I am building a 2400 sf. L shaped house and would like to use pre fab roof trusses. I was told I could save $2,000 over a custom built system. The builder is calling for 24" on center. Is this safe, and will I have any saging with the dry wall ceiling??. Also we will be using a 25 year roof shingles and was told this would hide any possible waves due to the crowning of the trusses. The roof pitch will be 7-12. I would appriciate any help.