Hanging a gutter on a flat roof is a good way to help drain water that can collect on the roof. A membrane covers most modern, flat roofs to prevent water from leaking into your home; however, if your roof has a slight slope or a tendency to collect puddles of water, gutters are a good idea. Gutters can help if you currently have to manually push water off your flat roof for it to drain properly.
Measure the perimeter of your roof, and write down the measurements.
Decide where you will put your downspouts and how many downspouts you need. Place downspouts in areas that can direct the water away from the house or into rain barrels. Measure the distance from your roof to the ground to determine how long your downspouts need to be, and then add at least four feet to each downspout, so that you can extend it away from the house.
Cut your gutters using tin snips or a skill saw. Wear gloves when cutting metal to avoid cutting yourself, and if you're using a power saw, put on safety goggles to keep bits of metal from flying into your eyes.
Put the gutters together on the ground. Gutters come in sections about 4 to 6 feet long, so you will need to put them together in order to get the length you need. Overlap each section's seam by at least two inches.
Drill at least six evenly spaced, 1/8-inch holes at each overlapping seam, about a quarter-inch from the edge of the seam.
Place rivets in the holes you drilled in each gutter section using a pop rivet gun to insert them. The rivets fit into the gun much like staples in a staple gun. You then clasp the handles of the gun together to push the rivets through the gutter.
Caulk the seam on the underside of each gutter with gutter sealant, using a caulk gun. Place a dab of sealant over each rivet for extra water protection.
Use tin snips to cut out the downspout outlet hole -- this hole will be outlined for you on the gutter -- in the underside of each gutter. Cut about 1/16-inch outside the outlet hole line on the gutter. Remove the outlet hole liner, and place a small line of gutter sealant around it. Replace the liner, and rivet the liner onto the gutter. There should be pre-drilled rivet holes, but if not, drill four holes, one in each corner.
Install the outlet rivets into the pre-drilled rivet holes in the downspout outlet liner.
Attach a piece of string to one end of fascia where you are installing the gutters. You need to do one side of the house at a time. Place a line level on the string. A line level is a small level that attaches to a piece of string. Pull the line tight to the end of the side on which you are working.
Measure the length of the gutter you need, and then calculate the slope. Gutter slope is usually 1/4 inch for every 10 feet of gutter. Divide the length of the gutter you need by 10. If you need 25 feet of gutter, then divide 25 by 10, resulting in 2.5. Multiply 2.5 by 1/4-inch, and the slope required is .625 or rounded to a 3/4-inch slope.
Measure 3/4 inch -- or your result -- underneath the string, and mark that line with a pencil. Remove the string and then hold out a chalk line from the edge of the roof line to the marked line at the end of the side on which you are working. Snap the chalk line on this slope. Attach the gutter on this sloped line.
Hook hangers to the front lip of the gutter, and screw the hangers into the fascia of the house. The gutters have to go up before the hangers, since the hangers attach the inside of the gutter to the house. Install hangers every 18 to 24 inches.
Install the downspouts into the downspout outlet in the gutter. The downspout drains the water out of the gutters and away from the house so it is running vertical at the end of your gutter. Drill holes for rivets, and then install the rivets to secure the downspout. Caulk the seam with gutter sealant.
Attach the downspouts to the side of your home using the downspout hangers. Screw the hangers into the side of the house, and clamp them around the downspout. While this step isn't mandatory, it will prevent the downspouts from blowing in bad weather.
Assemble the downspout so that it empties into your yard, a drainage tube or a rain barrel. The end of the downspout will fit inside the outlet hole you cut out. Seal the downspout into the hole with gutter sealant and rivets, drilling and inserting the rivets as you did for the gutter seams. Place the aluminum mounting straps around the downspouts, and secure them to the house with screws.