How a typical system connects

Tap point
A branch off the main supply, usually in the basement near where the water line enters
Indoor shutoff
A valve on that branch, often with a small drain cap or bleed screw
Backflow assembly
Usually outside, above ground, against the foundation; keeps irrigation water from flowing back into drinking water
Weak season
The first hard freezes in fall and late cold snaps in spring

Why the leak shows up inside

The pipe from the indoor shutoff to the outdoor backflow assembly passes through the foundation wall and the rim joist, one of the coldest spots in a house. If the indoor valve is closed but the pipe beyond it was never drained, water trapped in that short run can freeze and split. Nothing leaks while the valve stays closed, so the crack can go unnoticed all winter.

In spring the sprinkler line gets turned back on, and the split section sprays into the wall cavity or the basement ceiling. In a finished basement that may mean wet drywall, wet insulation in the rim area and soaked carpet before anyone connects the startup with the stain.

The outdoor backflow assembly can also crack or have its internal parts damaged by a freeze. That usually leaks outside, but a cracked fitting close to the wall can send water along the foundation and into a window well or basement corner.

Seasonal checkpoints

Early fall
Schedule the blowout before the first hard freeze; close the indoor valve and open its drain.
After the blowout
Leave the backflow test cocks and valves positioned as the irrigation technician directs, and insulate the assembly if recommended.
Midwinter
Glance at the indoor valve area for drips or frost on the pipe.
Spring startup
Open the indoor valve slowly with someone watching the basement ceiling near the foundation and the backflow outside.
First week of watering
Check for new stains on drywall below the tap point and for soggy soil next to the foundation.

If the startup reveals a leak

  • Close the irrigation shutoff, and the main if the leak is upstream of it
  • Photograph the cracked section and any wet drywall or carpet
  • Feel the insulation at the rim joist; wet insulation needs to come out
  • Check behind finished walls near the tap point for wet framing
  • Ask your insurer whether a freeze in an outdoor-feeding line is treated like other plumbing leaks

Common questions

Do I have to test my backflow assembly every year?
Many water providers require periodic testing of irrigation backflow assemblies, and some HOAs remind owners as well. Ask your water and sanitation district what applies to your address.
Can I blow out the system myself?
Some owners do with a suitable compressor, but too much pressure can damage heads and fittings, and missing the indoor drain defeats the purpose. If you are unsure, an irrigation technician is the safer choice.

General information for Highlands Ranch, not a quote or professional advice. In an emergency, call 911.