There are a few ways to turn a copper roof green. The first is time; the natural process of oxidation and the attack from airborne sulphur particulates allows untreated copper roofs to gain a blue-green patina that softens its bright pinkish colour. Depending on where you live and the humidity in the air, this process will be shorter or longer. Using certain chemicals you can speed this process up and give your roof a natural-looking patina in a matter of hours. This method is toxic and acidic, so wear protective clothing and a mask when mixing and applying the patina agent.
Clean your roof. For best results, get rid of all debris, dirt, oil and grease that is on the roof. Failure to do this will result in an uneven patina. Rain storms may be enough but you can make a simple cleaner with a mix of 3 parts concentrated lemon juice to one part salt. Mix it until the salt is completely dissolved then use the scrub brush to clean the roof. If the solution isn't strong enough, add some white vinegar to the solution. (Alternatively, you could buy a commercial cleaner from a hardware store.) Then rinse sufficiently to make sure none of the solution is left on the roof. If cleaned properly, there will be no beading on the surface. If there is, go through the cleaning process again.
If the roof has oxidised for six or months, remove the oxide film that has likely formed on the surface. The film will cause poor adherence of the patina chemical. To remove this, wipe the surface with a cold solution of 5 per cent to 10 per cent sulphuric acid. Wear rubber gloves and handle the acid very carefully to make sure you don't get any of it on your clothes, skin or adjacent materials.
Make the cuprous chloridelhydrochloric acid patina agent.
In the plastic tub dissolve all the chemicals in a small amount of warm water. Add small amounts of warm water as needed to help dissolve the materials. After the chemicals dissolve, dilute the solution with water to however many litres you prepared to make.
Plan on about 7.25 gallons or 27.5 litres of solution per 1,000 square feet of roof surface.
Now apply the agent by your method of choice. You can spray brush or, if you want a more decorative finish, stipple (use short strokes or dot the roof with a sponge or brush) the roof with the solution. The desired patina should be achieved in one application. Reapplying the solution could cause a chemical reaction (especially in direct sun) producing a film that is smooth, hard and colourless.
Store any extra agent in a sealed, non-metallic container.
Test out an area of your roof or some scrap from the roofing process to see how the copper will react to the treatment. If you don't like the result, you may need to adjust the chemical concentration, clean the copper better or check to see if it previously treated to keep it form ageing. To get the best results, be consistent in how you treat the roof.
At a minimum wear hand and eye protection when handling chemicals. When handling acid and the patina agent wear protective clothing and a mask to protect against the toxic fumes! Protect any adjacent areas, non-copper gutters, wood or brick from the chemicals being used. The chemicals can easily stain other materials.