I have a house from the 1970s and the joints between the stones are really starting to crumble. I want to do the repairs myself but I'm not sure if I should use lime mortar, cement, or something special. What would you recommend?
For a 1970s facade, it's pretty much certain that your original joints are in lime mortar, so you absolutely need to use lime as well and not pure cement - modern cement is too rigid and will crack the stone! I'd recommend a lime-sand mortar like NHL 3.5, you can find it easily at any DIY store, and honestly it's not complicated to apply yourself with a pointing trowel!
Your original mortar joints are indeed lime-based, so going back to lime is a good idea, but watch out for making the opposite mistake: 100% pure lime mortar can be too soft for some areas. What really works well is a lime-cement mix in the right proportions (like 3 parts lime to 1 part cement) - it gives you the flexibility and breathability of lime without losing the strength you need. The key thing is to avoid pure cement alone, which is too rigid and will crack the old stone over time, but lime alone risks wearing out too fast depending on your façade's exposure.
Honestly, if you're not totally sure about your ratios, you can also find ready-made restoration mortars (they make these product lines specifically for old houses) that already have the right formula in them. Before you get started, test a small inconspicuous spot to see how it behaves with your specific stone.
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