Since the stone and brick are dissimilar materials I would expect the mortar at the top of the intel to be raked back and sealant installed. Flashing behind the lintel should direct water from the wall cavity to weep holes or tubes at that location. I'm assuming this is a brick cavity wall rather than a solid double-wythe brick wall. You need to clarify that issue since it would change the detailing of the lintel; we can't see whats behind the face brick or guess how old it is.

It can't hurt to treat the broken lintel as if it were still good for now. Obviously sealing the crack would help prevent water from getting behind the top of the window.

The location of the crack directly below the end of a gutter is suspicious. It's possible water has been overflowing from the gutter down the face of the wall and getting behind the lintel and freezing, assuming, of course, you are in a northern climate.

Cross-posted in the Old House.

Thanks, JDS. It is a brick cavity wall, built in the 1920s, and we live in Indiana so it gets fairly cold.

I guess there's a chance the gutter caused the problem, but in evaluating the entire house it seems more like a settling issue to me. This corner has several stairstep cracks, the floors inside are a bit uneven, the ground outside isn't properly graded, etc. So I think the downspouts were probably not properly installed and that, combined with the water pooling on the ground, created settling issues for the entire corner and also for the lintel. Does any of that make any sense? And do you share the concern mentioned by others that caulking the crack will only trap water?

I didn't notice the gutter right away, but the water staining underneath it is the evidence the lintel is being affected by extra water. Try to have that addressed also. It is probable that water is not being caught by the gutter in that corner or... the slope is wrong.

That's a really smart observation about the water staining -- thanks, Vith! It's actually raining pretty hard right now so I looked at the gutter. I don't see any issues with it draining or tilting or clogging. But I'll keep an eye on it. Maybe there's a problem with the flashing or something like that?

It's x-posted in Home Repair. I suggested the gutter as a suspect on Saturday :)

The gutter issues may have already been addressed, but the damage was done. Or, the gutter only leaks in the winter when the downspouts are frozen and backed up, etc. That's also when freezing damage to masonry occurs.

I recall a temporary "weatherstripping caulk" that can be removed. This may be your best bet if you want to do something before the repairs are carried out in the spring.

http://www.homedepot.com/p/DAP-Seal-N-Peel-10-1-oz-Removable-Weatherstrip-Caulk-18351/100140056

I'm not sure if I'm doing something wrong with the cross posting, but the second sentence in both this post and the post in home repair mentions the cross posting -- I wasn't trying to hide anything or confuse anyone.

Anyway, my initial comments that the problems had more to do with settling in the foundation were based on a bricklayer who looked at the house -- that was the only reason I resisted the gutter explanation. But I guess it could be a number of things, like you said, Sombreuil. Thanks for the tip on the removable caulk. Unless anyone wants to talk me out of it I'm going to slap some of that in the vertical crack and along the horizontal gap -- a thick line I can shape so the water will run off -- and revisit this in the spring.

Just a courtesy to let people know so they are aware, that's all, just etiquette.