How to Design a Residential Irrigation System
Good irrigation design matches water output to plant need without exceeding your water supply. Three numbers drive the math: flow demand in gallons per minute (GPM), available supply GPM, and coverage radius of each head. Undersized zones lose pressure, overshoot zones waste water, and uncovered spots turn brown. A properly sized system uses 30 to 50 percent less water than overhead hose watering while delivering healthier plants.
Sprinkler Coverage Formula
Sprinkler heads spray a circular pattern. Heads overlap in a triangular grid (head-to-head coverage), so each head effectively covers 0.866 times its spacing squared:
Spacing should equal the head's coverage radius. A 15 ft radius rotor at 15 ft head-to-head spacing covers about 195 sq ft per head.
Zones Calculation
If total system flow exceeds available supply, you must split into zones. Each zone activates one at a time. Stay below 80 percent of available supply to preserve working pressure:
Head Types Quick Reference
| Head Type | Radius | Flow | Precipitation | Best Use |
|---|---|---|---|---|
| Fixed Spray | 5-15 ft | 1-4 GPM | 1.5 in/hr | Small lawns, strips |
| Rotating Nozzle | 8-18 ft | 0.4-1.5 GPM | 0.4 in/hr | Water-efficient spray |
| Gear Rotor | 15-30 ft | 1.5-6 GPM | 0.5 in/hr | Medium-large lawns |
| Impact Rotor | 20-70 ft | 3-20 GPM | 0.3 in/hr | Large lawns, fields |
| Drip Emitter | 1-2 ft (point) | 0.5-2 GPH | N/A - direct | Trees, shrubs, veg beds |
| Soaker Hose | ~6 in | ~0.5 GPH/ft | N/A - direct | Row crops, borders |
Finding Your Water Supply GPM
Before you can size anything, you need to know what your supply delivers:
- Open a main garden hose fully (no nozzle, no restrictions)
- Time how many seconds it takes to fill a 5-gallon bucket
- GPM = (5 / seconds) x 60
Typical residential GPM by meter size:
Runtime per Zone
Most lawns need 1 to 1.5 inches of water per week, split across 2 or 3 watering days. Apply the precipitation rate of your chosen head type to calculate minutes per zone:
- Fixed spray (1.5 in/hr): 20-30 min per zone x 2 days per week
- Gear rotor (0.5 in/hr): 30-45 min per zone x 2 days per week
- Rotating nozzle (0.4 in/hr): 35-50 min per zone x 2 days per week
- Water early morning (4 to 8 AM) to minimize evaporation and fungal disease
Zone Planning Rules
Group heads into zones by similar water need and sun exposure:
- Never mix head types in a zone - a rotor and a spray on the same zone cause the spray area to flood while the rotor area underwaters
- Separate sun and shade areas - shade zones need 30-50 percent less water
- Separate slopes from flat areas - slopes need shorter, more frequent cycles to absorb water
- Isolate drip zones - drip operates at lower pressure than sprinklers
- Stay below 80% of supply GPM per zone to maintain pressure
Winterization (Cold Climates)
- Before first freeze: shut off water at the main, drain backflow preventer, blow out zones with compressed air
- Air pressure: 50 PSI maximum, lower volume (not a regular air compressor - use a rental blowout)
- Spring startup: slowly pressurize, inspect each head for freeze damage, replace broken heads before first use