Soffit span question

Yasla

Hi, building a soffit in the basement to hide both the HVAC supply and return lines which are side by side. Also, the main gas and furnace air intake run alongside. In order to save as much headroom as possible, I planned to frame the vertical sides to ceiling, and attach them using 1/2" plywood. The plywood would fit up close to the ducts (withing 1/2"), and then I would drywall over that.

Question is does anyone see a problem with a 4' 6" plywood span sagging over time, there would be no central support. The soffit length is about 25 feet. Only the drywall and a single recessed light on one end would be hung on the wood. Is there a better solution?

Thanks,
Joe

Termite

This is a common challenge faced by DIYers and remodeling contractors alike. You can't support the furred down ceiling to the duct, so plywood isn't a bad idea. However, I don't think that you'll have much luck spanning 4'6" with plywood. I think it will deflect in the long run and you'll have trouble with the drywall joints.

How much headroom is currently under the ductwork that you're wanting to fur down? Knowing that, maybe we can find a solution.

If all else fails, I've seen a lot of people deal with this by leaving the framing/HVAC/wiring/plumbing in their basement ceiling uncovered and painting it black or dark green. It gives a sort of "bar" look, and can look good in even a high-end basement finish. Painting it dark kind of takes the focus off of it.

Yasla

The supply duct is insulated, would probably look pretty rough leaving it as is. The lowest height I measured is 6' 11" to the subfloor, only rug will be installed so I figured when done (1" for rug, 1" for plywood & drywall) the span would be 6' 9". I should have mentioned that the supply duct necks down along its length, getting smaller in cross section as it moves away from the furnace every 5 feet or so, this is in the middle space between the two ducts. So technically, I could install a central support easily for about half the run, possible but tight in the 8 feet, and then impossible for about the first 6-8 feet closest to the furnace.

Any new thoughts come to mind?

Additional question: Seeing how 1/2" plywood was questionable for sag over time, would 3/4" be better (stiffer) or worse (heavier)?? Thanks for the quick response.

Termite

Good idea to install center supports wherever possible. If you can support the plywood in the middle I think it will work.

Yes, 3/4" will be heavier. But, it might be a bit stiffer. I'd go for 3/4", but can't say for sure that it will work noticeably better. There's no space at all between the ducts at the wide section?

If you have no choice but to span the plywood that far and framing it down 4" is not an option, I'd use 1/4" sheetrock for the ceiling since it is being applied to plywood. It weighs half as much. You'll need some friends to help you support it while hanging though.