What is the total allowance for a ½" double hem?

In HVAC sheet metal work, a double hem is a common technique used to create a clean edge on metal pieces while also providing additional strength and rigidity. The total allowance for a ½" double hem is calculated by taking into account that the hem involves folding the metal over itself and then back again.

For a ½" double hem, the metal is initially folded ½" towards the inside. This first fold uses up ½" of material. Then, in the second fold, the metal is folded back on itself to create a finished edge, which requires another ½". Thus, you would initially think that the total would simply be ½" + ½", which equals 1".

However, when fabricating a double hem, you must also consider the additional material required to account for the material thickness when folding. This creates a small allowance that is typically an additional 1/16". Hence, the total allowance becomes 1" plus 1/16", which equals 1 1/16".

Therefore, for a ½" double hem, the correct answer is indeed 1 1/16", as it reflects both the total material used in the folds and the additional allowance needed for proper bending and finishing