Notes from Windward: #62
French Drain for the North Wall
The north side retaining wall
There is no substitute for good insulation, and nothing insulates like a couple of feet of dry earth, but the operative word there is "dry."
The dining hall is three times as long as it's wide, and the east/west orientation of that long, south-facing wall lets in lots of light. Even better, the south facing roof is designed for mounting a long solar array to provide most of the hall's energy needs, and all of that's great - except for the correspondingly long north wall and the need to make sure that it doesn't get water logged.
The backbone of the north wall is six feet of concrete retaining wall. That will allow us to back fill the foundation to a depth of five feet, more than enough earth to insure that the otherwise cold, north wall will never see the real cold of winter.
We've taken a series of steps to insure that water doesn't work its way through the retaining wall and into the dining hall, the first being the bedding of a thick PVC membrane in asphalt over the joint between the horizontal foundation and the vertical retaining wall. The material we wound up using was actually sold as "pond liner;" we figured that if it was good enough to keep water in, it should be good enough to keep water out.
The joint membrane rose one foot up the concrete wall, and was then supplemented by three feet of overlapping roofing felt. The two working together should form an effective barrier to the horizontal movement of ground water.
The perforated drain pipe
The dining hall is a sixty foot wide barrier to the natural downhill movement of ground water, so without a drain to gather and divert that water, a substantial hydraulic pressure would build up and create a perpetual wet condition inside the dining hall. Underground construction is good, so long as you respect the power of ground water to go down grade.
That should be enough to keep the water from getting into the concrete, so long as water isn't allowed to accumulate and build up hydrostatic pressure. To prevent that we've installed a three inch perforated pipe all along the bottom of the wall to provide a passage sufficient to drain water away from the wall.
That's the "drain" part of a "French drain;" no idea why this sort of subterranian drain is thought to be especially French.
Gina and Sarah backfill with gravel
The next challenge was to see that the clay in our soil didn't clog up the pores in the drain pipe. That was accomplished by encasing the drain pipe in a gravel bed to a depth of six inches, and shielding the side of the drain with treated wood to insure that the clay didn't intrude from the side.
With that in place, the next step was to cover the gravel bed with a fiberglass mesh fabric that's designed to let the water through but to hold back the clay.
That takes care of ground water, but the next step will be to install gutters to catch rain water dripping off the roof and divert it around the side of the dining hall and down towards the gardens. At some point in the near future, we'll want to install another septic tank to use as a rain catchment. The longer we go without rain, the stronger the urge to create as much water storage as we can.
The French drain installed and ready for cover
With both ground water and rain run off dealt with, we can look forward to the dining hall being warm and dry no matter how cold and wet the winter might be.
And as the snows settle in, and we brew a pot of winter tea with the water steaming in the kettle on the woodstove, we'll remember the summer's work and kick back content in the knowledge that the work was done right, and was done to last.
What we're going to do
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