Since a sirloin roast typically weighs more than 2 pounds and has an irregular shape, you may want to tie it before you cook it. A well-tied sirloin roast cooks more evenly and in less time than an untied roast, but it might take a few tries to get proficient at tying. If you find yourself struggling and spending more than a few minutes using the butcher's method, use the beginner's method instead. Practice the butcher's method on a rolled kitchen towel until you get it right.
The Butcher's Method
Cut a 6-foot-long piece of twine and wrap an end of it 1 inch inward from the left end of the roast. Tie a knot in the twine and loop the long trailing end of the twine loosely around your right hand. Loop the twine around the right end of the roast, and pull the piece of twine in your left hand until the loop reaches 1 to 2 inches from the first knot you tied. Tie another knot. Wrap the end of twine loosely around your right hand again, loop it around the right end of the roast, pull the twine and tie a knot 1 to 2 inches from the previous knot. Continue looping and typing until you reach the end.
The Beginner's Method
Wrap a piece of twine crosswise around the roast 1 inch inward from the left end. Tie the piece of twine in a knot, then cut the loose ends from it. Continue tying the sirloin roast at 1- to 1 1/2-inch intervals until you reach the other end.