Toilet plunger clearing a sink drain (C) Daniel Friedman Diagnose Clogged Drain vs Septic Backup or Failure
Blocked Drain Diagnosis for Building Owners/Managers

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Blocked drain or drain backup diagnosis: caused by drain blockage versus caused by septic system failure: here we explain how to determine if a blocked or slow drain is due to a problem inside the building or outside in a sewer line or failing septic system.

When a building drain or main sewer line is clogged or slow, or when there is a septic system backup, it's important to determine where the problem lies, since the repair steps can be quite different and costs can vary widely.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

DRAIN vs SEPTIC - Is the slow drain problem due to indoor plumbing or the septic system?

Plumbing drains (C) Carson Dunlop Associates

Is it a Plumbing Problem or a Septic System Problem - A First Look

Sewage odors, wet areas on the property, slow fixture drains, gurgling drain noises, or plumbing fixtures which "back up" or overflow back into the building may be the first signs of trouble at a property. Image courtesy Carson Dunlop Associates.

[Click to enlarge any image]

If there are sewage odors or soggy sewage-smelling wet areas at a property, the on-site waste disposal system is likely to be at fault. Even so, without further investigation we don't know yet if the problem is a simple repair such as a broken pipe underground, or a costly failure such as a saturated absorption field.

If building drains are slow or clogged, the problem could be the in-building plumbing drain-waste-vent (DWV) system or there may be a problem with the septic system. Without further investigation we don't know.

Also see these clues (and articles about them) that assist in diagnosing clogged drains, drain backups, and septic or sewage odors:

Leaky clamp patch drain line (C) Daniel Friedman

A simple initial step must be taken to distinguish between an in-building plumbing problem and an (outside) on-site waste disposal system problem.

Simply put, if a single building fixture is sluggish or clogged, but if other building fixtures drain properly, you should suspect a local clog or vent problem at the individual fixture.

If all building drains are slow or clogged, or if waste is backing up into the building from the lowest plumbing fixture, you would suspect the sewe line is blocked or there is a failing or blocked onsite waste disposal system (septic system).

Our client (left) is pointing to a leaky clamp patch on a building drain line. Odd and excessive slope, a mix of materials, and this patch were evidence of amateur workmanship that presaged problems with this drain system.

Curtain Drain - Design Sketch for Protecting Drainfields from Wet Soils

Curtain drain schematic (C) Daniel Friedman

Inspect the septic system absorption field: if the drain field (synonyms: leach field, leaching bed, soakbed, absorption bed, seepage bed, seepage trench) is wet or smelly we suspect a septic absorption field failure.

If the absorption fields are properly installed there is adequate clearance, typically 4.5 ft. between the bottom of the field and the top of the seasonal (spring) high water table on the property.

Otherwise in wet weather conditions your field is flooded, cannot readily accept effluent from the tank, and worse, you're also contaminating the local groundwater with pathogens from the septic tank.

If your absorption field is on a slope and is subject to high levels of surface or subsurface runoff, you may need to install an intercept drain or curtain drain (sketch above, source US EPA) up-slope from the absorption field.


Keep the intercept drain or curtain drain at least 10 meters from the absorption field perimeter. If the field is flooded and you do not have problems with surface runoff, subsurface runoff, or high water table, before you assume that the field is at the end of its life, check for constantly running plumbing fixtures such as running toilets or a water softener which is stuck in its "backwash" cycle.

If exploration of the on-site waste disposal system piping from house to tank, tank to distribution box, and distribution box to drainfield indicates that the failure is in the drainfield (or absorption system), a more extensive system repair is needed and significant costs are likely to be incurred.

Also see CAMPING & EMERGENCY TOILETS and also see TOILET ALTERNATIVES for a discussion of camping toilets, chemical toilets, emergency-use toilets, waterless toilets, graywater systems, composting toilets, home health care toilets, incinerating toilets, outhouses, and latrines.

Also see SEWER GAS ODORS and Sewer Line Replacement diagnosing a clogged drain leads to drain line replacement - step by step photo-illustrated guide to drain replacement.


Continue reading at CLOGGED DRAIN DIAGNOSTIC CHART or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see CLOGGED DRAIN vs SEPTIC BLOCKAGE FAQs questions & answers posted originaly at this article

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