Collecting rainwater from a fibre cement roof: uses and limits

If asbestos is suspected, a separate assessment of the old covering is needed, without test-damaging the material. Fibre-cement sheets from different periods and technologies are not a single category in terms of every performance characteristic. Declarations for a new sheet should therefore not automatically be transferred to a roof whose origin cannot be established.

Watering selected plants, supplying a utility installation and contact with food are different applications. It must not be assumed that roof water is fit for drinking or cooking. The mere absence of asbestos in the covering does not change this. Rain can collect contaminants from the air, and the roof slope can also collect them from the surroundings. Fibre-cement sheets do not perform the function of a treatment device. If the plan envisages more demanding use, an appropriate design, a quality assessment and solutions tailored to the purpose are needed. Information that a roof sheet is watertight is not evidence of the quality of the water leaving the gutter.

The quality of the run-off is influenced not only by the substrate of the sheet. The surface finish, previous painting, soiling and the preparations used may also matter. Before cleaning the roof, the handling of water flowing into the system must be established. Fibre-cement sheets should not be treated with any product simply because it is intended for other coverings. If there is no information on the compatibility of the preparation with the planned water harvesting, this question must be clarified before work begins. The tank should not become an accidental collecting place for residues from washing or renovating the roof slope.

Leaves, needles and sediment can block the outflow and increase the amount of contaminants passing further on. The harvesting system therefore requires accessible inspection and cleaning points. Fibre-cement sheets should discharge water into a correctly positioned gutter, without uncontrolled splashing onto the façade. Filter elements are selected to suit the installation and the surrounding conditions. It must be remembered that retaining larger fragments does not mean removing all contaminants. Every device has a specific function. Its presence does not justify extending the possible uses of the stored water at will.

The capacity should be matched to the catchment area, rainfall and actual consumption. There is no single size suitable for every property. Safe handling of excess water when the tank is full is also important. Asbestos-free fibre cement does not change this principle. The roof must still drain intensive rainfall without overloading the gutters and flooding unwanted places. The design should allow for cleaning, protection of the tank and winter conditions for the equipment. It is also worth establishing how part of the system can be disconnected during roof works. This prevents repair of the covering from becoming a problem for the entire retention system. The overflow point should remain known to the user. During heavy rain it must be possible to check whether excess water is draining in accordance with the design.

If rainwater is to be fed into the building, the solution requires proper separation from the installation for water intended for consumption. The type of protection should be selected by a qualified designer or installation contractor. Fibre-cement sheets are only the surface collecting the rainfall here. They do not determine hydraulic or quality rules. It is not worth building the system on the basis of a marketing diagram alone. The draw-off points, the method of topping up water and the operating conditions must be known. Clear labelling helps avoid mistakes during use and later repairs by people who were not involved in the installation.

Cleaning the gutter or inspecting the roof slope must be possible without walking directly on the sheets. Access to the filters should encourage regular servicing rather than require dismantling of the structure every time. Fibre-cement sheets should also be inspected for damage and changes at the flashings. In the system documentation, record which covering the water is harvested from and which agents have been used on it. A change of coating or replacement of part of the roof may require the assumptions to be checked again. A functioning retention system is a set of interacting components, not just a tank placed under a downpipe.

Before connecting the tank, confirm the composition of the covering and the intended use of the water. Then select the entire drainage, treatment and servicing system to suit the actual conditions of the building.