opening the plumbing cleanout Clogged Drain Diagnosis Chart
Drain Repairs for Building Owners/Managers

Table of causes & cures for blocked or slow plumbing drains shows how to fix a blocked or clogged drain: here we provide a slow or blocked plumbing drain diagnostic table for a quick and easy guide to diagnosing & fixing plumbing drain clogs and backups.

This article series explains how to troubleshoot & repair slow or blocked plumbing drains, diagnose & fix sewer line or septic system backups & how to distinguish between a probable septic system failure versus a probable blocked building drain or sewer line. Here we include a plumbing drain & septic or soakaway bed drainfield diagnostic table.

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Plumbing Drain Clog & Wet Soils Septic System Troubleshooting Chart

Watch out: disassembling old plumbing parts often means things fall apart. Don't try in-home plumbing repairs when stores selling repair parts are closed unless you have spare parts, tools, sealants, etc. on hand.

Watch out: Never lean over, smoke close to, nor ever enter a septic tank - the risk is rapid death. Also see SEPTIC & CESSPOOL SAFETY.

Drain Problem Check in This Order Clogged Drain Repair Action


Toilet, tub, sink, shower drain slow or backs up

Clogged fixture trap - only the individual fixture is having a problem Disassemble trap, remove clog; try a plumbing snake or kinetic water ram. Avoid heavy use of chemical drain cleaners.
Clogged building drain piping - multiple fixtures may be slow or backed up, but unless clog is at main drain, other fixtures will be working ok. Check for an outdoor cleanout access to see if water is standing in the pipe outside the building. If no water is found there, the blockage is inside the building. Try a plumbing snake; if no success you need a professional plumber.
Clogged drain piping between building and septic tank - standing water in the pipe between building and septic tank; may also be a problem in the septic tank, D-box, or drainfield piping.

Often caused by roots in the drain line or a collapsed or broken or partly collapsed drain line. Frequent power-snake operation can break old drain piping; trees too close to the building or to piping, especially with older pipe designs are often invaded by tree roots.

To determine where to try reaming the drain line, inspect the septic tank wastewater level. In a two-chamber septic tank if the outlet chamber is above the outlet pipe, the problem is probably downstream from the septic tank.

If the waste level is at the outlet, or if you have a single-chamber septic tank, measure the scum and sludge levels.

MEASURE SCUM & SLUDGE. If these levels are above the limit, pump the septic tank immediately.

If the septic tank inlet is clear, the pipe between building and septic tank is probably blocked and needs to be cleared. You need a professional plumber.

See DRAIN vs SEPTIC for more help in distinguishing these blockages.

Clogged drain piping between septic tank and D-box or drainfield - no backup in pipe between building and septic, backup visible inside the septic tank. As we described above, In a two-chamber septic tank if the outlet chamber is above the outlet pipe, the problem is probably downstream from the septic tank.
Broken drain line outside the building - rare but shallow pipes in cold climates can become blocked, freeze, and burst. You need a professional plumber. Snaking the line may discover the location of the collapsed or broken piping. You need a professional plumber.
Septic Dosing Systems - diagnosis, repair

Septic Dosing Tank Not Draining

Check for clogged piping leaving the septic tank, at the D-box, or in the drainfield.

Check for an inoperative siphon or pump.

Check the siphon vent pipe to see if wastewater is flowing into it. If not see the two causes listed just above in this table.

If water is entering the siphon vent pipe see topic C. below


Dosing siphon not working
Common problem, usually a cracked or broken siphon bell, siphon pipes, or a clogged pipeline exiting the tank

See "B" above. If wastewater is flowing into the vent pipe, pump the dosing tank & inspect/repair the siphon. A broken bell can block the siphon.

If the bell is undamaged, remove the bell and snake the siphon for blockage.

If the siphon is not damaged or blocked, see the final two diagnoses in "A" above.

If a blockage is found you need a professional plumber familiar with dosing system equipment.

If wastewater flows out of the dosing tank, pump the tank & inspect pipes & siphon components for damage you need a professional plumber to do this.


D: Dual dosing siphons not working Alternating or dual siphons or tipping pans, caused by cracked or broken bell, pipes, or pipeline

Dual siphons occasionally may stop alternating as they should. The septic system may continue to appear to function but you may be overloading one portion of the drainfield. If siphons only rarely fail to alternate, it's not a significant concern .

If one of the siphons is not working at all, see "C" above.

E: Septic Effluent Breakout Diagnosis & Repair - Wet soils in the drainfield area, puddles, sewage odors

E.1. Wet soils along or near sewer piping


Look for a broken or cracked waste pipe in this area Excavate gently, find the break, repair or replaced the damaged section. Diagnose the cause of the breakage: driving over the piping, frost damage to a clogged line, roots, buried rocks, poor original installation. Look for tree root invasion causing a break - you may need to remove nearby trees or shrubs.
E.2. Wet soils close to the septic tank or dosing tank
Broken or cracked pipe, cracked or damaged septic tank, or sewage backing up inside the septic tank. Determine the cause as in "A" above - probably the problem is downstream from the septic tank. Start by finding and inspecting the Distribution box. If it's not flooded, the problem may be in the piping between the septic tank and the D-box. Otherwise the fields and field piping may be damaged or flooded. This may be an indicator of a septic field at end of life, improper installation, local groundwater flooding, etc.
E.3. Wet soils over or close to the drainfield or soakaway bed

Failed septic drainfield, temporarily overloaded drainfield, flooding by groundwater from other sources

At mound systems remember to inspect carefully around the lower edges of the mound, especially at the lowest end of the leach bed lines.

Inspect nearby ponds, streams, culverts, ditches.

Failure to pump septic tank increases the risk of a field failure;

Inspect for external sources of water around the drainfield and re-route, install a curtain drain, or take similar measures if this is the cause.

A septic dye test can help distinguish between septic effluent failure and a drainfield flooding due to local groundwater or surface runoff problems.

Note: In addition to drawing on the USDA document cited below, that source cites the National Park Service, U.S. Department of Interior as a cooperator who provided advice and suggestions for this content.

Edited & expanded from original source: Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture (USDA), 8271 1302, 7100 Engineering, 2300 Recreation, September 1982

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.


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CLOGGED DRAIN DIAGNOSTIC CHART at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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