When adding a filler piece to shorten the duct to a certain length or filling a piece to keep the duct at a certain length, add an inch for each lap at s-cleat. Typically this is two inches for lapping over s-cleats. Just keep the thought in your mind to add an inch. If needed, draw a diagram on the floor to help reference the needed length.

Duct is usually made of 28-30 gauge metal in residential application and can be from 16-26 gauge in commercial applications. for Industrial applications, it can be even thicker. The thicker the metal the hard it is to make a hole. If the hole is for a cut out, there is not need to be careful as long as where the area is taken out is discarded. Use the back of a hammer (a masonry hammer or roofing hammer are the best for puncturing duct). If care is to be taken a screw hole can be put in the duct on a marked line and the cut can go from that point both ways. 

Hanging duct does not take two people. It is much easier is two people are used. One to stabilize the duct and one to install the drives and attach to the hanging supports.

The ladder should be place to where one person can easily install the drives and the other can stabilize the duct in the air.

The lap of a drive is one inch.

The duct should lip over the roof curb at least one inch. It also needs to be secured with screws so it does not fall under it's own weight.

every effort should be made to ensure that a curb is level in three directions. Horizontally two ways and vertically. Additionally, the gap between the roof and bottom of the curb should be shimmed so the curb does not bow under the weight of the mechanical unit. 

The seams of duct are sealed to keep the air from escaping from the duct as it is pushed by the furnace through the duct. The less holes in the duct, including at duct connections, the more efficient the air will flow. This means reduced energy costs for the end user and less maintenance on the system over time.