A subfloor that stays damp is worth investigating, because the causes are usually fixable and the consequences of leaving it are not.
Plumbing causes come first because they run continuously. A slow leak on a supply line, a leaking waste, or a failed seal under a shower or bath will keep a subfloor wet regardless of weather. The distinguishing feature is that the dampness does not vary with rain.
Drainage causes vary with rain instead. Stormwater discharging too close to the footings, downpipes not connected properly, or ground levels directing surface water toward the house rather than away all produce damp that appears after rain and dries slowly.
Ventilation is the third factor. Subfloors rely on airflow through vents, and where those are blocked by garden beds, paving or stored items, ordinary ground moisture cannot escape.
Establishing which of the three applies is what determines the fix, and the timing pattern is usually the strongest clue.
If water is entering the house faster than it can drain, turn off the mains water, keep clear of any electrical fittings that are wet, and call for help. Do not try to clear a surcharging sewer or stormwater pit yourself, because the water can carry contamination and the pit may be deeper than it appears.
