Rain gutters have to slope down to the downspout for proper drainage. If they are level or sloped incorrectly, rain water will collect in the gutters. The minimal slope required will allow for adequate drainage. However, for larger roofs and areas with regular heavy rainfall, increase the gradient of slope for quicker drainage during heavy downpours.
 Rain gutters have to slope down to the downspout for proper drainage.
Attach one end of the string where the gutters will be installed. If installing the gutter where a corner will join two sections together, measure the inside depth of the corner piece and attach the string to the fascia the distance of the depth of the inside of the corner piece. For example, if the inside depth of the corner piece is 5 cm (2 inches) attach the string 5 cm (2 inches) from the edge of the fascia.
Place the line spirit level on the string. A line spirit level is a very small level that attaches to string.
Pull the line tight to the other side of where the gutters will be installed.
Use the bubble on the line level to make sure the line is level the length of the gutter area.

Place the line spirit level on the string.
 Use the bubble on the line level to make sure the line is level the length of the gutter area.
Attach the level string to the opposite end of where the gutters will be installed.
Measure the length of gutter needed.
The minimal slope needed for proper gutter drainage is 1.25 cm (1/4 inch) for every 3 metres (10 feet) of gutter. Calculate the gutter slope by dividing the length of the gutter needed by 10. If the length of the gutter is 10.5 metres (35 feet), the amount of slope required is 1.25 cm (1/4 inch) multiplied by 1 metres (3.5 feet), which when rounded up is 2.5 cm (1 inch).
At the location where the downspout will be installed, measure and use a pencil to mark down from the string 2.5 cm (1 inch).
Use a pencil to mark where the string is attached on the other side. Remove the string.
Use a chalk line to mark the line by placing one end on the original starting point and the other end on the mark made for the slope. Attach the gutter using this line as a guide.
Increase the amount of slope for faster drainage on large roofs or for areas with frequent heavy rain.
Place the ladder on a level surface to prevent falls while attaching the string and using the chalk line.