A stoop or porch with a level slope allows water to accumulate and travel in the direction of least resistance. During heavy rain or snow melt, accumulated water can enter the home. If the floor slope of the stoop or porch is toward the house, the problem is exacerbated since the water will tend to flow in the direction of the house. The force of moving water allows it to enter the home more easily if it moves toward the home, rather than away from it.
The stoop or porch floor must slant down and away from the house and preferably follow the grade direction of the soil surrounding the house, assuming the home's foundation is correctly graded. Water that travels across the floor is moving in the direction of the grade and will continue to flow away from the house once it leaves the floor. The correct grade direction is perpendicular to the home's exterior walls and slopes down and away from the house.
Most building code ordinances have adopted a 2 percent slope on above-grade surfaces that include stoops, attached decks, porches and other floors on the outside of the home. A 2 percent grade is 1/4-inch over 1 foot or 1 inch over 4 feet. Some local building departments or code enforcement agencies set different slope amounts due to various local conditions or requirements. Check with your local building department for the slope requirements in your area.
The slope for steps will vary, depending on the number of steps and the distance from the ground to the stoop. Building codes call for a minimum tread length of 10 inches and a riser height of 4 to 7-3/4 inches. The rule of thumb is twice the riser height plus the tread length should fall in the range of 24 to 25 inches. The slope of the steps is what governs user safety rather than water movement.