The silicone that is used in baking moulds that you put in the oven and the silicone that you use to seal your kitchen tiles with, is not the same as the silicone that you use in the metal roofing industry. Instead, the silicone used to seal your roof, guttering or downpipes is designed for outdoors and to stick to metal, making it durable and perfect for the job at hand.
Using the incorrect type of silicone to seal joints on a roof or gutter will breakdown under the sun's ultraviolet light and the extreme conditions of the outdoors, causing the joint to leak.
After this happens, frequently the joint cannot be successfully re-sealed without taking the gutter down and cleaning it properly with solvents. If the joint is on the roof, then the affected piece of roof will also have to pulled apart and carefully cleaned with solvents in order to be re-sealed correctly. Both of these jobs are time-consuming and therefore expensive for the home owner. This is why properly trained and experienced tradesmen will spend the extra couple of dollars to buy the correct silicone in the first place.
Silicone should be used to seal the joint twice: the first time to stick the two pieces of metal together and the second time to cover the pop rivets, once they have been installed.
Note: Every pop rivet on your roof or gutter should be sealed with silicone to ensure waterproofing. This is because when temperatures change, metals expand and contract, placing stress on joints. Silicone is flexible and therefore copes well with the expansion and contraction, whereas pop rivets on their own, without the silicone seal, pose a potential problem.