How do we find the source of a leak?

You are going to have to use a flash light when it is raining to chase down the water. It can also be leaking thru the foundation, since your first floor is below ground.

First trace the water inside. Where is it coming from? It's ending up on the steps but is coming from the basement ceiling? Or the wall? Is the wall shared with the garage or is it an outside wall? If it's an outside wall it maybe what we had happen, when there is a deluge the water doesn't drain away from the house as fast as a normal rain. If there is no berm, or raised edge around the house where the dirt abuts the house the water just pours down between the foundation and the soil. We bought topsoil and mulch to create a berm around the whole house and solved our leaks. Also make sure your down spouts drain far from your home.

If the wall is shared with the garage there are a couple of things to look for. First is there water in the garage on the floor? Can you see stains coming down the wall in the garage? If that's a no then it's into the attic or what ever other way you can get up to see under your roof. Water can travel a long distance from the actual leak. I worked in a school library we had a leak that was 15' out from any wall. I climbed up a ladder and lifted the drop ceiling to look around. 10 or more feet from the wet spot was an area where an old roof was attached to a "newer" roof. The water was dripping from that seam, down an slanted beam, down the metal drop ceiling supports to a weld that caused the water to cascade off onto the ceiling tiles.

It's very possible it's a foundation leak right behind the steps like Dee suggests The way I trace roof leaks is use a garden hose turned on full with no nozzle. I hose down small areas of the roof and wait for it to leak. Most of the time it was a flashing that looked sealed but failed.

YOU don't havta wait till it's raining again. It works better if it's NOT raining so you will see what's going on. GET a hosepipe & zip-tie it to the gutter system. Turn the water on. Let it fill up the gutters. If it only happens when it's a heavy rain, the gutters will be a good starting point since the roof rains generally go into the gutters. There must be a point where the roof has separated from the house just enough to let the water in. I would start with the gutters, inside & out, behind them, under them, etc & see if there's a gap somewhere. Once you've found your leak, I would go & fill ALL the cracks behind the gutters with some clear caulk. Also, around the gutter nails too. Hope this helps.