Cause: just like concrete, mortar will do two things, get hard, and crack.
However, when it's all along the windows like that, you do need to watch it and be carefull.
In your first picture, there is white something next to where you circled. Is that your angle iron above the window? If so, that is supposed to be covered by the mortar all the way to the window. If it sticks out too far (as some masons will say) then it needs to be notched in the area past the window. Basically the typical angle is 3 1/2 x 3 1/2" and sometimes the stone or brick only goes about 3" deep. This is moreso the case with stone because you don't have a cavity typically. In any case, If that steel is exposed, then water for sure is getting caught there and going into the mortar/stone and causing it to crack. This could very well cause any and everything below it to have issues as well.
Picture two. I believe you are showing us the depth of the stone here. What I notice is that you have dryvit/stucco up against it. Be careful and watch that as well because I have seen it a million times where the mortar/brick/stone absorbs water, and the dryvit doesn't have a proper barrier behind it and the stone will then "wick" into the foam of the dryvit and cause moisture to come in. Best solution to this is to have a proper barrier behind the dryvit and also when they install the dryvit to coat the sides of that foam, and/or the brick to keep the moisture from transferring.
Picture three. I don't see any cracks, but certainly be sure to have GOOD caulk around all the windows.
Picture four. Also picture 5 that you posted. There seems to be no caulk between the window and the stone sill. It is hard to see if there is much pitch on the sill either. A sill is supposed to have adequate pitch to let the water run away from the house. Even so, it's supposed to be caulked to the window so that a driving windy rain still can't make it's way in.
If the original installers did things properly and had not only a good vapor barrier but also some sort of waterproof membrane (for up here with cultured stone they make us put two layers of roofing paper for code) and then also rubber flashing over all steels and under all stone sills. This also requires a weep rope or vent of some sort. Hard to see from the photos if you have that or not, but look under the sill and on top of the metal sill for sure. Without the weep or vent any water that DOES get in there, can't get out and just sits and either rusts our the steel, or freezes and cracks the stone. If they didn't do this, then there is a good chance there is no flashing either.
The ONLY way to repair cracked mortar is to take a grinder and cut it out and tuck point it back in. You then have to try and match the mortar as best you can. You can go to Menards or wherever and try one of their just add water mixes, but I'll bet with it being a bit older and with that much stone they used either a Brixment and sand mix, or a portland/lime/sand mix where you add everything separately. In which case one batch is a full wheelbarrow. We call this a "single" of mud.
If it were my house, I would look for the weeps first. I would maybe even pop off the stone sills (careful! they look pretty thin there man! Limestone sills are cheap very good replacements if needed) and check everything out and make sure there was flashing etc. in there. You can also get a mastic, which is a type of tar to seal up the flashing while you are in there. Oh, and make end dam's to keep any water that gets under the window/sill from wicking into adjacent stone and mortar.
I haven't seen that type of mortar striking before, it's very unique! A cool chiseled look to it. I would try and keep that if you can and try repairing what is needed. Taking off a sill isn't too bad as long as you don't crack it. Laying just that in a slightly different color mortar isn't so bad and won't be noticed by anyone but you and maybe a mason. Only other option is to take ALL the stone down and re lay it all. That's expensive and time consuming and messy. I don't know about over there, but we charge between $200-$250 a day for side jobs, and some of the better guys won't even come out for less than $300 a day. That's per man, and they will want to bring out like 2 bricklayers and a laborer. If you DO hire it out, do the demolition yourself and make everything as clean as possible. Get the mortar off the stone etc. so they just come in and lay it up.
FYI, I have been in masonry for 16 years, and am the 3rd generation. I am the head laborer at our company, Colvin Masonry. 75% of my job is usually doing the stuff that is broke, hard to work on, or just a royal PITA.
Please hit me up with any other questions! I could go on and on, but without seeing it in person sometimes this is the best you can do. Pictures are great though, really made it clear!
Lee Colvin AKA Speedling
