A couple of different issues may cause a roof to cave in between the trusses. When moisture gets into the plywood decking of the roof, it weakens the decking. In turn, the weight of the shingles may cause the plywood to begin caving inward. A roof may also cave in due to objects and debris hitting the roof in the event of a natural disaster, such as an earthquake or tornado, which may also damage the roof decking.
Prior to laying a roof deck, trusses must be installed using the proper spacing to provide adequate support for the roof and shingles. The standard placement for roof trusses is 24 inches. This spacing provides adequate support for the roof deck and most types of standard roofing, like asphalt or wood shingles. Some shingles, like slate, however, weigh substantially more than asphalt or wood shingles. The manufacturer of these shingles may recommend closer spacing of roof trusses. Heed any instructions regarding support prior to installing shingles to prevent a collapse due solely to the weight of the roofing.
The way to repair a roof that is caving in between the trusses depends upon the reason for the cave-in. If the roof decking sustained damage, you must replace the decking, but you won’t necessarily need to replace the trusses. If the trusses have improper spacing, though, you must remove all of the shingles, the roof deck and move the trusses on the top wall plates so that they sit closer together. If the trusses sit too far apart, you must add an adequate number of trusses to the roof to achieve the correct spacing.
In some instances, home insurance may cover the costs of repairing a roof that caves in between the trusses. If the cave-in occurs due to a storm, for instance, insurance may pay, so you should check with your insurance company before making any repairs. Insurance companies may also cover the cost of repairs in the event that a contractor failed to properly install the roof decking or the trusses, though you should contact the contractor since he is responsible for the damage.