The roof should be replaced before siding. It typically makes more sense to replace the roof first because it has a bigger impact on the rest of the house. An ineffective roof can cause damage to all other areas of the home. From leaks in the ceiling to wood rot on the siding, old roofs create more problems than old siding. If you replace the roof first, you will ensure that it can protect any new siding that gets installed. Plus, you will have taken a top-down approach to improving the efficiency and safety of your home. This ensures that your home is protected from the brunt of the elements while any other projects are happening.