Make a drawing of the structure. Go online to your local building department's website to find out the building code for constructing porches. You can also make a personal visit to the build inspections department. Speak with the building code inspector to find out the proper depth for the posts, the number of posts required and the recommended method for fastening the portico to the existing structure. Secure the necessary permit to build the structure before staring the project.
Remove the siding from the work area. Begin working from the top move down to the header located over the entrance door. The header consists of two 2-inch by 6-inch boards attached together and located behind the sheathing above the door frame. Check to determine where to attach the joist hangers for the portico.
Check your local codes. A 8-foot-wide portico usually requires one column at each end to support the front of the structure. Cut the sheathing away from the location with a circular saw to expose the area for attaching two joist hangers (for the portico's perimeter) to the studs. Measure up 8feet from the ground with a tape measure; mark this spot on the 2-by-4-inch studs. Secure the joist hangers to the studs.
Dig the holes for the posts a minimum of 18 inches deep. Local codes may require you to dig deeper, however. Prepare the bed of the holes with sand or gravel. Use the end of a post to compress the materials to ensure a solid bed.
Pour quick-set concrete. Put the post brackets in place before the concrete sets. Allow the concrete to dry completely. Secure the posts to the brackets with 1/2 -inch-diameter lag bolts. Attach 2-by-4-inch bracing to the posts to keep them in place. Cut each post to the proper height.
Secure the 2-by=6-inch boards for the side perimeter to the joist hangers with 16-penny nails. Make the top edge of each board level with top posts. Connect the boards to the posts with 1/2-inch-diameter lag bolts. Install the front perimeter board to the posts.
Place the 2-by-6-inch rafters 16 inches on center and parallel to the house; use 16-penny nails. Attach 1/2-inch sheathing to the rafters to form the roof. Apply the roof covering material. Use caulk to seal all joints between the house and the portico. Cover the underside of the roof with 1/2-inch sheathing. Reinstall the siding on the house.