Measure the slope as rise divided by run, aim for a grade of 2-8%, and build on convex ground that sheds water, not on concave ground that collects it. A drop of 1.2 m over 20 m is a 6% grade; anything steeper than 15% is too steep to build on safely, and flat valley floors flood, hold cold air and drain poorly.
Measuring slope and observing where water goes on a candidate site, so that buildings stay dry and fields do not erode.
Water Runs Downhill and Tells You Where Not to Build
Grade is the vertical drop divided by the horizontal distance, as a percentage. The carrier article’s targets: 2-8% grade gives natural drainage without erosion problems. Avoid flat valley floors (flooding, cold air pooling, poor drainage) and steep slopes over 15% (erosion, difficult building, landslide risk). Slope shape matters as much as steepness. Convex slopes (bulging outward, like the top of a hill) shed water; concave slopes (hollows) collect it. The carrier rule: build on convex, farm on concave.
Aspect matters too. In the Northern Hemisphere a south-facing slope receives the most winter sun; in the Southern Hemisphere it is the north-facing one. North-facing slopes in the Northern Hemisphere are cold with low winter light.
What You Need
- An A-frame level or a water level — see A-Frame Level, Water Level, and Grade Calculation.
- Two stakes and a tape or marked rope.
- Rain, or a bucket of water to pour.
Method: Measure, Then Watch
Step 1: Measure the grade Place two stakes a measured distance apart along the fall line, the direction water would run. Use the level to find the height difference. Grade % = height difference / distance x 100.
Step 2: Convert if needed 8% is an angle of about 4.6 degrees; 15% is about 8.5 degrees. Grade is easier to use in the field.
Step 3: Find the shape Walk the contour. Does the slope bulge outward (convex) or curve in (concave)? Hollows and swales collect water and cold air; spurs and shoulders shed them.
Step 4: Watch rain In or after rain, see where water flows, pools, cuts channels or soaks away. Dig a small hole, fill it with water and note how long it takes to drain.
Worked Example
Two stakes 20 m apart are set along the fall line. The level shows the downhill stake is 1.2 m lower.
- Grade = 1.2 / 20 = 0.06 = 6%.
- Angle = arctan(0.06), about 3.4 degrees.
- 6% is inside the 2-8% target. At the other end, a 3 m drop over 20 m is 15%, the carrier’s limit for steepness.
Common Mistakes
- Measuring slope once — slope varies; measure at several places.
- Building in a hollow because it is sheltered — it is also where water and cold air gather. See Cold Air Drainage & Frost Pockets.
- Ignoring drainage after building — the carrier names water damage from poor drainage as the leading cause of building failure in primitive construction.
Safety
Steep slopes slide
Cut slopes and steep ground can fail without warning after heavy rain. Do not dig into a slope or build at the foot of one with signs of movement: leaning trees, cracks across the slope, or fresh scars.
What’s Next
- Learn next: Drainage — draining a field once it is chosen
- Combine with: Watershed Position & Flood Risk — water arriving from far upstream