So the kitchen sink has a very slow drain. The kitchen branch drain is about 8 feet long and connects to a larger vertical drain that also drains the master bathroom upstairs. Everything upstairs drains fine to the street. A plumber snaked the kitchen sink branch from the trap adapter under the sink and it cleared it for about a day. I put a garden hose down the vent pipe on the roof that only serves the kitchen branch. The master has it's own vent pipe. When I turned on the hose, the kitchen sink backed up. This leads me to believe the clog is in the kitchen branch, between the vent pipe and the larger vertical drain pipe. Any ideas?

 

Maybe time for a new plumber.

So the kitchen drain is an 8' horizontal pipe that connects into the vertical stack.
If the kitchen has it's own vent..... it would be above the sink. Anything below the sink is wet.
The problem is between the kitchen sink and the vertical stack.

More than likely it's in the piece right behind the sink/wall that goes down and connects to that 8' run.

 

Kitchen drain pipes typically will eventually get completely gunked up with grease. An 1 1/2" pipe can often be so plugged you can hardly fit a finger in it... not that you would want to stick you finger in it... yuck!

If the pipe is accessible, replace it all the way from the larger vertical drain to the sink. If not, it needs to be snaked again and again and again. Remove the trap under the sink and do it from there.

 

I would also recommend a regimen of an enzyme type cleaner like ZEP Drain Defense. Follow the directions and you might find it clears up the problem. Regular use may keep it from happening again.

Snakes usually just punch a hole/tunnel in the crud, which clogs again rapidly. They need to use a cutter or whatever the actual term is that scrapes the gunk off the walls of the pipe.

 

I have run into this problem more than a few times. It most often caused by grease being put down the drain. The grease solidifies in the horizontal pipe forming a clog. Snaking passes through the goopy grease but the grease oozes around the snake and mostly seals behind it. If this is what's happening grease will be visible on the snake when the plumber removes it.

In the worst cases replacing the pipe is the best solution. Other than that it takes a lot of repeated snaking, running water down the line and more snaking. Each pass with the snake only removes a small bit of grease to it can be quite time consuming. You can also have the plumber repeatedly run his snake/auger through the pipe. Then install a balloon and try to blow the loose grease chunks out.

Running a lot of hot water down the drain can melt the grease in some cases but that grease will eventually cool and solidify somewhere else so it can just move the clog from one area to another.

If there was a hard obstruction in the pipe like a foreign object or rust buildup the plumber should have been able to feel it with the auger/snake. Galvanized steel pipes are the worst, especially when horizontal. They rust on the inside. Sometimes the rusting builds up reducing the inside diameter of the pipe. Sometimes pieces break off and can form clogs.

 

Do you know how old the house is, or what kind of pipe is behind your kitchen sink?
I too have taken out old galvanized pipes with literally a pencil-sized hole through the grease and crud. Normal snakes can poke a hole in the grease, but don't really clean it out.

You either need a better plumber or one with a larger-headed snake.

 

There is nothing better for greasy buildup in pipes than a good enzyme cleaner after flow has been established. The best I ever found is called Plumb Clean. If the buildup is especially bad be prepared to snake the drain the day after the first treatment because the buildup will often collapse from the interior of the pipe and stop the flow again. Treat 3 or 4 nights in a row and the pipe will be totally cleaned up. I use the Plumb Clean monthly and haven't had another problem in almost 30 years.

 

The house is 30 years old and the piping is ABS. I snaked the horizontal branch from the sink to the vertical stack several times yesterday, but it is still clogged (takes about an hour to drain a cup of water). There was also no grease on the snake. I am considering putting in a new branch behind the sink that drops under the house and ties in to the main sewer pipe. It is a corner sink base with plenty of room behind it. Would it be possible to vent the new pipe with the existing vent? I would have to connect my new drain pipe to the existing drain pipe at the wall.

 

Also, the clogged pipe is behind kitchen cabinets and runs through a 2x6 wall. I can access the pipe by removing some siding and plywood on the other side of the wall. My concern is if I cut into the pipe will I be able to get 2 new couplings and a piece of ABS installed. I can't imaging the pipe will have much play being that it is through the studs.

 

It would be difficult/impossible to fix your existing drain line with couplings if you decide to cut it open and look inside. There just isn't enough room to move the pipe and get the couplings in place. Instead use no hub couplings (rubber boot with a metal shield around the outside). The rubber couplings can be difficult to slide on the pipe but if you coat the pipe with dish soap they slide much easier.

 

get this:
https://www.amazon.com/dp/B073ZFC59R...v_ov_lig_dp_it

 

Before I removed siding from the other side, I'd exhaust all other possibilities. Since grease is the likely culprit, I would start by pouring a large pan of boiling water down the sink drain. If that didn't totally clear the drain, I'd go with the enzyme drain cleaner next.