The one solid rule that comes with a truss roof is that you should never cut a truss. To do so will usually void the warranty on the truss system immediately. Since you want to replace the trusses, losing the warranty probably won’t matter, but it’s essential to understand that you can’t go into the attic and start cutting trusses without risking roof collapse.
Trusses Vs. Rafters
The engineered shape of a truss member distributes the roof load to the sloped ends of the truss and then straight down through the load-bearing exterior walls. Vaulted rafters, which are necessary to construct a vaulted ceiling, distribute the load at an angle, from the ridge of the roof to the exterior walls. This means that the weight of the vaulted roof will exert outward as well as downward pressure on the exterior walls. To counter this pressure, it’s necessary to install a ridge beam at the peak of the roof.
Structural Ridge Beam
A structural beam must be installed beneath the inside roof ridge while the roof trusses are still in place. An engineer must design the beam to support the weight load of the new rafters. It could be a large steel I-beam or a massive wood beam. The existing trusses will not support the beam, so gable-end supports must be set in place before adding the beam. A large crane will lift and hold the beam during insertion, which requires cutting holes in the gables.
Gable End Supports
At both gable ends of the house you will need to construct a support system for the structural ridge beam. As with the beam, an engineer must determine the configuration of the support system, but it will likely involve pouring footings beneath the ground at the ends of both gables and installing vertical supports that run from the footings to the tip of the gable on both ends. These supports will hold the structural beam.
Once the beam is secure, you can install the rafters, but you still can’t tear out the trusses, which are bearing weight. Roof pitch determines the rafter size and spacing. Standard rafter spacing is 16 inches apart. Where the bottom of the rafters meet the top of the existing wall, a special notched cut will be necessary to maintain a smooth roof line.
Once the rafters are set, it’s safe to remove the bottom truss chords and the small triangular members that form the centers of the trusses. You’ll have to leave the top truss chords in place since the roof sheathing attaches to them, but they won’t be visible once you install drywall on the now-vaulted ceiling.
If you’re just trying to add living space, removing the trusses is an expensive way to gain a few extra square feet, and it’s definitely not a do-it-yourself project. An engineer must be involved every step of the way to reduce the risk of roof collapse. Since you’re changing the roof structure, you’ll probably need a building permit, and your homeowner’s insurance might require you to purchase a special rider to cover the work before beginning.