Rusted out pipe from where?

Our house was built in the 50s, and either they installed a double sewer pipe out of the house at that time for some reason, or someone installed a second one later.

I'm not sure why there's two and I haven't really cared - until an iron pipe to the smaller one decided to drop down and land on my dryer! It's rusted out on both sides, where it connects up it the ceiling (not in the photo, sorry) and where it connects to the drain going out of the house.

The problem now is that it's dripping from the pipe up at the ceiling, very slowly (one drip every 30 seconds or so). So I'll clean out the rust and replace the missing pipe to get everything back in order, but I'm confused.

Does anyone know why I have 2 drainpipes to the sewer to begin with? I systematically went through the house, using each drain and it seems that all of them drain into the main, larger pipe - using any water fixture in the house has no effect on the dripping.

So I'm guessing that it's a vent pipe that's dripping rainwater, clogged up with rust but I really have no idea. Any thoughts? Thanks!

--Chris

PS As you can see in the photo, there is one water fixture that is connected to the smaller drain pipe, it's the drain from my washing machine that is pumped up and then falls down into this drainpipe.

Could it be that originally, the house had a septic system, and eventually hooked up to the sewer system? Another possibility is that one is connected to the gutters for rainwater or a drain in the driveway? It might lead to the storm drain system rather than the sewer.

Thanks Jim. Anything's possible, if there was a septic they've done a good job concealing where it may have been. The gutters all flow out into the yard so I'm not sure it was that.

I still don't know where the water in the broken pipe is coming from, I guess I need to go up on the roof with a hose to verify if it's the vent pipe.

That would be a vent for the washing machine standpipe.

I suspect that at some time long ago the house was on a septic system and the washing machine being grey water bypassed the septic and went either directly to the leaching fields or, to a dry well. Of course when sewers were installed they tied both lines to the sewer system.

Not at all uncommon I've seen plenty like it.