A roof protects a house and porch from snow and rain, as well as the heat of the sun. Adding a covered porch adds exterior space to a house, and providing a roof for the porch moderates the temperature and conditions on the porch. Adding a roof can create drainage problems if the roof pitch is too shallow.
Minimum Roof Pitch
The minimum pitch for a roof is 1/4:12, which translates to 1/4 inch rise to 12 inches of run. However, you can only use this pitch with built-up roofing or specialized synthetic roofing. Covered porches that are near trees or in areas with heavy rains should not use a low-slope roof to avoid trapping debris and water on the roof surface.
Match Pitch of Existing Roof
The covered porch roof pitch should match the existing house. Buildings with multiple roof pitches do not appear cohesive. The porch should appear to be part of the structure, not an addition pasted on. If it is not possible to match the pitch of the existing roof, use a pitch that is shallower than the roof; do not use a steeper pitch, because the form of the covered porch will overpower or contrast with the main roof.
Match Roof Material and Use Appropriate Slope
Match the roofing on the covered porch with the main roof. Determine the minimum slope of the roof based on the roofing material. The minimum pitch for shingles is 1:6, which is equivalent to 4 inches rise to 24 inches run. The minimum pitch for a standing seam metal roof is 1:4, or 3 inches rise to 12 inches run. The minimum pitch for roll roofing is 2:12, or 2 inches rise to 12 inches run.
The function of the roof in weather and the environment determines the minimum pitch for a roof. For example, a shingle roof in a snowy region could be 2:12, but it is better to have a steeper pitch, such as 12:12 or even steeper, to shed the snow. Furthermore, a roof in a forest should have a slope of at least 2:12 to shed the fallen leaves and debris of the forest.