One weak zone while the others perform normally means the problem is in something only that zone has: its valve, its lateral line, its head count, or the elevation it covers. The supply, mainline and controller are all demonstrably fine, because the neighbouring zones prove it. The three most common causes are a lateral line leak, a valve not opening fully, and a zone carrying more heads than the available flow can serve.
The candidates
| Cause | Telltale |
|---|---|
| Lateral line leak | High flow on that zone, poor performance at every head, sometimes water surfacing while it runs |
| Broken or missing head on the zone | One open pipe consuming the zone’s flow while the rest of the heads go weak |
| Valve not opening fully | Everything on the zone runs, weakly and evenly. Often a flow control stem closed down |
| Too many heads for the flow available | Has always been weak, or got weak when heads were added for a patio or bed |
| Elevation change within the zone | High heads weak, low heads fine. Consistent, and not a fault |
| Mixed head types on one zone | Rotors and spray heads together — they need different pressures and run times, so one group always looks wrong |
| Clogged nozzles and screens | Several heads weak, usually shortly after a repair introduced debris |
| Pipe crushed or constricted | Reduced flow with no visible leak. Often root pressure or vehicle compaction |
Always weak, or newly weak?
This question separates a fault from a design limitation, and it changes the whole conversation.
A zone that has been weak since you moved in, or since the system was installed, is probably not broken. It is likely carrying more heads than the supply can serve at working pressure, mixing head types that need different pressures, or covering too much elevation change. Those are zoning and design issues, and the remedies are splitting the zone, correcting nozzle selection, fitting pressure-regulated head bodies, or grouping heads by elevation. No part is failing.
A zone that worked well and got weak has something wrong with it. That is where leaks, restricted valves, debris and crushed pipe live, and it is worth diagnosing promptly — a lateral leak is losing water every cycle.
There is a third case: a zone that got weak when heads were added. That is a design limit reached by accretion, and it is common on properties where a patio, a pool surround or a new bed prompted a few extra heads on whatever zone was nearest.
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How it gets measured
- Run the zone and walk it. Note which heads are weak and where they sit along the line. A break shows as a group of poor heads beyond a specific point.
- Compare flow against a healthy zone. Watching the meter while each zone runs is a crude but effective flow comparison. A leaking zone typically draws noticeably more.
- Measure operating pressure at a head. A pressure gauge fitted in place of a nozzle gives a real number to compare against what the heads require, rather than an impression.
- Check the valve’s flow control. Stems get closed down during repairs and never reopened. Free, and worth checking before anything else.
- Pull a nozzle and inspect the screen. Grit in the screens points at debris in the lateral rather than a pressure fault.
- Count the heads and compare with the other zones. A zone with fifteen heads next to zones with eight is over-subscribed, and that is the answer.
The elevation case, which is not a fault
Every foot of rise costs pressure. On sloped North Georgia lots — the Sugar Hill and Dawsonville end of our coverage especially, and anywhere falling toward Lake Lanier or a creek drainage — a single zone can span enough elevation change that the top heads are working well below the pressure the bottom heads see.
The homeowner sees a dry strip at the top of the slope and a soggy strip at the bottom, and increasing run times makes the bottom worse without helping the top. Nothing is broken.
The corrections are structural: group heads by elevation so each zone operates within a narrow pressure band, use pressure-regulated head bodies to even out what reaches the nozzles, select nozzles to match actual operating pressure rather than nominal, and use shorter cycles with soak periods so the low ground can absorb rather than shed. See low sprinkler pressure.
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Low Sprinkler Pressure
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Read more →Where we work
Irrigation Medics repairs irrigation systems across Hall County and the Lake Lanier communities, the Gwinnett corridor through Duluth, Suwanee and Buford, and North Fulton and Forsyth — Johns Creek, Alpharetta, Milton, Roswell and Cumming. Same rates throughout. See the service area.