What are you seeing?

Find where it starts. A trail from one crack, a pipe or a window is a single leak. A wet band along the bottom of a wall, on more than one side of the basement, is groundwater coming up at the floor seam.

Note how long after the rain it appears. Within the hour suggests roof water or a window well. A day later suggests the water level in the soil has risen.

The powder is salt left behind when water moves through masonry and dries on the surface. It brushes off and comes back. It tells you the wall is passing moisture even when nothing drips.

Low on the wall it usually tracks the wet soil outside. Higher up, check for a downspout or a flower bed on the other side.

That smell is mold or mildew growing on something damp: cardboard, carpet, the back of panelling, stored clothes. In summer it is often condensation, when warm humid air meets a cool wall or floor.

If the smell is strongest over a crawl space or in a room above one, the source may be under the floor instead.

It may simply be moving a lot of groundwater. It may also be pumping the same water in a circle: the pipe ends beside the wall, or the valve on the pipe has failed and water falls back into the pit.

Listen for how often it starts and watch where the water comes out outdoors. A pump with no battery backup that runs this hard is one power cut away from a wet floor.

Clear water rising in the drain during a storm can mean the foundation drain is tied into the sewer and the sewer is full. Dirty water or a sewage smell is a sewer backup.

A backup is not waterproofing work. Report it to your sewer utility, then find out whether the house has a palmer valve in that drain.

A window well with no drain, or a drain packed with leaves, fills up in a downpour and spills over the sill. The stain under the window is usually a clean vertical streak.

Cleaning the well, adding a cover and fixing the drain often ends it without touching the wall.

Look through the hatch after rain. Puddles on bare soil mean runoff or groundwater is getting in, or a pipe is leaking. Water that is there in dry weather points to plumbing.

A liner laid over standing water hides it. The water has to be given a way out first.

Cold floors and insulation hanging down mean the space below is damp and open to outside air. A floor that feels soft or dips may have joists that have stayed wet for years.

The damp is waterproofing work. Replacing or propping framing is carpentry or foundation work, so say what you feel underfoot when you call.

Soil that has settled along the foundation makes a trough that holds water against the wall. Downspouts that end at the corner fill it.

This is the least expensive kind of problem to have, and worth fixing before anything is done indoors.