How to prepare HVAC duct register opening?

[jbaratta]

Hi - I'm finishing my unfinished basement and running HVAC duct extensions out from the trunkline (which runs down the center of the house) out into the new rooms. I'm using 5" pull-offs and 5" round pipe to end at 4"x10" rectangular register drops. The registers will be installed into the cielings in the room through the overhead drywall (which I'll install/cut later). Do I have to "frame in" the register ductwork (the 4"x10" sheetmetal) with backer so there's something to screw the register fasteners (screws) into? Or should I bend 1/2 inch of the register duct over the drywall cutout (making a slight "lip") for the screw to penetrate? Or will the register screws simply go into the drywall once installed? Also, how far down should the register ductwork hang for the rough-in: flush with the bottom of the cieling joists, or flush with the drywall (like electrical boxes), or extend beyond the drywall cutout by some amount and fold over flat? Haven't done this part before (the rest of the duct work seemed quite easy). Thanks!

[Dean]

The register boots should be flush with the finished Sheetrock.
If they are a little short of being flush it won't really hurt anything but naturally if they overhang then the registers will never be tight up against the Sheetrock.
Register boots that wind up hanging down too low are a bear to trim off later.
A register screw into Sheetrock will never hold, you need sheet metal or wood above the ceiling.
A sheet metal angle attached to the short sides of the register boot 1/2" (or the thickness of the ceiling) up from the edge work fairly well to hold the register screws and also to span across from joist to joist to hold the boot in place.

If the registers are going to be running longways in the bay then I would use a couple of 1 X 2's that would be on top of the ceiling attached to the boots .

One thing to keep in mind is any screw heads inside the boots need to be up high enough so they don't hinder the registers from going up inside and for that reason I would use small shingle nails when using wood.

[jbaratta]

Thanks Dean!!

[imeduc]

In the basement runs we use a 6" pipe and take off to a 2"X12"-6" 90o boot. this gives you a much better throw down for heat in the basement nail the boot to a joist and yes cut up 3/4" on the side and turn the tabs out above the drywall this will give the registers screws some thing to hold to. 5X10 on a 5 " pipe wont cut it.

ED

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