I need to find the pipe on my roof that I can run water through with a garden hose to try and clear a clog in my pipes to the kitchen. There are two sets of pipes on the roof and each set has two pipes sticking up, one being shortter than the other.

The larger diameter is usually the main drain. Look at the proximity of the pipe to the main drain.

Running water down the pipe can backfire into a flood.

It's best to snake, but be VERY CAREFUL not to lose the snake.

Windows are a good point of reference.