Advice wanted on outside drainage project

[sbornia]

Another home improvement question for our resident experts...

What are some best practices/requirements for proper drainage around the outside a building structure? We have a detached, finished garage that measures 25' x 16'. It's on a slight slope, and I'd like to move rainwater away from it. Some contractors suggested digging a trench along the back 16' side and down one of the 26' sides, laying down a perforated PVC pipe in the trench, and then covering all of that with 3/4" rock. The big difference between estimates was in the depth of the trench -- one guy quoted 8"-12" and another 18"-24". The latter jives better with what I've seen online.

What do you all think? What else should I know about this kind of project?

Thanks!

[thollandpe]

French drain. You'll want to add some filter cloth to your detail.


Image Credit LDHP Cynthia J Steward.

You'll want to pitch the pipe 1/8" per foot. 18-24" sounds about right for the depth.

[sbornia]

Thank you!

[j44ke]

Usually these pipes "daylight" somewhere further downslope - meaning the pipe exits the trench and releases water out on the ground. You want to make sure wherever the pipe "daylights" you are happy with water being there. Erosion can be a problem. Animals can decide to stuff the pipe full of twigs so you need something to prevent that. Your neighbor might not like a marsh in their yard. But they can also present some landscape opportunities for a nice rock wash, a rain garden (plants that require a bit more water or bloom or green up when there has been rain) and so forth.

[rwsaunders]

What the two gents stated above and to take us into the modern world of instructions, here’s French Drain Man (not Lionel) on YouTube...

[Drew]

Regardless of the drain... just make sure the soil grade around the structure goes downhill away from the structure. Water flows downhill. For drain systems... you can do anything from a french drain to elaborate piping systems to just graded soil swails. All will work.

You can have the best drainage system in the world, but if the grade slopes toward the structure between the drain and the structure, you're going to have water intrusion issues.

There are a lot of snake oil salesman for drainage/crawlspace waterproofing/etc... every problem I've seen is actually a grading issue, not a drainage or waterproofing issue. Drainage and waterproofing are band-aids.

Make sure grading is discussed and included.

[rwsaunders]

I should have asked...does the garage roof have gutters and roof conductors or does the roof sheet drain to the ground?

[sbornia]

Thanks guys. The garage roof does have gutters, which may or maybe not be doing an effective job; something I need to ask more about.

For the drain pipe, our intention was to direct it down the driveway, which slopes towards the street. The length of pipe will depend on how much of a costly, invasive project we can afford. So far, the quotes I got were for the pipe to extend about 10' past the garage, at which point the water would need to run another 40+ feet down the sloped driveway.

Based on what you've shared, I have to ask one of the contractors for more info about how he handles grading and some other details.