Depends on what law applies to your situation, federal vs state. I would check your state law. In Texas I would not have to flag it. Of course the law is the minimum requirement, you can flag it or mark it if you want.
In Texas the state law would apply to me for a non-commercial load. Loads beyond 4' off the back are not allowed unless another law applies such as federal law for commercial vehicles. There is no state law requiring flags or markings for an extended load less than 4'.
Extended Loads
(Texas Transportation Code 621.206)
a. A vehicle or combination of vehicles may not carry a load that extends more than three feet beyond its front or, except as permitted by other law, more than four feet beyond its rear.
Federal applies for commercial vehicles. If so nothing is required as it is not an extended load.
ｧ393.87 Warning flags on projecting loads.
(a) Any commercial motor vehicle transporting a load which extends beyond the sides by more than 102 mm (4 inches) or more than 1,219 mm (4 feet) beyond the rear must have the extremities of the load marked with red or orange fluorescent warning flags. Each warning flag must be at least 457 mm (18 inches) square.
(b) Position of flags. There must be a single flag at the extreme rear if the projecting load is two feet wide or less. Two warning flags are required if the projecting load is wider than two feet. Flags must be located to indicate maximum width of loads which extend beyond the sides and/or rear of the vehicle.
[70 FR 48054, Aug. 15, 2005]
