This section is from the book "The Carpenters' Guide", by Harvey Miller. Also available from Amazon: The carpenters' guide.
Line and nail a 2x4 on the joists or sheething the thickness of the porch flooring lower than the door sill, the length of the porch, less the thickness of the headers. Cut the headers the length of the width of the porch, nail and square to the 2x4 nailed on the wall joists. Measure the length of the joists between the headers, next to the Avail, and place them the desired distance apart. (16 to 24".) Make the frame 3" shorter than the floor boards. Slope the frame to the front about 1" to every 6' for drain. Lay the floor two inches over the side of the frame. Line and cut the front the same as the sides to extend over the frame for facier board and cove. Locate the posts, deduct the depth of the beam from the height of the ceiling for the length of the posts, adding the drop of the porch floor.
Build the beam for the support of the porch roof that rests on top of the posts of 2" plank or boards. Then place the joists for the ceiling 2' or more apart the same way the floor joists run. (Fig. 48.)
The ceiling usually runs the same as the floor. Cut the rafters the desired pitch and style of roof, allowing for facier. Sheath and shingle same as main roof. Locate the height of the top rail; keep the bottom rail 2 1/2" from the floor and the side rails level, not following the slope of floor. Take the space between the rails, for the length of spindle, and the distance between posts for the length of rail, making allowance for fitting. Ceil the ceiling of the porch and put a cove in the angles of the ceiling and under the outer edge of the floor, a crown mould around the facier, and a bed mould in the angle of plancier and frieze.