Both floor drains backs up on a duplex when washer drains and toilet flushes. Had it snaked three days ago from a clean out located in my front yard back to the house and pulled out the smelly black sludge drain worked well until today. What is this black sludge? Is this the cause of my back up? How do I fix this?

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Had it snaked three days ago from a clean out located in my front yard back to the house

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It's a Bad plumbing practice to snake against the flow. You have no control over the snake and it could end up coming out of your toilet. A experienced plumber will always snake with the flow.

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The drain worked well until today.

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You still have a blockage that he missed. Call him back to complete the job,
[QUOTE]What is this black sludge?QUOTE] It's a mix of grease from the soap and matted hair combined with garbage if you have a disposal unit.

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Is this the cause of my back up?

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Yes! This could be part of it. Call him back to get the rest of it out. Good luck, Tom

Clean outs usually only let you snake in one direction and if yours was installed to go from the yard back to the house its been done wrong. You should also have a clean out on your basement floor or main verticle drain stack, is there one? How far from the stack to the street, that's how much snake he should run and usually using a cutter head if you have trees with drip lines near the sewer line