If your roof has been designed properly and built with quality material, it could last as long as 100 years. However, if you see your roof beginning to sag, it might be a sign that your roof will collapse. The most likely causes of a sagging roof include the following:
Reason #1 = Weak Roofing Material
Over time, exposure to moisture can begin to degrade the materials that compose your roof. If rain and snow are able to penetrate sections of your roof, it can lead to rot. If the roof stays wet for longer periods of time, mildew and mold can begin to grow, which further deteriorates the roofing material. Shingles and flashing will help protect your roof from water damage. If rainwater is not draining off of your roof properly, it can lead to excess moisture. If your roof lacks proper ventilation, any moisture that does collect will take much longer to dry, increasing the chances of damage.
Reason #2 = Faulty Roof Material
The quality of the materials in your roof make a major difference in how long it will last. It may be cheaper in the short run to use asphalt shingles, as opposed to slate, which lasts longer. You might find your shingles offer less protection and need to be replaced more frequently. The same applies to any other materials in your roof, from the nails to the beams. Installation makes a major difference as well. Even if you use quality components, unless they are properly installed, they will be just as ineffective as cheaper parts.
Reason #3 = Too Much Weight
Each roof is designed to support a maximum amount of weight. If your roof is exposed to frequent accumulations of ice, snow, or rain, the extra weight could cause damage. Take weather into consideration when purchasing a building. Excess snow can be especially problematic. On one hand, too much weight can stress the roof. On the other hand, scraping the snow off can damage the shingles. Making sure that your roof can support sufficient weight.
It is important to perform regular maintenance checks on your roof, usually at the start of a new season. This will ensure that you catch any problems before they become major issues. It is highly recommended that you have your roof inspected by a professional every three years at the minimum.