Site slopes: drainage minimums and accessible grades

One-line orientation

Site grading must drain water while keeping routes usable. Drainage minimums depend on the surface and use; accessibility rules separately distinguish a walk at 1:20 or flatter from a ramp with a 1:12 maximum slope.

Key points

• Drainage minimums vary by surface and use. Common planning slopes range from about 0.5% to 5% for sewers, roads, walks, lawns, swales, and paving. Use the value for the surface being designed.
• Walking / landscaping comfort (typical max grades):
  • 2–10% — comfortable for walking and planted/lawn areas.
  • Above ~10% — starts to feel steep; may need erosion-control measures.
  • 20–40% — challenging to walk; erosion concerns depending on soil.
  • 50% and higher — difficult to hold without structural support (walls, terracing).
• Accessible routes — the two break points:
  • ≤ 1:20 (5%): an accessible walk/route. Stays a walk, not a ramp.
  • Steeper than 1:20: counts as a ramp and must meet ramp rules.
  • 1:12 (~8.33%): the maximum running slope a ramp may have.
• Lay out paths along contours so the running slope stays gentle and water is directed off the path — preventing puddles and ice.

Site slope thresholds pair drainage, access, and stability

ORDERED · NOT TO SCALE

Use rise over run to translate ratios and percentages, then match each threshold to its purpose

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Key

• rise ÷ run

1:20 (5%) is the walk-to-ramp dividing line; 1:12 (8.33%) is the ramp's own maximum.

Confusions / comparison

Slope	Value	What it means
Minimum to drain	Depends on surface and use	Check the requirement for the actual condition
Comfortable walking	2–10%	Walks, lawns, planting
Accessible route threshold	1:20 (5%)	At/under = walk; over = ramp
Ramp maximum	1:12 (~8.33%)	Steepest compliant ramp run
Steep / structural	>10%, then 50%+	Erosion control; then retaining/terracing

Related

→ Swales vs berms (this module): the grading elements that move the water · Runoff strategies (this module): steep- vs shallow-site drainage · Code: accessible ramp handrails, landings, and slope limits in detail.