BACKGROUND OF THE INVENTION.

The present invention relates to a device suitable for collecting and at least temporarily storing water on a supporting surface, to be at least partly covered by a multi-layer structure, the multi-layer structure comprising a plurality of super-imposed layers as described in the preamble of the first claim.

DESCRIPTION OF THE PRIOR ART.

Until now, water buffers build on top of a roof often consisted of roof vegetation systems. In the mean time roof gardens have gained increasing popularity partly because of the heat insulation effect they provide. To allow culturing vegetation on top of the roof a wide variety of systems have been developed. As the systems must meet specific vegetation requirements, take care of supplying sufficient water to the plants and be resistant to the roots of the plants, most of them are quite expensive. However roof vegetation systems show a number of unwanted disadvantages. First of all it has been observed that damage to the roofing by the plant roots, cannot be completely avoided. Repairing of the roof, following damage by the plant roots, is laborious and expensive. A second major disadvantage is that absorption and release of water proceed relatively slow. The vegetation, the vegetation growing substrate and the need to large volumes of stored water have been found to seriously increase the roof load, thus necessitating the construction of a more expensive roof substructure.

There is thus a need to a system with which water may be collected when precipitation takes place, at least temporarily stored after having been collected and be released to any desired circuit, if required.

In FR-A-2.677.062 a supporting panel is disclosed for the construction of drainage flat roofs trafficable to pedestrians, capable of capturing the excess water falling down in case of heavy rain or storm. The supporting panel covers a supporting surface of a rigid or slightly porous supporting structure on top of the roof. The panel comprises a core of a

thermoplastic honeycomb material with a plurality of vertical spaces for receiving the water. The honeycomb material is covered on both sides by a non-woven water permeable textile. The above described structure in turn is covered by a top layer of for example concrete, stones, wood, synthetic grass, etc.

DE-U-9401805 relates to an automatic irrigation system for roof gardens. The system is installed to a substantially flat roof, the gutter of which has been raised somewhat to allow storing the water on top of the roof. Within the framework of the raised gutter, a drainage layer is applied, which is responsible for the transport of water from water storage volumes provided therein to the ground layer containing the plants, the water storage volumes forming the water reservoir. On top of the drainage layer, the ground layer for growing the roof garden is applied. The water level within the drainage layer is controlled through a detector, which activates a water supply from a water reservoir if the water level in the drainage layer gets below a pre-set level. If the water level in the drainage system raises above a pre-set level, a float is opened and water is transported from the roof to the reservoir.

The system disclosed in DE-U-9401805 is specifically designed as an irrigation system for a roof garden, into which water may be temporarily stored to have it available for the plants. In the system of DE- U-9401805 however the presence of the garden is essential so as to fix the system to the roof. The system disclosed in DE-U-9401805 is not at all designed for capturing rainwater, temporarily storing the rainwater and quickly deviating it to a water reservoir for further use, thereby limiting evaporation.

W096/11313 discloses to cover a roof with a structure which functions as a temporary water reservoir, from which water may evaporate in a fast manner, with the aim to improve the climate in the neighbourhood as well as inside the building. The water-retaining structure of W096/11313 is intended for use with industrial buildings, with the intention of reducing the dimensions of the buildings'drainage system and the water purification needs. It is usually mounted on top of a water impermeable film that covers the top of the roof, and comprises a first layer attached thereto which is made of a material capable of collecting and retaining water. The water-retaining material is made of a honeycomb material covered by a filter layer to avoid that impurities end up in it. The filter layer may be permeable to water and water

/ vapour, be substantially impermeable to water but improve the mechanical resistance of the structure, or it may be a UV resistant material to improve the influence of UV to the water-retaining structure. The presence of the honeycomb structure ensures fast water capturing from the environment as well as a fast release into of stored water into the environment through evaporation.

The water-retaining structure of W096/11313 however has a limited water storage capacity, it is designed to provide for a fast evaporation of water but not to function as a water storage device.

In EP-A-266.701 a substrate assembly for irrigating a roof garden is disclosed. The substrate assembly comprises a plurality of parallel sheets to be applied to an essentially flat roof structure. A drainage layer of porous polyurethane foam is applied on top of the roof structure. The drainage layer is covered by a filter fleece, which supports a carrier plate into which a plurality of water storage volumes is left out. A hole in the bottom of each volume allows for a water flow from the storage volume to the lower drainage layer. The drainage layer functions essentially as a sponge, which absorbs water until saturated. Excess water supplied after saturation of the drainage layer is returned to the water storage volumes. Insufficient water in the vegetation layer is overcome by a water suppletion from the drainage layer to the water storage volumes and further to the vegetation through capillary action. An anchoring fleece on top of the carrier plate ensures receives the roots of the vegetation.

The increased water consumption in the recent years in industrial as well as domestic environments, the increased costs involved in the purchasing of water and engagements in environmental awareness, have implied a need to a system in which water may be recovered from the environment, preferably stored on a longer term basis for further use. As roofs provide a free surface area with significant dimensions, which with most buildings is not used, they constitute a readily available surface for storing water.

SUMMARY OF THE INVENTION.

It is an object of this invention to provide a structure or a device for collecting precipitation from the air or any other liquid that ended up on the surface, deviating the captured precipitation towards a reservoir, the risk to evaporation of the liquid being minimised.

DESCRIPTION OF THE INVENTION.

This is achieved with the present invention with the technical features of the characterising part of the first claim.

The device of this invention comprises a multi-layer structure suitable for at least temporarily storing water, part of the water remaining available as bulk water. The multi-layer structure comprises (a) a water-capturing layer comprising a plurality of hollow, water capturing volumes for receiving and storing water. At least part of the hollow volumes in the water-capturing layer has a top face, which is substantially open and capable of capturing water. At least part of the capturing volumes is dimensioned such that they are capable of capturing precipitation seeping through (b) a filter layer covering the water-retaining layer. The function of the filter layer is to prevent that impurities end up in the bulk water, to provide a quick transport of water to the hollow volumes of the water-capturing layer. By the presence of the filter layer the liquid stored in the water-capturing layer is shielded off from the environment, which assists in minimising evaporation of the liquid from the structure. The hollow volumes in the water-capturing layer are further provided for storing and containing bulk water; (c) between the supporting surface and the water-capturing layer a layer of a water absorbing material is provided. The function of this layer is more or less fixing the multi-layer structure in its position, to counteract displacement of the multi-layer structure with strong winds or heavy weather.

In the water-capturing layer a plurality of holes is left out to allow for a water flow from the water-capturing layer to the layer of the water-absorbing material. A function of the water absorbing material is namely to guarantee a fast transport of water from the water-capturing layer to the drain which is responsible for discharging excess water present in the multi-layer structure to a reservoir.

The presence of absorbed in the water absorption material and the capacity of this material of at least temporarily retaining the absorbed water, allows to substantially increase the weight thereof and. As a consequence this water-absorbing layer is automatically kept in its position due to

its heavy weight and does not necessitate further fixation, although this may be done.

The device of the invention further comprises a water level detector for measuring the level of the water contained in the water-capturing layer or in the multi-layer structure. The water level detector is connected to a water discharge system for discharging water from the multi-layer structure to a water reservoir or a water consumption circuit, in case the water level in the multi- layer structure exceeds a predetermined value. Water is discharged until a first predetermined level is reached.

Water seeping through the filter layer is collected, filtered and stored as liquid bulk water within the hollow volumes of the water- capturing layer. This water which is stored as liquid water, is immediately available for further use. The fact that the water can readily flow through the filter layer to the water-capturing layer, without necessitating any capillary activity or having to pass vegetation, increases the rate with which water may be captured by the multi- layer structure. The presence of the water-absorbing layer, which is capable of quickly discharging water to a drain allows further increasing the water capturing rate of the device of this invention. In that way the device of this invention may function as a buffer in case large volumes of liquid end up on its surface. An example is a case of heavy storm. Besides the fact that the water layer has a heat isolating function, it is also sound-proofing.

Monitoring the water level and discharging of the water from the multi-layer structure to a water consumption circuit allows controlling the level of the stored water so that on the one hand it does not exceed the multi-layer structure and, on the other hand, the water-absorption material is kept sufficiently wet. Monitoring the water level further allows preventing overloading of the surface supporting the multi-layer structure.

The water consumption circuit may be a water storage volume, for example a pond or a container, a sanitary equipment of the neighbouring building or a combination thereof, or any other water consumption circuit known to the man skilled in the art. The excess of water may at least temporarily be stored in the storage container. In that way a buffer is provided which monitors periods with a lot of precipitation and dryer periods, the water in the buffer being available for further use.

The device of this invention is suitable for use with any supporting surface known to the man skilled in the art, for example a roof, a ground surface, the surface of a car parking place, preferably in the vicinity of the building intended to make use of the stored water.

In case the supporting surface is not water impermeable, the multi-layer structure preferably contains as a lower layer or base layer a sheet or layer of a material which is substantially impermeable to water.

This base layer is provided to be applied to the supporting surface, for example the roofing, to shield this surface off from the stored water; to minimise water absorption by the supporting surface; and to ensure the water transport from the multi-layer structure towards the drain and the water consumption circuit.

To shield the water stored in the holes of the water- capturing layer from daylight and UV-light, to minimise the risk to algal growth or development, and to minimise evaporation of water from the structure, the multi- layer structure is on top covered by a layer of a particulate material. The particulate material is preferably a solid non-porous material or a material with a small porosity, preferably selected from the group of marble split, gravel, shingle, cobble stones, plastic material etc. However any other non-porous or slightly porous material known to the man skilled in the art may be used. The above mentioned particulate material are preferred because they are colour stable, frost, wearing and UV-resistant.

A supporting layer on top of the filter layer will preferably take care of receiving and supporting the particulate material. The supporting layer will mostly be a net or web like material or sheet with holes for receiving at least part of the particulate material in them.

If so desired, two or more of the base layer, the water absorbing layer, the water-capturing layer, the filter layer and the supporting layer may be comprised in one single multi-layer structure to facilitate commercialisation and mounting of the device. Preferably, the water absorbing layer, the water- capturing layer and the filter layer are comprised in a multi-layer structure.

In a preferred embodiment of the invention, the water level detector is coupled to a monitor for controlling a water flow from the water storage volume to the multi-layer structure if the water level in the water-capturing layer is lower than a second predetermined level. This is not only done to maintain

insulation properties or to avoid degradation of the multi-layer structure or parts thereof when dry, but mainly to guarantee that the water-absorbing layer always contains sufficient water and to immobilise the multi-layer structure. The first predetermined water level may be approximately equal to or different from the second predetermined water level.

The water level monitor may also be coupled to a monitor controlling the water need of sanitary equipment of a building neighbouring the multi-layer structure. In case a signal is received from the sanitary equipment, the monitor ensures a water transport from either the water- retaining structure or the water storage volume, depending on the water level in the water-retaining structure, until the amount needed is replenished.

The above described technical features of this invention provide a flexible system for capturing water or any other liquid in a water retaining structure and storing part of the captured water in the water retaining structure, the evaporation of water from the structure being minimised.

The excess of water may be discharged to a water consumption circuit, or to a water storage container. In case the water level in the water retaining structure is insufficient water can be replenished from the water container to the water consumption circuit.

The connection to the water consumption circuit ensures that the water-retaining layer can be substantially emptied of water in case temperatures get low, to prevent damaging of the material by frozen water.

With warm or sunny weather the water contained in the multi-layer structure heats up, which involves evaporation of stored bulk water and implies a cooling effect to the supporting roof surface and the building covered by it.

The invention also relates to a roof or a supporting surface at least partly covered with the device of this invention.

The invention further relates to a multi-layer structure for use with the device of this invention.

The invention will be further explained in the appending figures and description of the figures.

BRIEF DESCRIPTION OF THE DRAWINGS.

Figure 1 is a cross section of the multi-layer water- retaining structure of this invention.

Figure 2 is a schematic view to the device of this invention applied to the roof of a building.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION.

The device 10 of this invention for collecting precipitation water shown in figure 2 is positioned on top of the exposed upper surface of the roofing material of a roof 17 as a supporting surface. The water- retaining device 10 may however also be positioned on top of any other substantially flat or horizontal surface area of the desired dimensions. The supporting surface may be substantially flat, but may also slant somewhat.

The water-retaining device comprises a laminar multi-layer, water-retaining structure 10 built up of a plurality of super-imposed, preferably substantially parallel extending layers or sheets 1-6. The multi-layer structure may be made as a mat of a large surface area covering the supporting surface 17, but may also be made in the form of longitudinal strips having a predetermined length. The use of strips facilitates storing and mounting of the multi-layer structure 10, and allows storing while being rolled-up. The use of strips further allows providing a drain between adjacent strips to improve discharging of water from the supporting surface to the water consumption circuit. This will usually be done by positioning adjacent strips at a distance from each other on the supporting surface 17.

Two or more of the layers 1-5 may be but need not be connected to each other to improve the strength of the multi-layer structure and to prevent displacement of one or more layers with respect to the others. It will also be understood that two or more of the layers 1-5 may be incorporated into a laminar, multi-layer material to facilitate mounting.

The multi-layer structure 10 may along one or both longitudinal sides be provided with an upright profile 21 to substantially prevent discharge of water along that side. The upright profile 21 preferably extends to below the lower surface 22 of the multi-layer structure, and may be connected in a water-tight manner to the supporting surface 17. The multi-layer structure 10 may however also be received within a surrounding upstanding rim of the non-water

permeable material, to provide a closed volume and allow discharging of water around the said rim.

In a preferred embodiment, the multi-layer structure 10 comprises a base layer or sheet 1, provided for positioning on top of the exposed upper surface 17 of the roofing. The base layer 1 is preferably made of a material which is substantially impermeable to water to prevent water from leaking from the multi-layer structure to the roof 17. Suitable materials include polyolefin, polypropylene, polyethylene, rubber, PVC, polyamide films or any other films or sheets of a water impermeable materials known to the man skilled in the art. In general, the material of the base layer 1 will be adapted to the material of which the other layers of the multi-layer structure are made to prevent that the weight of the multi-layer structure amounts too high. The dimensions of the base layer 1 may be selected such that the base layer 1 covers the sides of the surrounding rim, to provide a closed volume. In case the supporting surface is already impermeable to water, for example in case of a water-impermeable roofing, the base layer 1 may be dispensed with.

On top of the base layer 1, a layer or sheet of water absorbing material 2 is positioned. This material always contains absorbed water, and functions as a positioning layer for the multi-layer structure by permanently increasing its weight. The other function of the water absorbing material 2 is to provide a fast discharge of water received from the hollow volumes (7) of the water capturing layer (3). The water absorbing layer 2 may be made of various materials, but is preferably made of a non-woven material, for example a fleece of a non- woven material as these are capable of fulfilling the above-described functions.

Suitable materials for the water-absorbing layer 2 include non woven sheets of polyolefin, polyethylene, polypropylene, ethylene- propylene copolymers, polyester fibers, natural fibres or any other non-woven material known to the man skilled in the art. An example of a suitable material is SSM 45 supplied by Zinco, the Netherlands, which is a polypropylene fleece with a density of approximately 470 g/m2 and a water absorption capacity of approximately 51/m2. This material is available in sheets of 2 x 50 m. However any other material, with higher or lower water absorption capacities, weight and dimensions, known to the man skilled in the art may be used. An example is a fleece including mineral fibres.

If so desired to improve the water-absorbing and water-discharge properties of the water-absorbing layer 2, the layer 2 may comprise two or more laminar sheets of the same or different materials.

On top of the water-absorbing layer 2, a sheet of a water-capturing layer 3 is positioned. This water-capturing layer 3 comprises a plurality of water storage volumes 7 for receiving bulk water resulting from precipitation falling down. To fulfil this function and provide a sufficient capturing capacity, the water-capturing layer 3 will have a larger thickness as compared to the other layers or sheets, and will in general thus also function as a spacing layer.

The water storage volumes 7 may be contained in either one single or in several levels of the water-capturing layer 3. The water storage volumes 7'in the upper level are directly accessible trough the filter layer 4, to allow capturing and storing of precipitation water therein. The water storage volumes 7"in the lower level are not directly accessible from the environment, but are connected through connections or holes 20 to the water storage volumes 7'. In that way the possibility of water transport from the upper level 7'to the lower level 7", as well as the water storage function of the lower level volumes 7"are ensured.

Additional connections 20 are provided between the lower level 7"and the upper level to allow for an escape of air and to prevent floating of the sheet 3 when filling the lower level 7"with water. However, any other structure which contains water- capturing volume may be used.

The water-capturing layer 3 is further provided to allow for a water flow from the hollow volumes 7,7'7"to the water absorbing layer 2, and thus for a discharge of water from the multi-layer structure to a drain or reservoir. The material of the water-capturing layer 3 is preferably chosen such that it is sufficiently rigid to be walked on.

An example of a very suitable material for the water- capturing layer is Floradrain, a polyethylene sheet supplied by Zinco, the Netherlands. This sheet has a density of 2.2 kg/m2 and is available in sheets of a dimension of 0.96 X 2.08 m. Other suitable materials include polyethylene, polyproplene, polycarbonate, polyolefin, polyamid, thermoplastic materials, hardened resins or any other material known to the man skilled in the art.

The water-capturing layer 3 is preferably covered by a filter layer 4, to prevent that impurities end up in the water-capturing layer and to

minimise evaporation of water from the structure. The filter layer 4 may be made of any material known to the man skilled in the art, which is capable of providing the filter function, which is permeable to water and allows a fast permeation of water through it. This is important as the present invention aims at providing a water storage function and at minimising evaporation of water from it, thereby ensuring that a sufficient amount of water ends up in the water-capturing 3 and water- absorbing material 2, and minimising direct drainage of water from the filter layer 4 in the drain. The filter layer 4 is preferably made of a material with a dark colour so as to minimise incidence of day light and UV radiation to the structure 10 with the aim to minimise UV degradation and algal growth.

The filter layer may for example be made of a felt-like or a felt material, any other suitable non-woven material made of natural or synthetic fibers, any other filter or geotex material or woven material known to the man skilled in the art. An example of a suitable material is Isotex, supplied by Bouwen en Milieu, the Netherlands. Isotex is a geotextile made of woven polypropylene strips. It is a highly water permeable material, which is impervious to light, so that algal growth may be prevented. Since it has a coarse woven structure, it can easily dry when it is not contacted with water. Another example of a suitable filter material is Fibertex, supplied by Bouwen en Milieu, which is a non- woven polypropylene-based material. If so desired, to improve the filtration properties of the filter layer, the filter layer may be built up of two or more sheets of the same or a different material.

The filter layer 4 is preferably covered with a fixation layer 5 for receiving a solid particulate material 6. The fixation layer 5 comprises a three-dimensional structure or net or web, comprising cavities or webs for receiving the particulate material. The material for the web layer 5 can be any material known to the man skilled in the art, for example metallic wires, polyethylene, polyamid, if so desired coated with a UV and/or light resistant material as there is a risk that the layer 5 gets exposed to sun light.

The position of the web 5 will mostly be ensured by having part of it extended with respect to the multi-layer structure 10, folded down to lie on the roof 17, and by covering the extending part with the particulate material 6. The web 5 may extend along all sides of the multi-layer structure 10 or along one single side or two or more sides.

It will be understood that the material of which the individual layers 1-5 are made may be adapted to each particular situation, e. g. the density of the material may be varied depending on the available roof load.

Care should be taken that the material of which the layers 1-5 are made is water- resistant, i. e. does not degrade when contacted with water for prolonged periods of time, and that it is resistant to micro-organisms.

The particulate material 6 may be any particulate material known to the man skilled in the art which is preferably non porous to prevent degradation when freezing and minimise algal growth. Examples of suitable materials include cobble stones, gravel, marble split, etc or chopped brick stones, Argex (E), etc when coated with a non-porous coating. The particulate material functions to shield the water-retaining layer from daylight and to counteract algal growth and is therefore chosen such that it is resistant to discolouration.

Preferably, either the individual layers of the multi- layer structure 10 or a plurality of the layers 1-6 are secured to the supporting surface 17 or the upstanding rim. This may be done in various ways well known to the man skilled in the art taking into account the specific function of each layer.

In a particular example, with the preferred materials disclosed above, the base layer 1 has a density of 1000 g/m2, the water absorbing layer 2 has a density of 470 g/m2, the water-retaining layer 3 has a density of 2200 g/m2, the filter layer 4 has a density of 0.1 g/m2, the upper particulate layer 30000 g/m2. The over-all density of the above described multi-layer structure is approximately 4120 g/m2. With a water level of 5 cm, the density mounts to 84120 g/m2. This is comparable to the weight of a gravel layer with a thickness of only 5 cm. In general the weight of the water-retaining structure 10, contained water included, will be around 80 kg/m2. This is small as compared to vegetated roofs, the weight of which is virtually always higher than 80 kg/m2.

The multi-layer structure 10 may be applied to substantially the whole roof surface 17. It is also possible to cover only part of the roof, depending on the desired water storage capacity. In case part of the roof is covered, care should be taken to contain the multi-layer structure in a closed volume to provide the water storage capacity.

In case use is made of strips of predetermined dimensions, a plurality of strips 10 will be positioned one next to the other on top of the supporting surface. Often care will be taken to position adjacent strips of the multi-layer structure such that a drain is provided between them, to improve the water discharge rate from the water retaining structure. The drain between adjacent strips will in general be connected to the drain leading to the water reservoir. This may be achieved by having the filter layer 4 extended with respect to the multi-layer structure 10 such that the extending part covers at least part of the surface between adjacent strips. The extending parts of the filter layers 4 of adjacent strips will be positioned in an overlapping manner and will in general be covered by the particulate material to fix the position of the extensions.

It is also possible to provide a drain which surrounds the water-retaining structure 10 and guides the water towards the discharge unit 14. Drains may also be formed by folding the layer 5 covered with particulate material 6 down to the water impervious layer 1 and leaving the other layers 2-4 out. Such drains may for example also be provided along the sides of the water- retaining structure.

The device of this invention comprises at least one water discharge pipe for discharging water from the water-retaining structure 10 to a water consumption circuit. The inlet 9 towards the water discharge pipe is located somewhere at a position within the thickness of the water-retaining structure 10, but below its upper level. A water-level detector 8, for example a float, is mounted in the water discharge pipe, for detecting the level of the water stored in the water-retaining structure 10. The position of the float 8 is adjustable in height direction to allow controlling the water level. The float 8 is provided to be opened and allow a discharge of water through the discharge pipe if the water level in the water-retaining structure exceeds a first predetermined level. Water will in general be discharged to a second predetermined level. Discharged water is collected in the water consumption circuit, which may comprise a sanitary equipment, a water storage volume 11, for example a container or a pond, etc.

The water-level is preferably adapted to allow a virtually complete emptying of the water-capturing layer 3 to ensure minimising evaporation.

The water flow within the device of this invention is preferably controlled by a water level monitor 12 as will be described hereafter.

The water-level detector or float 8 is coupled to the monitor 12 to ensure that water is supplied from the pond 11 to the water-retaining structure 10, if the water level in the water retaining structure becomes lower than a third predetermined value.

This will usually be done by means of a pump.

The monitor 12 may also be coupled to the water consumption unit, for example a sanitary equipment, to provide a water flow from the water-retaining structure 10 towards a sanitary equipment, upon a signal from the sanitary equipment that water replenishment is required. The monitor 12 detects from the position of the water level detector 8, the water level in the water- retaining structure 10. If this is sufficiently high, the monitor 12 controls a water flow from the water-retaining structure to the sanitary equipment until it receives a signal from the sanitary equipment that the water is replenished. If this is insufficient, the monitor 12 ensures a water flow from the water storage volume 11 towards the sanitary equipment.

The water storage volume may be a container 11 or a volume in which water evacuated from the water-retaining structure 10 is at least temporarily stored. As can be seen from figure 2, it may be preferred to capture the water coming from the water-retaining structure 10 in a first pond. The first pond forms an overflow to a water reservoir mounted below the pond, for storing the water for a longer period of time.

Preferably the multi-layer structure 10 is fixed to the sides of the roof with appropriate fixation means known to the man skilled in the art.

It is furthermore possible to mount one or more solar panels to the roof, and to have the water flowing to the reservoir flowing trough it, to heat the water. It is also possible to have the water flown through a heat- exchanger to recover the heat energy contained in the water.

The multi-layer structure may be covered with floor tiles or any other material suitable to walk on.

The water which is recuperated in this way is suitable for further consumption, in applications such as for example garden irrigation, sanitary equipment, cleaning, washing machines, etc. It will be understood from the above given description that the device of this invention has a simple construction, may easily be installed and is cost efficient.

The functioning of the device of this invention can be described as follows. Precipitation falling down from the air or any other liquid ending up in the multi-layer structure, seeps through the particulate layer 5-6, quickly passes the filter layer 4 and is captured in the water storage volumes 7'of the water-capturing and spacing layer 3. The water passes from the water storage volumes 7', through the connections 15,20 to the water storage volumes 7". Air is allowed to escape from the water storage volumes 7"through the connections 15.

Water is absorbed by the water-absorbing layer 2 until saturated.

As the inlet to the water discharge system 14 is somewhat countersunk with respect to the water retaining structure 10, water flows to the discharge system 14 as soon as the water level reaches the level of the inlet. As soon as it is detected by the float 8 that the water-level in the water- retaining layer 3 reaches a first predetermined level, the float is opened to discharge water to a second level. The water is discharged towards the container or to the sanitary equipment, if there is a need to replenish water in the sanitary equipment. If the water level gets lower than a third predetermined value, the monitor 12 controls for a back flow of water from the pond or container 11 to the water retaining structure. If the monitor receives a signal from the sanitary equipment that there is a need to replenish water, water is supplied from the water retaining structure 10 or the pond 11, depending on the water level in the water retaining structure 10.

The above described device is suitable for use on a substantially flat supporting surface, for example a flat roof. The device may however be adapted to render it suitable for use with more or less slanting surfaces. This can for example be achieved by providing along the lower rim of the slanting surface, an upright rim 21 with a larger height as compared to the upper rim, and by having the height of the side rims increasing towards the lower rim.

The water level detector will than be adapted to provide for a water discharge as soon as the water level in the vicinity of the lower rim gets higher than a preset level. This can for example also be achieved by continuously circulating the liquid from the lower rim to the top rim and back.

With a slanting surface, in general care will be taken that the bottom hollow volumes of the water-capturing layer run approximately horizontal.