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I've had that problem with my roses, and sometimes the brown spots are from powdery mildew or rust, not necessarily black spot - it depends on what they look like (if they're powdery, if they have rings, if they're only on the undersides). What does work in any case is what's mentioned: removing the bad leaves, improving air circulation and using fungicide, but before spending money on a strong one, try copper sulfate or baking soda, which sometimes is enough.
It's almost certainly black spot, which shows up when there's humidity and poor air circulation between the leaves - first thing is to prune the affected branches and let more air circulate. If it keeps happening, spray with sulfur fungicide every 10 days, but honestly prevention is what works: water from below, never wet the foliage.
The first thing you need to check is whether the spots have a defined dark border or if they look more like grayish powder, because that'll tell you if it's black spot, powdery mildew, or rust - they need different treatments. In my case I had roses that got covered in little brown spots with that characteristic ring of black spot, and what really worked was combining: removing the infected leaves (without leaving them on the ground), watering only at the base without wetting the foliage, and letting it dry fast between waterings. Fungicide helps but if the plant stays wet all the time anyway it won't do you any good.
I've seen this a ton in gardens and what nobody mentions is that those brown spots can also just be simple water burn if you water at night or early in the morning - water on the leaves under direct sun damages them, and that gets confused easily with a fungal disease. Before you apply fungicides, look closely at whether the spots have well-defined dark edges (then yeah, it's black spot or rust) or if they look more like burns without any specific pattern.
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