The USGS calculates, (https://water.usgs.gov/edu/activity-howmuchrain.html
), that 2 inches of rain on a 60 x 40 ft. roof area will produce over 2,900 gallons of water, and all that water either falls directly along your homes foundation, or if you have rain gutters without
storm drains, directly onto the corners.
Southern California is currently experiencing record drought conditions, yet we still get 7-9 inches of rain. That's a whopping 13,500 gallons of water causing havoc on your foundation.
That amount of water landing directly next to your foundation will cause what is known as soil consolidation
, which can cause substantial settlement.
If you home is constructed on a 'cut and fill', or hillside lot, the potential for erosion and downhill creep is greatly increased.