Does your roof look like a giant ball fell from the sky and put a dent in it? If so, there can be many causes, some are dangerous and some just look bad. In every case, your roof sagging needs to be repaired before it leads to bigger issues.
Shingles are not designed to be a waterproofing barrier. They are designed to shed water and as a result, if they are installed improperly or have something impeding their ability to shed water they can begin to leak. When water gets underneath the shingles it can cause extensive damage to the deck underneath. Both OSB and CDX Plywood absorb water and swell. If this problem isn’t addressed quickly it can cause permanent damage to the deck and that wood will begin to sag.
First, this should be pretty self-explanatory but you would be surprised at the roofing contractors I see that have no regard for the design limitations of a structure. When an engineer designs a house he creates load-bearing calculations. These are to guide the installers on what type of materials to use. Many contractors will use cheap or inadequate materials to save money. This is extremely dangerous and can lead to collapses.
Secondly, I often see where a contractor installed an additional layer of decking without consulting an engineer. This can also lead to sagging because of the stress being put on the support underneath the deck.
Third, the code for most places says you need to remove the existing roofing system down to the deck. But an easy way to lower the price of a roof is to just install a second layer of shingles over the old system. This can lead to all kinds of problems and a sagging roof deck is one of those.
The least common cause of a sagging roof is the materials. Sometimes it is caused by cheap and unscrupulous contractors cutting corners. As a contractor, we have the responsibility to inspect what we are installing and in some cases raise a red flag if we think something is off. This can save our clients thousands of dollars in the long run! But sometimes even if we have the best intentions and inspect our products meticulously a defective batch of materials will slip by the manufacturer’s quality control department. In this case we better hope we kept out receipts and that we can track down