A substantial share of Washington properties outside the incorporated cities are on septic rather than municipal sewer, particularly in rural Pierce, Kitsap, Skagit, Whatcom, Thurston and Yakima areas.
How the system works
Wastewater flows into a tank where solids settle and separate. The liquid flows out to a drainfield — a network of perforated pipes in gravel — where it disperses into the soil and is treated by natural processes.
The tank needs periodic pumping to remove accumulated solids. The drainfield needs to remain able to absorb liquid, which is where most serious failures occur.
What kills a drainfield
A failed drainfield is the expensive failure, and the causes are largely avoidable.
Overloading. More water than the system was sized for, whether from added fixtures, more occupants, or a leaking toilet running continuously.
Neglected pumping. Solids carried into the drainfield clog the soil and it stops absorbing. This is the most common preventable cause.
Compaction. Driving or parking on the drainfield compresses the soil. Do not do it.
Roots. Trees planted near the drainfield seek the moisture and intrude.
Surface water. Drainage directing roof or surface water onto the drainfield saturates soil that needs capacity to absorb wastewater. In Western Washington’s rainfall this matters, and in high water table areas like the Skagit valley it matters considerably more.
What not to put in
The tank relies on biological processes, and things that disrupt them cause problems.
Wipes — including those labelled flushable — do not break down and are a leading cause of blockages. Fats and grease. Excessive chemicals and bleach. Anything not intended for a drain.
Garbage disposals increase solids substantially and are worth avoiding on septic, or at least using minimally.
Signs of trouble
Slow drains throughout the house. Gurgling. Sewage smell inside or outside. Wet or unusually lush patches over the drainfield. Standing water above the tank or field. Backups.
Wet ground over a drainfield during dry weather is a clear signal. During Washington’s wet season it is harder to interpret, which is another reason to notice what normal looks like on your property.
The renovation connection
This catches people out and is the most useful thing to know before planning work.
Adding a bathroom, adding fixtures, or increasing the number of bedrooms can exceed what the existing system was sized and approved for. That involves the health department alongside the building department, and it can affect whether a project is feasible at all.
If you are on septic and planning a remodel, an addition or an ADU, raise it at the design stage. Discovering a capacity constraint after plans are complete is expensive; discovering it after work has started is worse.
Buying a property on septic
Have the system inspected as part of the purchase, separately from the general home inspection. It is a substantial piece of infrastructure whose condition is not visible, and drainfield replacement is a significant cost.
Ask for pumping and maintenance records, and establish the system’s age, capacity and approved bedroom count.
Routine care
Pump on a schedule appropriate to your tank size and household — a septic professional can advise for your specific situation. Keep records. Know where the tank and drainfield actually are. Keep vehicles off the field, direct surface water away from it, and be mindful about what goes down the drain.