A solar roof would be particularly suitable if you have a roof that needs repairing or replacing. A solar roof is cheaper than solar panels, and acts as weatherproofing as well as a solar heater. Its disadvantage is that it is less efficient than conventional solar panels.
The solar roof, or trickle collector, consists of a black metal sheet over which water trickles from a perforated pipe at the top of the sheet. There is a layer of glass over the sheet which becomes hot in the sun and heats the water flowing down it. The hot water is collected in a gutter at the bottom of the roof, pumped through a heat exchanger in a hot water storage tank, and then back up to the top again. The system needs a pump, but it can never freeze because there is no water on the collector unless the sun shines and the pump switches on.
Build the system on a suitable south-facing roof at an angle of 300 to 600 to the horizontal. To give access to the roof you will need two extension ladders which you should tie to screw eyes fixed into the rafters or fascia at the eaves and to stakes driven into the ground. You will also need a roof ladder which has a special end that hooks over the ridge of the roof. There is a lot to be said for building a trickling collector on a single storey building or extension to reduce the problems of access and possible accidents. If you are keen to build on a higher roof you should perhaps hire a set of scaffolding and have it erected to provide easy access to the whole roof, or maybe reconsider the whole project.
Preparing the roof
Remove any existing roof covering to expose the rafters and give them three generous coats of wood preservative. If they are not at about 400-450mm centres nail in a few extra ones. If the roof is old check it for any signs of rot and replace any damaged timber with new treated pieces. Then cover the rafters with a layer of 18mm roofing grade chipboard, nailed down with annular nails. You can buy chipboard ready finished with a layer of roofing felt: this is worth using as it is waterproof once you have taped the joints, so you will not have the problem of rain damage.
Lay roofing felt on the chipboard surface to provide protection should the collector leak and to prevent vapour from the hot water on the roof diffusing into the roof structure. The solar roof is an efficient generator of water vapour because the water is open to the air, and you must take a lot of care to keep this vapour out of your house. Starting at one edge of the roof, trowel on a thin layer of Ruberoid (or similar) cold applied mastic, which you can buy when you buy the felt, to cover the chipboard to a width slightly greater than that of the roll of felt. Unroll the felt and fix the bottom edge in place with a few galvanised clout nails. The job is best done on a warm day so that the felt, which should be a medium grade, will unroll easily without cracking.
When the first piece of felt is in place, firmly bedded on the mastic and fixed down with a few clout nails round the edges, put another band of mastic on the chipboard, overlapping the felt by 100mm. Lay the next piece of felt with a 100mm overlap on to the piece already there and continue along until the whole roof is felted. Make sure that the whole roof is well covered with mastic and that the felt is smoothed down before nailing so that there are no wrinkles. Now lay another layer of felt, again bedded on mastic. This time it should be laid horizontally, starting at the eaves, with each layer overlapping the one below by 100mm. Both layers of felt should overhang the edges of the roof by about 100mm.
Fitting the collector – The aluminium sheets
You are now ready to make the heat-absorbing part of the collector. This should be made from corrugated aluminium, painted black. The easiest way to do this is to buy pre-painted aluminium such as Granges Essem TRP 20 profile with Metallack black finish. The fixing screws for fastening the aluminium to the chipboard roof can be bought from the same manufacturer. They should be of a type suitable for wood and up to 20mm long. It is essential that the screws should be stainless steel or aluminium alloy and that they have suitable sealing washers to prevent water leaking under them; if you buy them from the makers of the corrugated sheet you will be sure to buy the correct ones.
You will also need a tube of lap sealing stuff to seal the joints between adjacent sheets of aluminium. When you order the aluminium tell the manufacturer the length of sheet you require (up to a maximum of 13 metres) and the width of the area to be covered. Each sheet is 1123mm wide but it covers only 1050mm because of the overlap at the joint with the next sheet.
Choose a calm day to put up your aluminium and have a friend to help hold the sheets in place while you drill them. The first sheet should come in line with the side of the roof and should be carefully positioned so that it is at right angles to the line of the ridge. It should overlap the bottom edge of the roof by about 50mnn. If this first sheet is not positioned correctly the whole roof will come out crooked.
Each sheet will need a row of screws down the edge and down the middle, with the screws about a metre apart down the roof. Fix the first screws of the sheet at the top of the middle row, making a dent with a centre punch to stop the drill from sliding about. Try to drill only a short distance, say 5mm into the chipboard so that the screw has a good fixing. The drill diameter should be very slightly larger than that of the screw. Finally tighten the screw firmly with a spanner or a socket spanner to compress the sealing washer against the aluminium. A socket spanner on a ratchet handle makes this job very much easier.
When the first sheet is fixed down at its centre line and its outside edge, smear some of the lap sealing compound along the edge where the next sheet will overlap it and then put the next sheet in place and fix it down, again starting at the middle row of screws. It will take some care to make the sheets line up properly so do not be tempted to hurry the job.
When the sheets are all in place, attach the aluminium purlins that carry the glazing bars which hold the glass. The purlins can be aluminium Z section about 38mm high. Space them at about 1200mm centres across the aluminium sheets and fasten them to the sheets with aluminium lmex rivets which are obtained from the suppliers of the corrugated sheets. It is very important to use these rivets and not steel ones so as to prevent corrosion. The purlins should be drilled to take the rivets at about 600mm centres (be sure that the holes you drill match the spacing of the corrugations in the aluminium sheets).
Stretc.h a string across the roof to provide a straight line to show where the purlins should go, and mark the first hole, near one end, on the aluminium. Centre punch the mark and drill the hole; then put a rivet into your rivetter, and push the end of the rivet into the hole in the purlin and the aluminium. While your helper holds the purlin in place squeeze the handles and the purlin will be rivetted in position. If you repeat the operation at the other end of the purlin it will then be fixed in place while you drill and rivet all the other holes. There should be a purlin at the top and bottom of the roof, and intermediate ones as required.
Now is the time to put up the pipe that distributes the water over the aluminium sheet, and this is where the main problem with this type of solar water heater occurs. If aluminium comes into contact with copper, or even has water dripping on it off a copper pipe, it corrodes. It follows that the distributor pipe should not be copper, although in our experience solar roofs have been built with copper pipes and they have not corroded yet. It may be that the pipe furs up inside with mineral deposits from the water, and these prevent the water from dissolving enough copper from the pipe to affect the aluminium.
Stainless steel pipe is an alternative to copper. It comes in the same sizes as copper and is about the same price, but even if you use stainless pipe you cannot avoid the copper coil heat exchanger in the hot water cylinder. If you want to do the job properly you should use only stainless steel pipe between the heat exchanger and the top of the solar roof, with stainless steel fittings. There should be a corrosion getter between the outlet from the heat exchanger and the stainless steel pipe containing a sacrificial element of aluminium for the copper in the water to react with. However, we are all less than perfect and we suspect that a system using stainless steel pipe with normal copper joints will perform very well, although it is probably worth making the corrosion getter.
The distributor pipe should be drilled with 4mm diameter holes at 75mm centres so that the holes coincide with the valleys in the corrugated aluminium. Mark the holes with a centre punch and drill them using an electric drill, but do not press too hard or the bit will become red hot. Take care that the holes are in a straight line along the pipe. If the collector is wider than 4 metres the pipe should be jointed so that no piece is longer than 2 metres, as this will help give a more even flow of water through the holes.
The ends of the pipe should have removable access caps so that you can poke a stick through if any of the holes become blocked up.
The manufacturers of the screws for holding down the aluminium sheet say that they are weatherproof but if you are worried at the idea of hot water pouring over them you can omit the holes in the pipe at each row of screws. The connection to the distributor pipe can either come up from below through the aluminium, or can be brought in from one end. The perforated pipe should be about 150mm below the top edge of the aluminium sheets to prevent water running behind the sheets. Fix the pipes to the sheets with strips of aluminium about 15mm wide bent to form saddles, drilled and rivetted through the corrugated sheets. Space them about 900mm apart.
The fascia board
Finally make a Tanalised softwood fascia board for the bottom of the roof, using 25mm thick wood planed all round. Before you fix the board in place, screw and glue (using waterproof glue) some blocks of Tanalised wood 50mm x 100mm and about 200mm long to the fascia. These should be fixed at 900mm centres. They are to support a second fascia which will carry the rainwater gutter. It is not worth painting the inner fascia, because you will not be able to reach it to redecorate it, but the outer one, which should be made from 25mm thick Tanalised wood (19mm when planed) can have three coats of preservative stain before it is fixed.
Nail the inner fascia into place and fix a 75mm diameter half round PVC gutter to it to collect the water from the solar roof. The aluminium should overlap the edge of the gutter, and so should the felt which must be trimmed with a Stanley knife as required. The gutter will need an outlet, the position of which will depend on the layout of the building and the placing of tanks. The gutter should be supported by gutter brackets at 500mm centres to prevent it drooping when full of hot water. Do not fit the other fascia at this stage.
The glazing bars and glazing
You are now ready for the glazing bars. These should be aluminium greenhouse bars. Before fixing them you will need some pieces of thin aluminium sheet, say 16 gauge, bent at right angles and fixed to the end rafter of the roof. These are to seal the ends of the solar roof and should be well smeared with mastic and fixed to the rafter with round head galvanised screws. You can have these pieces made up by any local small metal workshop, or you could go to evening classes in metalwork and make them yourself. The pieces of aluminium should be overlapped by 50mm at the joints and sealed to one another with mastic.
Drill holes for the glazing bar bolts using a 6mm drill. The first glazing bar must be fixed through the aluminium edges described above and bedded on mastic to seal any gaps underneath. The glazing bar should be long enough to overlap the outer fascia board by 25mm at the eaves. At the top of the roof the glazing bars should be cut so that a piece of no 6 lead sheet can be fixed over the top edge of the glass to keep the water out. Fix the bottom edge of the lead with strips of lead about 25mm wide fixed to the wall or to the ridge board with galvanised clout nails. Bend the ends of the lead strips to hold the bottom edge of the flashing.
If the other side of the roof is tiled or slated, you can then mortar a row of ridge tiles into place over the top of the flashing, taking care not to break the glass.
Finally, before you are finished, you must fix the fascia board in place under the glazing bars at the eaves. Fix it with cups and screws (plated of course) so that it can be removed easily to give access to the solar gutter when required. Fit a gutter of your choice under the glass, arrange it to discharge the water into a rainwater butt or soakaway and the solar roof is nearly finished.
Behind the chipboard put at least 100mm of insulation: be sure that you do not use expanded polystyrene in case the roof ever becomes hot enough to melt it. Choose glass fibre or mineral wool. If the collector is on a roof with a ventilated attic you need do nothing else, but if the back of the roof is a ceiling it will need a vapour barrier of heavy duty polythene, foil-backed plasterboard or special vapour barrier foil, to protect the insulation.
If you want to build a really cheap solar roof you can use corrugated clear PVC sheet for the glazing instead of glass. This will not last so long, and it may have a tendency to melt, or at least sag, in very hot weather, but it does allow you to put up a solar roof very easily. If you want to use PVC, follow the construction of the solar roof as far as the aluminium sheet; but instead of aluminium purlins, use wooden purlins so that the PVC can be screwed to them. The purlins should be of 25mm x 50mm hardwood such as iroko if you can find some, or else of Tanalised 25mm x 50mm softwood. Screw them through the aluminium into the chipboard with countersunk head no 12 screws 63mm long. The screws must be bright zinc-plated or cadmium-plated to prevent corrosion.
Space the purlins about one metre apart down the roof, and fix the PVC sheets to them with the special screws and waterproof caps sold for this purpose. Drill the holes for the screws in the plastic very carefully so that you do not crack the sheet, and do not overtighten the screws. You will need to flash over the top of the PVC with a piece of lead sheet just the same as the glass, and you will need some aluminium edges as used on the roof with glazing bars. Alternatively you can screw the edge of the PVC sheet into a 25mm wide Tanalised softwood board at the edge of the roof.
The PVC-covered solar roof will work perfectly well, and will provide hot water, but after about ten years you will probably find that you must replace the PVC as it will have become brittle and discoloured.
Connecting the system to the plumbing
Once the roof is glazed you can connect it up to the plumbing. Do not be tempted to connect up the solar gutter before the roof is glazed because you will flood the house if it rains. The gutter should have a conventional downpipe discharging over an expansion tank with a capacity of about twenty litres. Support the downpipe with clips to prevent it sagging when full of hot water. When the pump is switched off all the water in the system will drain back into this tank, so if your collector is large you had better calculate the volume of water involved by using the pipe volumes.
This overflow tank is also the water supply for the collector circuit and it should be connected to a conventional cold water header tank up in the roof by a 15mm pipe with a low pressure ball valve in the solar tank. This is done to prevent possible contamination of the mains supply; if there is any problem the header tank alone — which is used only to supply this solar tank and should be of the same size — will be affected.
The header tank will need a mains water supply through a high pressure ball valve, and an overflow pipe to warn of any problems with the ball valve.The solar tank should also have an overflow pipe.The ball valve for this tank should be about halfway down the side rather than near the top, so that there is room for the extra water when the pump switches off. Put a nylon kitchen sieve under the outlet from the gutter downpipe to catch any bits and pieces that may fall into the system and clog the holes in the pipe at the top of the collector.
Near the bottom of the solar tank fit an outlet to take a 22mm pipe and connect up to a central heating pump of a suitable head capacity for the system you have designed. The pump must be at the lowest point in the circuit so that it is always full of water, otherwise it will not work properly. From the pump the water goes through the indirect hot water cylinder (buy one with a coil type heat exchanger) and up to the perforated pipe at the top of the collector. All pipework should be in 22mm stainless steel, and you can use conventional compression fittings on all the pipes from the tank to the hot water cylinder.
When the pipe comes out from the cylinder heat exchanger it should go into a corrosion getter. This is a plastic bottle trap as used for wash basins and sinks with a 75mm seal. You will need fittings to connect 22mm stainless steel pipe to both sides of the trap. Bring the water in at the top of the trap and out at the side, just as if it was being used in a sink, but cut off the projecting pipe inside. Then make a muslin or nylon bag and fill it with little bits of aluminium, or aluminium shavings from a workshop. Put the bag, like herbs in a stew, in the base of the trap and screw the base back on. The idea is that any copper in the water will react with the sacrificial element in the trap rather than with the collector itself. You should check the trap occasionally when the collector is not operating and renew the aluminium as required.
Between the trap and the perforated pipe all joints as well as pipe ought to be in stainless steel. For the perfectionist stainless steel pipe joints are available and have the added advantage that they are assembled with glue which makes the whole job very easy. The special glue and primer are supplied by the makers of the fittings together with instructions on their use. The important thing (as with all pipework) is to cut the pipe square using a proper pipe cutter and to clean it well with emery cloth. Clean the inside of the joint as well, then spray pipe and joint with the special primer and allow to dry for a minute or two.
Squeeze a bead of adhesive round the end of the pipe and the mouth of the fitting. Insert the pipe, pushing it right into the fitting, and give it a very slight twist. Leave it to set at room temperature for five or ten minutes before you handle it. Leave it to set overnight before running water through it. The only stainless steel fittings that seem rather expensive are the stop ends with access caps for the ends of the distributor pipe, so you could use a normal copper stop end compression fitting and take a chance.
The rest of the solar roof plumbing is the same as that for a solar panel system. You will need a connection to the existing hot water cylinder and a pump controller. Fit the temperature sensor from the controller to the top of the collector plate with an aluminium strap, or wedge it in behind with a piece of wood.
Now the solar roof is ready to run. It has the advantage over solar panels that you can actually see the hot water coming out of the gutter into the tank, you can put your hand under the stream of water and feel all the heat coming from nothing but the sun. It is a great way to convince sceptics who say that solar energy does not work.