Ok I have a cast iron vent stack in my home. I have a leak in a spare bed room where the stack comes up through a wall and has a 90 degree bend to take it from vertical to horizontal. It is leaking at the joint between the bend section and the first straight horizontal piece. I have read all the post on here about cast iron repair and had a plan but wanted to see if you guys had any advice on this. Here is my plan,

1. Tamp down the joint gently with a blunt tool to make sure that the lead is tight against the oakum
2. Use epoxy putty on the leak to form a new water tight seal
3. Use a bell seal clamp or rubber boot clamp on the joint to ensure that it doesn't start leaking again
4. Install a "mushroom cap" on the open vent end that is going through the roof and currently doesn't have any cap on it at all to prevent rain water from entering

Do you guys think this will be a good long term fix for the problem. I don't want to fix the hole that I have made in my ceiling only to have it start leaking again in a few months.

Thanks so much for any advice.

Or you could simply rent or purchase a set of inside and outside caulking irons and recaulk the joint,(see image) Mark gave a detailed answer, along with a drawing, so great I'm going to steal them for your solution.

You could take an inside and outside caulking iron (purchase at home or plumbing supply store for about $20.00 total) and just peen the lead down if you are concerned about leaking in the future.

Check out the picture below to see how to place each caulking iron... be careful not to hit too hard or you could crack the fitting! Thanks Massplumber! Tom

Yea I saw that and would like to but it is in between two floor joists and I can only get to the bottom of the joint.

Suggestion: this has worked many times. After tamping, use a wire brush to thoroughly clean the area. Use "J-B Weld" to cover the lead. It is an automotive grade 2-part epoxy that will adhere to the lead and the cast iron. It's available at all auto parts stores.