How do I treat this?

[Manalto]

The trim above the masonry damage has been patched, so that's a clue as to what's causing the problem, isn't it? I've seen this horizontal trim (I used to call it a skirt board, but that term refers to an element of stair construction) with prominent bullnose drip molding at the top. It helps deflect water off the foundation wall and, to my eye, is an attractive design element.

[Gothichome]

One of the smart folks may correct me but I think the corner boards should fit ove the foundation wall this would not allow water to get between the foundation and the sill plate and behind the paint. I’m afraid to mention it Corcetiere but you may have an issue beyond the rotting cement.

[GibsonGM]

You're not wrong, Gothic. I see this as a two-possibility repair, and it's hard to know how far to go without seeing the INNER surface of the foundation there, and being able to note how strong the cement is. Without evidence of imminent failure or other danger, I stick to "Level 1", personally. Unless you have 12 to 20 grand to burn.

"Level 1": what has been suggested...stabilize the 'rot', fill with parge, stucco, re-paint. Yes, get some flashing up behind that 'skirt board' (I call it that too) to keep water out! Check the sill there. Repair skirt as best you can, repaint, done. I assumed that part to be self-evident...if a sill repair is needed, that could make this a bigger job of course, but still not too bad, IMHO, esp. if you can get in that corner from inside. The durn paint is what holds the water in the cement - it shouldn't be painted at all, but that's what people like to do...I'd think this is oh, from $50 to $300 if you have the skills to do it., depends how extensive the sill may have been impacted

Can you get a pic of the inside of that corner?

"Level 2": Consult an engineer and/or qualified builder to suspend that corner of the house...remove some number of feet in either direction back to solid cement and re-pour the foundation corner, with reinforcement. Remove cribbing and repair the things in the home that have shifted and cracked from moving it. Can also be done with block on a new footing depending on which way is faster/easier.

Even if that corner WERE 'smoked', many people would do something inside ('lally column'...) rather than bring in the excavator. I wouldn't get worried without more data....

[Mick_VT]

One of the smart folks may correct me but I think the corner boards should fit ove the foundation wall this would not allow water to get between the foundation and the sill plate and behind the paint. I’m afraid to mention it Corcetiere but you may have an issue beyond the rotting cement.

I see a channel in the join of those boards above leading to the area of issue - my bet is that water has been running down there and soaking in. On the good news front, if I am correct then the damage should be localized

[phil]

what ya need there is a board stretcher looks like someone cut a chunk out.

[phil]

I picked up a bit of free Hardiboard recently. at first I thought it's not very original looking for my house but then got to thinking that maybe for a place like that near the ground maybe it would be OK at least it won't rot. in that case built out a little with a spacer between and a drip edge on top, then the water would stay away from the wall itself and there would be a little a little air space so the mortar could breathe. Not sure how that might fit with the rest of the house or if it would hide any original detail. around my house I sometimes have issues if the gutters plug or something then I get water free-falling and splashing up around the perimeter.

[Corsetière]

Yeah the whole porch needs rebuilt honestly. There are several rot issues due I think to no one repairing a gutter for several years before me. And the more thorough repair will get done eventually, but I am currently just focusing on dealing with cleaning things up as best as possible and making sure all paint is stabilized before I schedule the lead risk assessment. The name of the game is "Good Enough" right now. lol!

[Gothichome]

Corcetiere, well enough is good enough for what your trying to do, get a pass on your lead issue.

[Mick_VT]

at least caulk the crap put of that gap to cut off the water supply