In some instances, you might be able to install new roof decking over old roof decking. However, it's usually not a good idea to do this. The old decking might not be attached properly, which could cause the new decking to come loose.Also, installing new decking over old decking can cause the roof to sag under the additional weight. If installing new decking over old decking, your contractor may need to reinforce the rafters with additional bracing.