Merging French Drain with Solid Drain Questions
Merging French Drain with Solid Drain Questions
(OP)
I’ve a few questions about French Drains I can not find answers to anywhere… yet. Hoping you can help.
I am running underground solid and French drains around nearly all four sides of my house. All four sides will get a solid drain for downspouts and some surface drains, but part of the front of the house, one full side and part of the back will have a French drain – the area where subsurface water accumulates due to being downhill from my neighbors. The French Drain will be 4” perforated pipe, holes down, in 12 inch wide gravel trench, appropriately deep. The pipes will then run out my front yard, about 50 feet ling, to the street. Assume I have the slope correct to gravity flow everything. My questions are:
1. Do I have to drain each pipe separately, or at some point, can I merge my perforated pipe into my solid pipe?
2. If I merge my pipes, do I have to worry about water building up in the trench after the merge? Where is the best place for the Merge?
3. Where both solid and French share the trench, do I run these pipes side-by-side or can I put the Solid on top of the Perforated pipe? Does it matter?
Thanks in advance for any info
I am running underground solid and French drains around nearly all four sides of my house. All four sides will get a solid drain for downspouts and some surface drains, but part of the front of the house, one full side and part of the back will have a French drain – the area where subsurface water accumulates due to being downhill from my neighbors. The French Drain will be 4” perforated pipe, holes down, in 12 inch wide gravel trench, appropriately deep. The pipes will then run out my front yard, about 50 feet ling, to the street. Assume I have the slope correct to gravity flow everything. My questions are:
1. Do I have to drain each pipe separately, or at some point, can I merge my perforated pipe into my solid pipe?
2. If I merge my pipes, do I have to worry about water building up in the trench after the merge? Where is the best place for the Merge?
3. Where both solid and French share the trench, do I run these pipes side-by-side or can I put the Solid on top of the Perforated pipe? Does it matter?
Thanks in advance for any info





RE: Merging French Drain with Solid Drain Questions
Next consider the time that there is a significant rainfall and the downspouts are running full with some pressure head on them. Connecting st any place to the perforated pipes will then be an outlet for those roof drains, in part. Then you will have water coming in backwards to your "French Drains", feeding your basement walls with water. I suspect you won't like the result.
This then tells of the requirement for never connecting the two together no matter where in the system.
In a firm where I worked, we had a case this type of thing was done to dispose of roof water, supposedly saving two systems, a decision of an architectural firm, not us. Unfortunately this was a place where high plasticity clay underlaid the building and the roof water followed backfill to utility trenches into the building and boy did those floors heave up. Very difficult job to correct.
RE: Merging French Drain with Solid Drain Questions
RE: Merging French Drain with Solid Drain Questions
I would consider not running the perf pipe all the way to the road. Once you get past the area near the house you are trying to drain/dewater you could consider changing to a solid pipe, with a minimum 2% slope. This will be less expensive, will increase flow out of the pipe and will make it easier to connect to downstream storm sewer (if necessary). But as stated above, you do not want to combine this pipe with the other solid pipe, but you could put in the same trench.
RE: Merging French Drain with Solid Drain Questions
Mike McCann
MMC Engineering
RE: Merging French Drain with Solid Drain Questions