As commonly understood, gutters are elements installed along the edges of roofs and balconies to channel rainwater. Their primary function is to collect water flowing over the roof's surface and direct it to the appropriate downspouts. Proper sizing is crucial regardless of the material—whether aluminum, PVC, concrete, galvanized steel, or others. Depending on the climate in which the project is situated, the damages caused by incorrect application can be considerable.
Rainwater not directed to the proper location can overflow onto the sides. This excess water on the walls can lead to mold and leaks, while water accumulation at the base of the structure can result in soil erosion. Moreover, this system offers a practical and sustainable solution for harvesting rainwater and facilitating its use for various activities.
Next, we will present a basic guide for pre-sizing gutters in different projects.
Determine the Intensity of Rainfall
The first step is to understand the geographical context of the project because climatic and geological conditions are crucial in defining the rainfall intensity (mm/h) that will affect the building. This information is typically available in Rainfall Charts specific to each region. However, it's worth noting that some professionals have noted that many existing references are outdated due to significant climate change and consequent alterations in regional rainfall patterns. Therefore, it's important to stay vigilant and consult the most up-to-date Rainfall Charts. Some authors in Brazil recommend using 0.067 liters per second per square meter as a safe estimate.
According to a Brazilian standard, gutters should drain rainwater with an intensity corresponding to a 5-year return period (rainfall that has the probability of occurring once every 5 years) over the contributing area of a roof. However, the standard divides this period into three levels of risk based on the characteristics of the area to be drained:
- 1 year, for paved areas such as driveways, sidewalks, open areas, etc., where puddling can be tolerated;
- 5 years, for roofs and/or terraces;
- 25 years, for areas and roofs where puddling or overflow are not tolerated.
Identify Descent Points
The downspouts are the openings in the gutters that direct the collected water into the piping system until it reaches the stormwater sewer or other storage systems. Downspouts should be free from obstructions such as windows, doors, etc., and their determination is important as they define the lengths of the gutters. In areas near dense vegetation and regions with tall buildings where wind can commonly carry plastic bags and other lightweight objects, it is relevant to consider an additional vertical conductor, anticipating that occasional blockages may occur in any of them.
Calculate the Contribution Area
The contributing area refers to the part of the roof responsible for gathering rainwater that flows into the gutter being sized. The larger this area, the more water it collects, requiring larger gutters accordingly. It's important to highlight that NBR 10844 differentiates between the dimensions of the roof, its slope, or the presence of walls at its edges.
This value is necessary for us to calculate the flow rate of the project, which is determined by the formula: Q= I.A/60
Where:
Q = project flow rate (liters/minute)
I = rainfall intensity (mm/h) – indicated in rainfall tables and charts
A = contributing area (m²)
Size the Gutter
There are various gutters available with different materials and designs, and their selection depends on the architectural characteristics of the building. However, it's important to note that the flow capacity of gutters is directly related to the shape of their cross-section, which can be rectangular, or semicircular, among others.
The Brazilian standard provides a table considering some standard gutter sizes and adopting a slope of 2% (with a minimum tolerance of 0.5%). The chosen gutter should have dimensions sufficient to accommodate the calculated volume of rainwater according to the size and shape of the roof. For other gutter dimensions, the Manning-Strickler formula should be applied. It's worth noting that the standard also indicates the possibility of recurring variations in the roughness coefficient determined by the chosen material for the gutter.
Q= K.S/n.Rh²³.i¹²
Where:
Q= project flow rate (liters/minute)
S= rea of the wetted cross-section (m²)
n= roughness coefficient – indicated in the table
R= hydraulic radius (m)
PH= P/S wetted perimeter (m)
i= slope of the gutter (m/m)
K= 60.000
For more information, access NBR 10844.
