Should I fix the problem right away, or wait until the case is resolved?

If the defect is actively causing ongoing damage to your property, addressing the repair now, rather than waiting for the litigation to fully resolve, is generally the right call, since delaying necessary repairs while a lawsuit proceeds typically allows the underlying damage to worsen and become more expensive to eventually fix. Before beginning any repair work, however, photograph the defect and its surrounding area thoroughly, from multiple angles and at multiple stages, and retain every invoice and receipt connected to the repair.

If it's genuinely feasible given your timeline, arranging for an independent expert to inspect the defect in its original, undisturbed condition before any repair work covers or alters the evidence is valuable, since firsthand expert observation of the original defect is considerably more persuasive evidence than photographs alone once litigation is underway.