Lot grading, water percolation & french drain on the property line along the fence..What is a correct drainage for this property?

Question

I am now trying to understand if the current water drainage on my lot is wrong. The previous owner excavated the soil around the house and he probably spread the soil around the house in the backyard. Also, my neighbor's property is on a higher grounds and I suspect his drainage is not 100% fair. Below you have a representation of the rear side of the house, the left side house is mine.
The purple line is the general slope of the terrain, as mentioned my neighbor's house is on grounds slightly higher than mine.
The blue and brown lines show the backyard general grading. The 8' strip that you see along my patio is not sloped correctly and I intend to fix that.
The patio drains towards the red spot where I intend to install a catch basin and I plan to drain that to a dry well south east of that corner.

Problems

• the yellow line on my neighbor's garage is a gutter that he extended on the soil and that currently drains on his property on the paved walkway that you see there
• by design both properties seem to drain the water toward the property line and that should at least affect the fence. As a matter of fact one of the poles is completely cut (rotten) at the base
• I believe that under the soil (subgrade) the water moves towards my house anyway due to the general slope of the terrain

Questions:

1. Looking at the measurements of the space between the garages do you think that the water that comes out of his gutter (yellow on the figure) will infiltrate the soil and move laterally towards my property and my garage? Just next to the garage the former owner built a small parking lot, paved with precast concrete blocks and I suspect that the base of that parking spot will suck all the water that makes it to it. Am I too worried about this?

2. Looking over the fence at my neighbor's patio one can easily see that when he built that he accentuated the slope and that now all that water drains toward the fence. I think that the only remedy here is to build a french drain along the fence which I should drain in the dry well that is in the plans for the patio. Is this a correct plan? Will this french drain affect the fence in any way?

3. Is the distance between the fence and my house long enough to keep my house safe from that water which probably collects underground on the property line? (not sure where it goes from there).

Answer 1

I'm not sure how much "fair" applies to geology. If you have the downhill lot, gravity is just going to do what gravity does. :-)

It looks like water is definitely draining off of both yours and your neighbor's lots into the depression that the fence sits in. If the soil can absorb all the water quickly enough, there's no issue. Your neighbor's hard surface parking lot, and your hard surface patio, both reduce the amount of soil available for water to absorb into and cause more surface water to flow wherever it flow. Very little water will absorb into the ground under those hard surfaces. (Engineering for this kind of run-off is a serious engineering issue when facilities are built with large parking lots and large roof areas).

If your fence posts are rotting at the base, be sure you replace them with pressure-treated posts. Maybe the original posts were not pressure treated? If not, even modest amounts of moisture would rot them fairly quickly.

The same principles apply to water that seeps into the ground and migrates in your direction. You're further downhill. There's just no getting around that. Once the ground is saturated, the water will pool up on top and rapidly flow downhill. And of course if there is more high ground on your neighbor's side of the fence, gravity is going to tend to move the underground water in your direction. You definitely want good drainage away from the foundation of your house. Otherwise, you'll eventually end up with seeping basement walls, or a crawlspace you could use a canoe in.

Is the planned dry well the only option you have for drainage? Is there a slope in any direction that would let you run a perforated 4" pipe (ABC or PVC) to the street to drain excess into the city storm drains (essentially extending the french drain)? The blue line in your diagram shows water flowing away from the house and garages toward what I presume is the back of your lot, but you don't specify what's back in that direction (an alley? Another lot line? A street with gutters and storm drains? A field?).

Generally speaking, though, if you have those french drains built right, with perforated pipe buried in gravel, you can tie them together with additional underground pipe sloping downhill away from the house. If you bury them in plenty of sand and gravel, that will create a lot of additional absorption for water, and provide extra time for the water to absorb into the surrounding soil.

I'm curious if you are on city sewer, or if you have a septic system and a drain field. If you have room in the back of your lot, and if drainage is really that big an issue with water pooling around your house on a regular basis, you could even potentially build something like a septic system drain field--without the septic tank--to direct more water away from your foundations and give it more underground area to absorb into the soil. But that's kind of the big issue. The ground can only absorb so much water in a given amount of time. That varies widely based on the composition of the soil. Dense clay soils can't absorb as much water. Soil that is already saturated can't absorb more water. So the water is going to run somewhere unless you can spread it out underground and expose it to a bigger area of soil. Even then, if the flow is too high, those french drains will back up and you'll have water pooling up above them.

I worked as a design tech for civil engineers once upon a time, and I remember stories of a city sewer system that was under-designed to the extent that when a "100 year storm" would occur, water pressure would back up and water would flow back up out of manholes, with the manhole covers literally pushed up off the ground riding on top of the water flow out (that's a lot of pressure).

Answer 2

Good drawing. Ideally, a drain running along the fence comprised of a trench with a minimum 1/4-inch fall per foot, lined with an impervious fabric, and finished with no.1 surge stone will work fine to drain the water away from the houses. Its not clear from the drawing if you have to make a turn to follow the purple line to ultimately discharge the water away from the property. The fence posts should be pressure treated to begin with - however, providing a drain should help the fence rather than hurt, and the distance between the houses is insufficient if the ground is relatively impervious, amount of rain, etc. If the ground is spongey to walk on after a rain, then you are justified to worry.

Answer 3

Q1 Hard to tell. A survey with elevations would help. I imagine though that this water would flow down the driveway toward the street.

Q2 The french drain percolates into the soil and will add moisture to the soil. As such it could affect posts that are directly buried rather than sitting in or on concrete. Q3 Underground water is probably okay. It's once the ground gets saturated and you have sheet flow that you'll have an issue.

Answer 4

If you are only looking for answers to your three questions then here you go

Q1- The yellow line showing your neighbours downspout from his roof drain is showing you have a considerate guy beside you. He's sending the water out front in direct contravention of the cities request and is heading for the street not your yard.

Q2- If after it rains you see lots of water sitting along the fence line then I guess the only option might be the French Drain but if not, then get together with your neighbour and put in sauna tubes where each fence post is then go with pressure treated posts on top, or cement in pressure treated post and prevent the rotting. just make sure they are meant to be burried in the ground since pressure treated lumber has all changed.

Q3- This can't be answered properly since from the drawing, all the grading on your side seems to be sloping towards your house and not away as it should be. If however you are correct and it all drains towards the red spot all should be good. It's virtually impossible for anyone to tell you where water goes below the surface, it could go straight down fifty feet or there could be a layer of clay three feet down. Since your main concern seems to be the fence try digging a trench down two feet grading it towards the front, drop in a 6" bog "O" and cover it with 1" or 3/4" clear limestone and it will solve all your issues. Use no dirt in the trench just Limestone. It will act as an exceptional drainage chanel along both of your properties. as long as your property is sloping towards the fence.