The metal material really was intended for lightweight roofing, and one major caveat aside, its weight is really a big advantage. For your information, the 24 gauge residential standing seam metal roofing weighs only around 59.1 kg per square, or per 100 square feet of roof cover. Compared to a conventional asphalt roof which is heavy and bulkier in nature.
A good Asphalt shingle weighs for about 109.1 kg per square (for the Owens Corning roofing brand, 30 year durability). For the Owens Corning roofing shingles, it weighs as much as 211.4 kg per square. When you are thinking of replacing your roof or installing brand new ones, you have got to be more careful of the weight that you already have on your current roof deck prior to adding another new layer for the roof. You could opt for the standard practice of adding one new layer over the older layer.
Meanwhile, if you already have two layers of the roof, it is not really necessary or even required for you to add a third layer for the roof since this will increase the weight of the roof, and that by adding too much of a layer, your roof may collapse and damage the house foundation. Also, adding a third layer to the roof may be against the code in your area because the weight may be too great for your structure to support and handle everything at once. Also, a heavy roof can be problematic during a disastrous events such as fire. The fire, or a severe storm, has the ability to weaken your structure up to the point that will make the heavy roof collapses. This can cause more damage to both the roof contents and structure, not to mention any victim who gets trapped under the collapsed metal roof and needs help. To avoid such events, always remember to not add any more weight.
Although it may be lightweight, its reliability and its durability are enough in protecting your house from critical outdoor weather condition or indoor mishap. This is also the main factor as to why you are not recommended to install a slate or cement tile roof since they are way too dangerous under several unexpected situations. The weight of a metal roof could also mitigate the risk involved and the increasing amount of your installation cost. You need to construct a more substantial truss system for the added weight of these products.