Rainwater Collecting Systems

Water-butts have, for a long time, offered an example of recycled materials: usually an empty wooden beer barrel, tapped at the bottom, this is set up on enough bricks so that a watering can may be comfortably placed under the tap; and it is covered to keep out leaves that would otherwise sink to the bottom as they rotted, eventually blocking up the tap. So pervasive is this image that the standard 170 litre plastic water-butt for sale in every garden and hardware shop retains the barrel shape. If you are satisfied with storing a little rainwater for use in the garden, allowing the surplus to flow to a ditch or soakaway, then two water-butts, one at the front and one at the back of the house, will be all you need. However, if you aim to provide the recommended storage volume of 25 per cent of the estimated collected rainfall, you will be talking of a storage volume of about 4000 litres, which would be equivalent to about twenty-four plastic water-butts, an expensive and impractical idea. In this case some kind of large underground cistern might be the best solution.

Using oil drums

The cheap alternative to the water-butt is a 250 litre oil drum, direct descendant of the old recycled beer barrel. Our oil drum was left by a friendly builder, but we have seen them advertised in the small-ad column of a local paper and you may know of a source anyway. If you can, get one from which the top has already been cut. Our knowledge of metalwork is limited and we could not think of an obvious way of removing the top, beyond using a cold chisel and a hammer. Do not use any form of hot cutting tool: this would be disastrous if there was any oil left in the drum.

The tap

To fit a tap in the bottom of the drum, the first step is to make a hole. Use a normal tank cutter (sold for cutting holes in the side of a cold water cistern) on a brace and bit. You will need a vertical bibtap, either chromium-plated or brass if you can buy it, and a back nut to fit. The wall of the oil drum is thin so there will be sufficient thread projecting inside it on which to fix the back nut. Alternatively it is possible to buy an urn or wash-boiler tap which comes complete with back nut and has a long thread which makes it easy to fit it into an urn or wash-boiler.

Fitting the tap is a job for two people. One of you stays outside the oil drum and holds the tap in the hole whilst the other person crawls inside with the back nut and the stilson. Before the tap is inserted wind PTFE tape round the threaded shaft. Resisting the temptation to roll your friend or life partner over inside the drum, and this is the hard part, hold both the tap and drum steady whilst he or she tightens up the back nut on to the PTFE tape in a very confined dark space. If you want to prolong the life of the oil drum water-butt, paint it inside and out with bituminous paint.

Setting the drum up

Set the drum up on three or four courses of brick and arrange a shoe on the bottom of your downpipe so that the water from the gutter discharges into the drum. It is wise to provide some kind of lid made from scrap boarding or exterior grade plywood with a hole cut out so that the water discharges through it. This will have to be weighted down with a couple of bricks to prevent the wind blowing it off.

Many alternative vessels may come your way that can be used as water-butts: galvanised tanks, especially those which were once used for cold water supplies in the older type of plumbing system; or plastic cold water cisterns, which can be fitted with taps as described and which are available with a capacity of up to 227 litres. In fact, anything that will hold water can be used and you do not even need the refinement of a tap if you are prepared to dip your watering can directly into the top.

The site

The most obvious place to site the water-butt is directly under the existing discharging downpipe but if you are planning, or have built, a conservatory along the south side of the house, it would be worth considering rearranging the guttering to bring the water-butt inside the conservatory. The mass of water inside the conservatory, apart from being handy for watering the melons and tomatoes, helps to store heat which is released when the conservatory cools down at night.

It can also serve to avoid wasting energy: on clear sunny days, especially in spring, the air temperature inside the conservatory may reach a point where you need to open ventilators to avoid overheating. If you have arranged for hot air from the conservatory to be vented directly into the upstairs rooms of the house you will be able to use the heat this way, otherwise it is so much energy lost. A large volume of water inside the conservatory could be heated up by this surplus, releasing the heat once the sun goes down and the structure begins to cool.

It is hard to judge exactly how much heat can be stored, but experiments have been carried out, especially in America, to store heat in this way in conservatories and greenhouses. The vessels used have included black-painted containers with capacities ranging from 4 litres to the more familiar 250 litres of the oil drum, and transparent containers filled with dyed water. These have succeeded in keeping the temperatures up within the structure on cold, overcast days. A single 170 litre water-butt is not going to make a major contribution to heat storage but ten of them or a single tank of equivalent size probably would. And the lid will also provide a warm spot on which to stand your germinating seedlings so that they do not suffer too wide a temperature fluctuation.

Fitting the down-pipe

If your conservatory has brick ends then it will be a simple matter to guide the downpipe through a hole in the end wall. To make the hole you will need a lump hammer and a cold chisel.

The speed at which you make the hole will depend on the age of the brickwork and the type of mortar. If you are worried drill a few holes through the wall first, in the place where the pipe is to pass, using a masonry drill; then knock out the brickwork left between the holes with the chisel. Once the downpipe has been fitted it should be mortared into the brickwork.

If your conservatory has glass ends it would be best to replace the pane of glass the downpipe will pass through with a sheet of WBP plywood or perspex cut to the same size. A hole to take the downpipe can be cut in either of these materials with a padsaw once a pilot hole has been drilled first. Make the diameter of the hole match the external diameter of the downpipe as closely as possible, and seal around the gap when the guttering is assembled with silicone mastic to stop the draughts.

For serious heat storage the water-butt should be a dark colour — the black-painted oil drum is ideal — and set so it intercepts the light through the vertical panes of glass. If it receives light only from the top, only the top layer of water will be heated and as hot water rises there will be no mixing of hot and cold water within the drum. The aim is to set up convection currents within the water-butt so that all the water becomes as uniformly heated as possible and thereby stores the maximum amount of heat.

Using a series of butts

If you want to store large quantities of rainwater and use the mass as a heat store it is possible to connect up tanks or oil drums in series. To cope with the flow of water from the roof during heavy rain the tanks will have to be connected with a wide diameter pipe and Bartol Plastics Ltd make 41mm external diameter waste pipe and a tank connector to fit. You connect all the water-butts either at the bottom or at the top. The former will mean that all the butts will fill simultaneously and only one tap need be fitted to draw water from them all; but you will have to crawl inside to wrench on the back nuts. It is easier to make the connection at the top of the butts when they will fill one after the other; but each will then have to be emptied with its own tap.

The latter is probably the better solution from the point of view of heat storage as once the tanks have been filled they can be emptied in turn starting with the one directly under the downpipe which will be the first to be refilled. The others, meanwhile, sit there quietly storing more heat in their filled state than if they were only half full.

If you are using old flat-sided galvanised tanks then make holes in them using a suitable diameter tank cutter on a brace and bit. Ideally the tanks should stand as close together as possible: the surface area of the storage should be minimal so that it loses heat to its surroundings more slowly, which may be important in periods of prolonged cloudy weather. In practice, unless you are good at lining up tank connectors on a horizontal level to within 1-2mm, this will be impossible and it will be easier to space the storage tanks about 300mm apart. You can always grow tomatoes in pots between them. Having cut the hole, push the tank connector through from the outside, wind PTFE tape around the threaded part, screw on the back nut and tighten it into place. Now push the 300mm length of plastic waste pipe into place if you are using the Bartol system (other systems may require solvent welding).

In the next tank cut a hole the same height above ground level. With the tank connector fitted on to the end of a short length of pipe, manoeuvre the tank into position so that the connector passes through the hole. Since both tanks will probably be supported on bricks if they are fitted with taps, you will need help at this point. The tanks can be levelled with pieces of broken slate placed on top of the bricks and, although the 300mm length of pipe will absorb some discrepancy, it would be best to line the tank connectors as near level as possible before screwing on the last back nut; and do not forget the PTFE tape.

If you are using oil drums rather than flat sided tanks you may find that the curvature of the drum prevents you from obtaining a watertight seal. Take a mallet and bash the side of the drum to give a flatter surface against which to tighten the back nut. This will help relieve the frustration of trying to juggle large tanks so that the holes you have cut in them line up.

Using a storage tank

As an alternative to the series of connected butts, doing double service for storage of rainwater and heat, it might be worth considering building a purpose-built rainwater storage tank. If featured inside your conservatory and fitted with a few water plants it might also be called a pond. Unless you are a proficient amateur concreter and preferably already own a concrete mixer it would be better to stick with the old oil drums, but even with the method described below you could at least supply the labour whilst employing a builder to do the concreting. If you do intend to go to the expense and trouble of building a concrete tank you will have to be fairly dedicated about the need for storing rainwater.

Digging the hole

From the example quoted earlier the volume of storage required is about 4000 litres or 4m3, that is a tank 2m x 2m a 1m. Although it is possible to build a concrete tank of this size above ground, it would be more sensible to make use of the ground as a support for the mass of the water. So the first job will be to dig a hole the appropriate size. Depending on the ground water table of the area in which you live, you will generally find that if you make the excavations and do the concreting in summer you will be less likely to be digging out the last 300mm under water.

The exact dimensions of the tank will depend on how much water you want to store and whether you want to use it as an optional indoor paddling pool/boating lake for young children (under supervision) in which case it should only be 300-400mm deep. Whatever the proposed size of the tank the bottom of the excavation should be 200mm greater all round to provide the space in which to build the concrete block walls of the tank. The type of soil will determine how much wider the hole will be at the top than at the bottom as the sides of the excavation may have to slope outwards.

When you have done the digging and disposed of the soil (a skip which is 7m3 in volume is useful for this purpose but a rock garden somewhere nearby might be an alternative) you will have to level the bottom off reasonably accurately. Fix timber pegs across the width and along the length of the tank. These should be 250mm above the ground — 100mm for the hardcore and 150mm for the concrete slab. By laying a level across the tops of the pegs you will be able to line them up. Once the pegs are fixed in place put in a 100mm layer of broken brick and ram it well down. The pegs will inevitably get knocked so check that they are level again, but it is still easier to put them in first rather than trying to bash them through the hardcore.

The cement

The concrete for the tank should be a mix of one part by volume cement to two parts sand to three parts coarse aggregate. This can either be ordered ready-mixed or if you have a concrete mixer you can make it in batches. Fill the excavation slightly above the top of the pegs and then tamp it down with a length of say 50mm x200mm timber used on edge to compact the concrete. You will have to fit handles on to this timber so that it can be used from the top of the excavation; otherwise you may find your wellingtons concreted into the bottom of the tank. Do not worry about smoothing the outer 100mm around the edge of the tank as this should be left fairly rough so that the concrete which forms the sides bonds with the bottom slab.

The day after making the bottom of the tank you should be ready to build the concrete block walls, It is important that the work be done on consecutive days so that the whole thing bonds together properly. Use solid dense concrete blocks 400mm x 200mm x 1 00mm and lay these in a mortar of one part cement to three parts sand. If you cannot face doing all the blockwork and pouring the concrete side walls on consecutive days you could do the work in stages, building a course of blockwork and pouring the concrete behind, dampening the junction between the old and new concrete. The courses must be laid on consecutive days. The concrete should be left for at least a week to cure and the surface of the concrete blocks should then be sealed with a mortar of one part cement to one part fine sand which should be brushed in by hand with a small brush.

Alternative versions

Another method for making a concrete rainwater tank: The slab at the base is formed in the same way but after it has set overnight cardboard boxes are placed on it so that there is a margin of approximately 150mm between the sides of the boxes and the sides of the excavation. The boxes are then filled with soil and concrete is poured around them. When the concrete has set after a week the soil is dug out and the cardboard removed. Finally the inside surface of the tank is given a rendering of three parts sand to one part cement. The disadvantage of this method is that having completed the initial excavation you have to shovel a great part of the earth back and then dig it out again, but it would be very invigorating exercise.

If you really cannot face the bother of making a permanent concrete rainwater storage tank you could just dig the hole and then line it with a garden pond liner, preferably one of butyl rubber, which would have to be held down around the edges with stone or thick baulks of timber. Polythene sheet is unsuitable as it degrades in sunlight. Providing you were never careless with your garden fork such a construction should have a reasonable life.

However you form the underground tank arrange your downpipe to feed into it and provide some kind of cover lest you step backwards unthinkingly. The cover can be formed by building your greenhouse staging over the tank, thereby bringing the plants directly above the water heat store.


Building an overflow system into your underground tank is difficult, and impossible if you have used the hole and liner system. The easiest plan is to keep an eye on the level and to bale the tank out with a bucket if it becomes too full, pouring the water on the garden. The excess water should not really be poured down the drain as this is usually designed only to take foul water. For normal use watering cans and buckets can be dipped into the tank. If you are going to the trouble of providing underground storage it is likely that your tank is going to be as large as possible, otherwise all the effort is not going to be worthwhile; as long as you use the water it is unlikely to become so full that it starts to overflow.

An overflow should be provided if you have only one or two water-butts to take the whole of the flow off the roof. It should discharge into a nearby ditch or a purpose built soakaway, designed so that the water permeates into the surrounding soil. The normal diameter of pipe discharging into a soakaway would be 75mm; but as the pipe has to be connected to the top of the water-butt it will have to be 41mm as it is limited by the size of tank connector available. Because of this it would be best to provide two such pipes, perhaps discharging from separate water-butts to the same soakaway. Check with your local building inspector to see what they will accept: they will probably also have some suggestions for the way to make a soakaway acceptable to your particular local authority.

Typically it is a hole 1m x 1m x 1m (yes, more digging), filled with broken hardcore and covered at the top with 150mm of earth. The 41 mm pipe from the water-butt travels under ground and discharges into the hardcore somewhere near the top of the soakaway. The soakaway should not be dug anywhere near the foundations of the building it is to serve, or any other building, and it should also not be dug in ground which is higher than that on which the building stands.

The inevitable pile of soil you will be left with presents the usual problem; by now the rock garden is probably beginning to resemble a tumulus.

Leave a Comment