Any time, but the wet spring is both the busiest season and when leaks are easiest to diagnose because the source is actively showing. Interior work runs year-round indoors; exterior excavation is weather-dependent and best in drier stretches. Booking ahead of the spring rush helps.
More detail
Basement waterproofing can be done in any season in Cincinnati, but there are practical timing considerations. Interior drainage, sump work, crack injection, and encapsulation all happen indoors and are essentially weather-independent, so they proceed year-round; the only weather sensitivity is that the slab pour-back on an interior system needs above-freezing cure conditions, which a hard cold snap can briefly delay. Exterior excavation is genuinely weather-dependent: you cannot leave an open excavation along the foundation through a multi-day Cincinnati soaker, and frozen ground complicates digging, so exterior work is best scheduled in drier stretches. The seasonal pattern worth planning around is demand. The wet spring (roughly March through May) is when most Cincinnati basements leak, which makes it both the busiest time for waterproofing crews and, usefully, the easiest time to diagnose a leak because the basement is actively showing where and how water enters; a contractor seeing the basement wet during a spring rain gets better diagnostic information than one seeing it dry in August. The tradeoff is that the spring rush tightens scheduling, so if you already know you have a water problem, booking the assessment in late winter or early spring (or even getting ahead of it in fall) secures a better slot than calling during the first big April storm along with everyone else. For a basement that is actively flooding, of course, the best time is now, and we prioritize active-flooding emergencies within capacity regardless of season. For planned, non-emergency waterproofing, scheduling around the spring peak rather than into it is the practical advice.