No fitting for a 7-inch tap on a round trunk? A printed notching template does the job

In a video published this week, the crew runs a 14-inch trunk through the attic, reduces it to 10 inches and feeds four living-room registers with 7-inch branches, about 500 CFM in all. Each branch lands on a curved pipe, so its end needs a saddle-shaped cut that follows the trunk.

Blocklayer's pipe notching calculator takes the parent and branch diameters and draws the cut line at full size. The site's instructions: print at 100% printer scale, wrap the paper around the branch pipe, trace the line with a marker and cut. Entering zero for wall thickness makes the cut sit tight against the pipe's outer surface, which suits thin sheet metal. The Stud Pack crew also cuts tabs about an inch apart to fasten the branch to the trunk. MEP Academy describes a similar method for start collars on rectangular duct: the tabs are bent alternately inside and outside the main, screwed, then sealed.

The International Residential Code (section M1601.4.1) says every joint, seam and connection in a duct system must be mechanically secure and sealed, using mastic, gaskets, welds, liquid sealant or tape. Tape and mastic on metal duct must meet UL 181B and carry its marking. A field-cut tap is a connection under that rule, so it gets mastic or listed tape like any factory fitting. MEP Academy notes that saddle taps work best on a main that stays one size, while a reducing tee-wye is the better choice where the main steps down.

The crew's other habits: longitudinal trunk seams always face up, foam saddles carry the trunk in place of hangers, and any change to the duct route goes to the system designer before installation.