Rain gutters have to slope down to the downspout for proper drainage. If they are level or sloped wrong, rain water will collect in the gutters. The minimal slope required will allow for adequate drainage. However, larger roofs and areas with regular heavy rainfall may increase the slope needed for quicker drainage during rainy seasons.
Attach the beginning of a string at one end of where the gutters will be installed. If installing the gutter where a corner will be joining 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 2 in., attach the string 2 in. from the edge of the fascia.
Place the line level on the string. A line level is a very small level that attaches to a 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.
Attach the level string to the other opposite end of where the gutters will be installed.
Measure the length of gutter needed.
The minimal slope needed for proper gutter drainage is ¼ in. for every 10 ft. of gutter. Calculate gutter slope by dividing the length of the gutter needed by 10. If the length of gutter needed is 35 ft., the amount of slope needed is ¼ multiplied by 3.5 or, 7/8 which can be rounded to 1 in.
At the location where the downspout will be installed, measure and use a pencil to mark down from the string 1 in.
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.