The picture shows the stump. We ground up the main stump, but there are parts of it and the root under a block wall/fence dividing my property and my neighbor's property. We suspect that this is pushing the block wall and causing it to lean. (I show a men's size 8 shoe in picture for scale).
I'm able to dig under the root/stump a bit.
What is the best way to remove this and any other remaining roots under the wall.
update
Added note that the wall is actually a block wall fence separating my property and my neighbors property. We suspect the root is causing the wall to lean towards his property. We hope to correct the lean once the stump is gone.
Best Answer
Easiest? Saltpeter. If you drill holes with a spade bit, fill them with a liquid stump remover, then wait for a week or so it should convert the wood to a punky black consistency that's much easier to dig out. If the chemical you used is saltpeter you will have converted the wood to an oxidizer. Should the stump somehow become saturated with kerosene that was left in the holes for a day or two and set alight it would burn even underground, destroying all of the roots.
Your best bet is probably a reciprocating saw. It won't take a week and a half or attract the fire marshal. It's also safer, in my opinion, than using a chainsaw in very tight quarters.
There are wheels that would allow you to grind the rest with an angle grinder. I'm not too sold on them from a safety perspective, so it'd be a last resort for me personally.
On the professional side of the house, makers of large oscillating saws for masons offer general purpose blades that cut wood. (ArborTech's AS170, for example) Unfortunately it's probably not a common enough tool to be available at most rental places. I'd call and ask, but not get my hopes up.