with holes along the bottom. Usually there will be a second row of holes diametrically opposite and therefore uppermost.

Clogging of filter cloth with dirt is retarded if some gravel is put in the trench first followed by the filter cloth and then more gravel and finally the drain pipe.

A sump pump is mandatory if there is any possibility that the lead out pipe could back up and fill up and cause any section of the perforated pipe to become completely full of water.

 

have watched a million youtubes to prepare

You can't believe everything you see on the internet. There's a whole lotta stuff people post vids and talk about that they really don't have any idea, they just copy what they've been told without thinking about it, it works somewhat, so the method or myth is just carried on. Unfortunately for me, what I'm going to say is on the internet as well, so it's hard for anyone to evaluate even if I'm correct or not.

how to correctly slope the drain.
Does EVERY inch have to be sloped and how would this be enforced?
explain it to me like I'm 5

First, understand a french drain can have ZERO slope and still perform it's function. House plumbing pipes full of doogies and wads of paper, yes then there must be slope and a specific one at that to function. But not a small-scale storm water drain. Realize that water seeks level. So as long as there is some outlet somewhere at the end of a pipe that is lower, water will drain towards that lower area. If you don't believe me, take a straw, fill it with water and hold your finger over both ends. Level the straw perfectly over the sink. Remove fingers, and the water will drain out. Gravity works wonders. This is the concept of basement footing drain tiles, where the pipes are installed pretty much level. The water will drain to the discharge pipe which is lower going to the sump pit.

The second thing to understand (though there isn't a question about it) is that a french drain is just an underground gutter. All you need is some material that allows more movement of water than the adjacent soil. So you could fill a trench with gravel, and water will move through it better than the soil. If you then want to move more water, and quicker, then instead of gravel you can leave a void by installing a pipe into that trench, so now that huge void has lots of capacity. If you then want to make that huge void move water even quicker, with a bonus of carrying any debris that might have entered (like a house plumbing pipe), then you can put a slope on that pipe. Then, if you want to allow a more efficient collection of water, then carrying it away, put the pipe within gravel.

So how much slope and how do you measure? A simple level, whether box, string, or laser. For a small scale yard drain, each segment does not have to be exactly the same. Most home yards will have a set HIGH elevation height at the beginning of the pipe, and a set LOW elevation height at the end of a pipe where it discharges. You really just have to connect the dots, as the factor of how deep a trench needs to be will play in how much slope you want, because a homeowner doesn't want to dig a 3' deep trench that is 50' long.

I could end up with standing underground water causing mosquito issues.

This is the next thing for the most optimum performance. Use solid pipe, not corrugated, for both perforated collection sections and solid transport sections. The benefits or solid pipe are not only in the flow of water, tight joints with standard PVC fittings, more resistance to crushing, self-elimination of debris, and elimination of standing water, but it is also easier to install and establish a slope. The only advantage that corrugated has is that you can make horizontal turns and it is cheap.

If your main concern is installation of slope, all you do is set each end of a solid 10' long piece, and then just backfill the trench.

I also see some YT's that claim you dont need geotextile at all

See above about YT. If you have gravel or perforated pipe, use geotextile. Non-woven needlepunched is the correct material.

i am not sure about squeezing fabric into a 6" trench.

It takes up no room. Set it in the trench folded out over the top sides, throw your gravel in and the fabric will hug the sides.

As I said, if the project gets installed and does nothing it wont turn out to be the end of the world.

If you go to the trouble and spend the money to do something, do it right, once.

I'm led to believe you do not necessarily need these massive 2+ feet deep frost line (no concern here) trenches you do in Michigan or somewhere.

A french drain is not about getting it below the frost line. It's purpose is collecting ground water below grade, so you need to put it below grade. If you only need to control surface water, then you use swales, not french drains.

 

If you daylight your FD properly, you will not end up with standing water for mosquitoes.

 

If it's corrugated pipe, there will always be water sitting in the corrugations. Unless, that is, the corrugations are filled with debris like sand size particles, asphalt shingle granules, etc (it doesn't have to be leaves and twigs). Any debris caught in a pipe will invite further collection of debris at that location (look at sandbar in a river phenomenom).

 

A lot was already covered. I will add:
Non-corrugated gets more important if debris will get in there such as from surface inlets. If there's no inlets, and wrapped in fabric, there's little concern about debris.

The more turns and bends your french drain has, the more installing PVC instead of corrugated will annoy you and cost you.

And the less debris, the less the slope matters. Having said that, if you can slope the trench well most/all the way, then if the pipe or something fails in some way in the future, the trench itself will still have good slope. And there will be less standing water in the trench if that is a concern.

Setting slope with a string line sounds like torture.
The cheap laser level method is pleasant. You level the level, point the dot down your trench line, and then hold a yardstick wherever you want along that line to check the slope depth. Pretend you're Harrison Ford in Raiders of the Lost Ark when he's in that room using that staff on the small replica of the city.

And then as time goes on you might be surprised at how often you use the laser for other projects. Or using it as a fancy laser pointer for cats.

 

A properly constructed french drain will not get any of that junk in it.