The Standard for Slopes in Porches

Porches and Drainage

• Porch structures are attached to the house, and are usually covered with a roof. Moisture is an issue with porches, because their construction involves multiple points where water can penetrate without evaporating readily. The foundation needs to be kept dry, and the location of the porch next to or above it creates the potential for water entry into the basement. Porches are usually built with concrete, a porous material. Exterior concrete structures need to be pitched slightly to shed water.

Slope for On-Grade Porches

• Building regulations prescribe surface drainage measures adjacent to the foundation that affect on-grade porches. The ground has to slope away from the structure for a specified distance, usually 5 to 10 feet. For soft surfaces, the specified grade is generally 5 percent -- 6 inch drop over 10 feet. Exceptions are made for hard surfaces, reducing the percentage to 2 percent. The percentage can be reduced because hard surfaces are more stable, so water runoff isn't impeded.

Slope for Above-Grade Porches

• Above-grade porches also have to slope, even when they're covered, to shed water away from the house walls and avoid moisture conflicts with the siding and foundation. Porches should be sloped to the same specification as landings and steps, at 2 percent. Hard exterior surfaces are sloped to avoid icing in cold climates and to shed rain. Pitches steeper than 2 percent are difficult to walk on, and increase the risk of tripping.

Additional Considerations

• The subgrade for an on-grade porch has to contain suitable well-draining material, and be compacted according to local recommendations. Paved structures next to the house have a tendency to slope towards the building over time, but with adequate preparation, a well-constructed porch structure may help keep water flowing away from the foundation. The ground for an above-ground porch needs the same quality of preparation, as the site is difficult to access once the porch is built, if moisture problems arise.