Determine the Slope of the Eaves
Determine the slope of the eaves on the long porch roof to confirm the natural direction for the gutters to route the water. Use a small line level attached to a string to create a "level" - as long as the length of the main porch - in this case 36 feet. Have a helper stand on a ladder at one end of the porch and hold the string taut as you balance the bubble in the level. The slope is about 4 inches on a length of 36 feet, a significant drop that will facilitate channeling the rainwater into a downspout installed at the far left side of the short porch roof. Another downspout will accommodate the side of the porch.

Determine Placement of Downspouts
Use the line level to determine the slope of the eaves on the short roof. The side that intersects the long roof is nearly level, so we snap two chalk lines: one to mark level elevation and one to mark a 1/4-inch drop to the end of the roof. The 1/4-inch drop denotes the mounting line, because all gutters must slope downward at least 1/4- inch for every 10 linear feet in order to drain properly. You will need to consider edge of the porch roof to determine placements of downspouts and types of corner joints.

Measure Straight Sections of Gutter
Take measurements for the straight sections of gutter. The run on the short roof will need an "interior" elbow fitting (90-degree turn) in the corner where it meets the long roof and an "exterior" elbow on its opposite end where it turns and parallels the front of the house. (In this project, we will cap the other exterior corners.) Hold the elbows in place and mark their ends on the eaves at the mounting line.

Subtract 1/4-inch for the Seams
Subtract a 1/4-inch gap from each end for the seams when we join the straight gutter to the elbows later with a "seamer" strip. In our project, the required length for the straight gutter on this dimension of the roof is 67 inches.

Measure and Mark 67-inchs on Gutter Section
Measure and mark 67 inches on the gutter section, after supporting it temporarily on saw horses. The homeowners have chosen aluminum gutters - these seem to be most popular, although they come in copper, plastic, and wood. The straight pieces usually come in 10-foot sections that you must measure, cut to length, and then join with special connectors.

Carry Measurement all the Way Around
Use a combination square to carry the measurement all the way around the gutter, marking it clearly with a pencil.

Cut Along the LIne
Cut along the line with heavy-duty tin-snips. Start at the top edge of the gutter and cut down the first vertical face, stopping at the flat bottom. Make a similar cut downward on the opposite face, again stopping at the bottom.

Crease and Cut Bottom of Gutter
Crease the bottom of the gutter, bending it back on itself along the line marked earlier, and then cut at the crease. The edges will be razor sharp, so be careful. Repeat steps 1 through 8 on each separate face of the porch roof to ensure the proper slope, measurements, cuts, and joint type for each.

Mark Hanging Bracket Positions
Mark positions along the back edge of the gutter for hanging brackets to mount the gutters to the edge of the eaves. The manufacturer recommends placing brackets about 6 inches from each end and at 18 to 24 inch intervals between those points.

Snap Brackets into Place
Snap the brackets into place.

Mount the Long Gutter
Mount the long gutter by driving the screws on the hanging brackets through the fascia board on the eaves. You and a helper will probably need to stand on ladders to hold the gutter along the mounting line while you drive the screws with a power drill.

Prepare to Install the Downspout
Prepare to install the downspout on the correct gutter section by first holding the un-mounted length of gutter in place and transferring the downspouts -  position, tracing around it on the bottom of the gutter.

Position the Square Sleeve
Position the square sleeve that will join the downspout to the gutter in the tracing just made. Use the bottom of the sleeve, itself, as a template to draw on the bottom of the gutter to transfer its exact position.

Breach Each Corner of Sleeve Tracing
Use a metal punch to breach each corner of the sleeve tracing.

Drill Starter Holes
Drill starter holes.

Cut the Opening
Cut the opening with a jigsaw. Insert the saw blade into one of the starter holes and work around the perimeter of the transfer lines. The starter holes will help make neater corners on the cutout.

File Cutout Edges
File the edges of the cutout smooth.

Apply Silicon Sealant
Apply a generous bead of silicon sealant to the underside of the sleeve at its lip.

Insert Sleeve into Cutout
Insert it into the cutout, working from the interior of the gutter, and press it gently into place. Expect some of the sealant to ooze at the edges.

Secure Downspout Sleeve
Secure the downspout sleeve with short sheet metal screws.

Slip and end Cap on
Slip an end cap on the end of this gutter section, after adding some sealant to waterproof the joint. In our project, this section is the last in the run and has a downspout, so we cap it rather than joining it to the next section.

Crimp the Cap Edges
Crimp the edges of the cap with pliers to hold it securely.

Hang the Gutter
Hang the gutter, after marking positions for the hanging brackets in the correct intervals. Drive the screws into the fascia board to secure them.

Install the Angled Downspout Fittings
Install the angled downspout fittings to make the transition from the horizontal gutter and square sleeve to the straight downspout.

Cut Length for Straight Portion of Downspout
Measure and cut the length for the straight portion of the downspout - our measurement is 75 inches, allowing several inches of overlap on each end to couple with the angled fittings. The two ends on a piece of downspout look the same at first glance, but one end tapers so that it will slip into the end of the next section to make an easier joint. Cut off the larger end with the jigsaw to avoid any damage to the tapered end that will fit inside. Also, be sure to install the downspout with the larger ends of the joints upward and the tapered ends downward to prevent water leakage at the joint.

Assemble and Test Downspout
Assemble the downspout and test the fit. In our test, we discover that the downspout hits the concrete porch.

Brace Downspout
Add small blocks cut from pressure-treated two-by-four lumber to brace the downspout against the wooden porch supports and to space it outwards slightly to avoid the concrete floor. Add aluminum mounting-straps on top of the blocks and then screw the blocks and the straps to the porch column at the same time.

Slip Downspout Over Square Sleeve
Slip the downspout assembly over the square sleeve and secure it with screws.

Secure Mounting Straps
Fold the aluminum mounting straps around the downspout and secure the lap with sheet metal screws driven into the downspout.

Connect Remaining Sections of Gutter
Connect the remaining straight sections of gutter to each other and the 90-degree corner pieces (interior and exterior) using specially made 2-inch strips of aluminum called "seamers". They overlap the joints and fill in the 1/4-inch gaps we left earlier during measurements.

Apply Sealant
Apply a heavy bead of sealant on the inside of each joint to make it watertight. Let it dry overnight and then test the entire gutter system with water from a garden hose to ensure that the water flows from each section to its respective downspout without leaking from the joints.

Add Leaf Guards
Add leaf guards to keep leaves from the overhanging trees out of the gutters.