Internal French Drain

internal french drain
internal french drain in a home

Internal French Drain

An Internal French Drain is used to capture water from getting into crawl space.

Water in the crawl space: what fun! Instead of sinking your hard-earned money into something showy and sexy–something that you will actually see–such as kitchen counters or flooring, you get to sink it into water mitigation.

The procedure I am about to describe is very close to me, as I just paid several thousand dollars to have this done. For reasons I will outline later, I chose to mitigate this problem of water under the house by hiring a company rather than doing it myself. But that doesn’t mean that you cannot do it yourself.

Main Problem: Standing Water in Crawlspace

I live in a very wet area, Seattle, Washington. So, under the house is a permanently wet area. After a hard rain, the wet area increases in diameter to about 10-12 feet. The ground, instead of merely being muddy and spongy, begins to pool up with water, about 1/2″ deep.

During the dry months of June to October, the pool of water mainly dries up or percolates away–but it never completely goes away. Does any of this sound familiar to your situation?

Related Problem: Mold

With excessive water in the crawl space, you have the related problem of mold. Mold loves to grow in places that have: a.) Moisture; b.) Lack of light. Both conditions perfectly describe a crawlspace.

Mold will grow on the piers, joists, ducts, and other hard surfaces. But that’s not all of the problem. Mold also grows on the “soft” materials, such as fiberglass insulation and on the plastic vapor barrier. When fiberglass insulation develops mold, it’s impossible to clean off. Rip it out and replace it.

Process of Ridding Crawl Space of Water

There are many ways to get rid of the water. Before you even attempt what I am about to describe–water mitigation within the crawlspace perimeter–you’ll want to first take care of all of those common basement waterproofing tactics.

The process described here basically sets up a perimeter within the crawlspace and seeks to capture the water trying to infiltrate the perimeter. When water hits this border, it is diverted into gravel-filled channels and fed by gravity to a sump pump. The sump pump then siphons up the water and purges it from the crawlspace. Let’s look at it in closer detail:

  1. Protect: Unless you can access your crawl space from the exterior, you will need to bring in materials through the house and down an access door. You will need to protect all walk areas with plastic. Or, to prevent slipping, you can use builder’s or rosin paper.
  2. Remove Vapor Barrier: Your crawl space may already have a plastic vapor barrier. Remove this and discard.
  3. Dig Trench: Dig trench around entire interior foundation, using foundation as a guide. Trench should be between 8″ and 24″ from the foundation. If not, it may undermine the foundation.
  4. Lay Pipe: Lay down 3″ socked flex perforated pipe through entire perimeter trench.
  5. Cover with Gravel: Cover with 1 1/2″ drain rock.
  6. Spread Soil: What to do with all of that dug-up soil? Spread it evenly around crawl space.
  7. Install GFCI: Install a GFCI outlet in crawlspace.
  8. Install Sump Pump: At the low end of the perimeter trench, install a sump pump as recommended by manufacturer’s directions.
  9. Discharge to Exterior: Sump pump to discharge perimeter trench water to outside.
  10. Install Vapor Barrier: Install 6 mil vapor barrier to prevent vapor transfer to crawlspace.