Considerable knowledge is gleaned from inspecting existing roofs. Time and time again we see roofs that are 25 years old and the only means of determining roof age is by the condition of the metal flashing. After reviewing over one and a half million square feet of roofing for one client in 1997 we found one of the best roofs to be 27 years old. The conventional type roof assembly consisted of a steel deck, vapour retarder, 1-1/2" fibreboard insulation, 4 ply felt roof membrane, and a double pour of asphalt and gravel.
Our experience with single ply PVC membranes indicates that the membrane becomes a little brittle over a 12 to 15 year period. The brittleness is often accompanied by splitting of the membrane when under stress. In answer, suppliers typically state that the average life expectancy of a roof as per the National Research Council (NRC) is 12 years and one should be looking at some maintenance work after approximately 10 years of age.
Inverted roofs (protected membrane roofs) have earned their place in the industry but they can be a problem especially when trying to detect a leak. Ask your local roofer for the horror stories. Ensure there is good reason to select an inverted roof, and make sure it is well drained and select a system with a good history. Typically the exposed roof ballast and metal flashings always look good but keep a good eye on the condition of the roofing below!
When inspecting roofs we not only look at the roof, we also observe changes in the adjacent roof elevation, and interface condition at walls and projections. We record anything that might have an effect on water entry especially if it could be construed to be a latent roof leak.