It is a common practice among employees engaged in fire protection activities to relieve employees on the previous shift prior to the scheduled starting time. This practice will not have the effect of increasing the number of compensable hours of work where it is voluntary on the part of the employees. If the practice is required by the employer, the time involved must be treated as compensable hours of work.
Iowa Admin. Code r. 875-220.18
29 CFR 553.225.