How to Prepare Your Roof for Harvesting Rainwater

PUBLISHED 8 DEC 2016   

Rainwater can be harvested from the roof of your house or any other roof surface - including your garage, shed or patio roof. Before you start collecting the water that falls onto your roof, there are a few things to consider to ensure the roof surface is properly prepared,  especially if the collected water will be used for drinking.



Type of roofing material


Where collected rainwater will be used will determine the level of water quality - water quality for garden irrigation or pool top-ups will not be on the same level as that of drinking water.

If you are using it in the garden or pool then any type of roofing material can be utilized as a catchment without any prior treatment. The exception to this is unsealed asbestos, which is generally considered an environmental hazard that poses a health risk. 

It is recommended that asbestos roof sheets should be painted with a lead-free sealant before being used as a catchment surface, to prevent any possible contamination of groundwater and soil.



If the rainwater that you collect is going to provide your family with drinking water, you will need to take additional steps to prevent pollutants from contaminating your water supply.

The first thing you need to ascertain is whether or not the actual roofing materials are manufactured from products that may contain hazardous chemicals or other toxic substances. Roofing materials that contain known toxins such as lead-based paints or primers; unsealed lead flashing or asbestos, need to be sealed with a non-toxic sealer in order to prevent these toxins from contaminating your water supply. 

The same applies to any treated wood that is exposed on top of the roof. The toxic chemical compounds used to preserve wood can leach out into your water supply if the wood is not properly sealed. Bitumen, which is toxic, is often used to waterproof roof surfaces; if your roof has been waterproofed using a bitumen agent rather refrain from using the collected water for drinking.

Overhanging vegetation


Regardless of what the water will be used for, you should try to prevent leaves, dirt, and debris from entering your storage tanks. This can be done by installing a mesh screen onto your gutters to prevent leaves from collecting in the channels, and by fitting a leaf shedding rain-head and/or a first-flush diverter to the downpipe leading to your water tank. 



If the water is going to be used for drinking, it is recommended that any branches or other vegetation hanging over the roof should be trimmed right back. This will prevent birds from using these as a perch or overnight roosting area, which could result in bird droppings accumulating on the roof surface that could be washed into the tank and pollute the water. Also make sure that any trees or plants that overhang, or are in close proximity to your roof, are not poisonous, as these could get into your tank and introduce toxins that could pose a health risk if ingested. Poisonous plants should be trimmed right down, or better still, removed completely.

Other potential sources of contamination

Other potential sources of contamination include chimneys from fireplaces or indoor braai areas, as well as overflow pipes from a hot water cylinder, solar hot water system, or air conditioner. Should an overflow pipe flow onto your roof, either divert the water off the roof or avoid collecting water from that section of your roof if it is going to be used for drinking.

Gutter pitch


If the angle from the gutters to your downpipes is insufficient it will allow water to accumulate in the gutters, which can provide conditions conducive to mosquito breeding and algae growth. 

To prevent stagnant water from pooling in your gutters, ensure that the pitch of the gutter leading to the down-pipe that flows into your storage tank is at least 1% — adjust this until the water flows efficiently. By getting the pitch right you will not only prevent build up of stagnant water but you will also maximize the amount of rainwater collected from your roof.