2.1: Catchment Surface

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The catchment surface is the area that the rainwater falls on to be captured. The catchment surface in a built (active) system is typically a roof.

The main questions when designing a catchment system are usually the following:

When can we catch the rainwater (i.e., what are the seasons)?
How much rainwater can we catch?
How much water do we need?
What can we afford in money (initial, operation, and maintenance, etc.)?
What are our overall goals?
What can we afford in time (initial, operation, and maintenance, etc.)?
What expertise and equipment are accessible?
What do we need it for?
What are the social, cultural, political, and other appropriate constraints?

The catchment surface is important to some of the main questions, e.g., to determine how much water you can catch, how the water can be used, and what end uses are appropriate. The surface material and size is part of determining the total catchable volume. The surface material also affects what the water can be used for, as shown in Table 2-1. The surface height is used to determine where you can use the water or whether you will need a pump.

Some typical roof materials are listed in the following table:

Table 2-1
Roof Material	Catchment Potential	Water Safety
Thatch	Low	Low
Tile	Medium	High
Asphalt	Medium-High	Low
Concrete	Medium-High	Medium
Corrugated Metal	High	Depends on Coating
Standing Seam Metal	High	High

Roof material impacts are still being studied, and in specific cases, where purity is critical, it is suggested that water be tested periodically7 for various pollutants and environmental data such as pH, BOD, TSS, fecal coliforms, etc. The following images show various roof materials such as asphalt shingles (Figure 2-2), concrete (Figure 2-3), clay tiles (Figure 2-4), and metals (Figure 2-5 and Figure 2-6).