Ask a friend to help you. Gambrel roofs such as those on a barn require that each side carry an equal weight load, states the Vermont Agency of Agriculture. Having someone on the other side of the roof cleaning off snow at the same time you are prevents the roof from having an unequal distribution of snow, which can impair the roof's integrity.
Stand on the ground and remove as much snow as you can reach with an extendable snow rake. Work from the bottom of your roof toward the top removing the top layers of snow with your rake. Leave 2 to 3 inches of snow to prevent damaging your roof with the rake. Pull the snow from the roof to the ground beside you. Stand on a ladder when you cannot reach the snow from the ground.
Attach safety lines to a sturdy roof structure such as a chimney if you cannot reach the rooftop with a snow rake and ladder. You may need to purchase and install a roof anchor to have a place to attach the line. Climb on the roof and shovel snow only if you have no other way to reach it. Shovel snow by removing layers of snow like you did with the rake. Toss snow onto the ground.