For wetter climates and underground water, make a ditch drain. After making the ditch, you may want to observe the drain functioning under wet conditions before you fill it in. If so, set up walkways so people and wheelbarrows can get across easily, and put up barrier fences so no one falls in.
Test holes and observing the results after you've dug the ditch will help you see whether you've gone deep enough and whether it flows all the way. (Does the floor stay dry when it's rained a lot?)
Your test holes will also give you an indication as to how the water flows under the surface. Ideally, the bottom of the drain will be lower than the bottom of the foundation. (See illustration page 55.) The deeper the drain, the safer you are, and the more gravel you will need to fill it. If your drainage ditch is quite a way uphill from your home on a steep slope, it's impractical to make the ditch deeper than the foundation. You'll have to use your judgment about how deep to go to catch any water that might otherwise end up at the house. You might get away with simply making it deeper than the foundation in relation to the surface of the ground.
For proper drainage, you will need to slope the ground level away from your home in every direction. To achieve this, dig on the uphill side of your homesite until you've created a slight downhill angle from the house, then dig your drain at the bottom of that slope. You will be doing three things at once: creating drainage, making a place for a walkway or patio, and getting dirt for building. While you're digging, put the dirt somewhere handy for later use. (Hint: Inside the building is a convenient place to mix cob. Leave enough room to maneuver a wheelbarrow.)
The drain can be right under the foundation, or it can be anywhere from 1-10 feet or more away. If you want to leave the drain open while you're cobbing, it'll be more convenient if it's far enough from your house to be out of the way while you are building. Make sure the water will flow from the house to the drain!
It needs to be wide enough to fit a 4 inch perforated pipe and 2 or 3 inches of gravel on either side of the pipe. You will need enough room to get your arms in to lay the pipe. (See tips on page 31.)
You can either do this step before you start the house, or wait until you're sure the drain will function like you want it to, after a series of heavy rains. If your soil has a lot of clay, sliding a shovel along the sides of the ditch will make the clay smooth and slick. This can harden and create a water resistant barrier that blocks the water from entering the drain. Try to avoid this by roughing the sides to allow water in.
Lay 2 inches or more of 1-2 inch diameter round river gravel in the ditch. (Crushed gravel takes a lot more energy, big machines and gas to make, and its flat sides sit closer together, leaving less space for the water to flow in, but if that's all you can get, it'll do.) Next, lay the perforated 4 inch plastic pipe on top of the gravel. If you don't like plastic, you can go the old fashioned way and use cylinders of ceramic tile laid end-to-end. You will be creating an empty space for the water to flow along after it runs in through the perforations or between the tiles.
Start at the high point in the ditch and run the pipe or tiles at a slight slant to where you want the water to go. Here's a trick to make sure the pipe or tiles will drain the way you want them to. (You'll need at least a 1/4 inch drop for every 10 foot length.) Starting at the high point, lay a long, reasonably straight 2x4 on top of the pipe or tiles. Place a level on top of it. When the air bubble in the level is off center to the uphill side of the ditch, you know the water will flow. Hold the pipe or tile against the 2x4 and fill in underneath the pipe or tile with more gravel. Then move along the pipe or tiles until you get to the outlet. This ensures that you won't have low points where the water will pool instead of happily flowing down its new course.
When you have the pipe or tiles where you want them, fill the ditch almost to the top with round clean gravel. Fill it the rest of the way with 3 inches of straw and/or a 1/8 inch stack of newspapers to help filter out the dirt particles that might clog the spaces between the pieces of gravel, or block the holes in the pipe. (See illustration on page 55.)
Pathways or garden walls can be built right on top of the drains or cover the drain with topsoil and plants. Do not cover it with a clay soil because the water will have a hard time getting through it and into the drain.