Cross bracing 4 new sub floor - duct in way!

[bethanyc]

Good Morning my friends!
I call you this because so many of you have helped me in your own problems and solutions on this forum. Love it, love it, love it!
I need help and can't find my exact problem on here, but I am probably missing it.
I have a 97 Clayton La Vista (single wide) and needed to replace the vinyl as well as part of the sub floor. We have the sub floor repair area (about 2 1/2 feet wide by about 3 1/2 feed long) mostly pulled up. The length of this cut runs perpendicular to the joists. We pulled up small sections of the sub floor between joists to work on one at a time. We are placing 2x4 cross bracing under the old sub floor, allowing for some of the 2x4 to show to act as support for the new sub floor. This area, if you can imagine, is the center walk area of the bathroom. The bathroom is about 5 x 5 on floorspace. The tub sits at the back of the room and it's length runs parallel with the joists. The toilet and vanity are to the right of this repair and a wall is only inches to the left.
Here is my problem. We have soft spots because pieces of the sub floor met in between joists. On top of this, one corner of the sub floor met in between joists at the floor register. That section of floor became incredibly weak. I **thought** that the air duct (sheet metal box) was below the floor and a collar of some sort came up to meet the floor. I planned on bracing on each side of what I thought would be there. Oh no! This duct - cross over from the main trunk- simply has a hole cut into it and was somehow attached to the underneath of the sub floor. It rests in between joists which I need to cross brace for the new sub floor. The vent was positioned, in all their infinite wisdom, directly in front of the tub and toilet so you MUST walk on it. I don't see not bracing it being an option.
So....how in the heck do you manage cross bracing when the air duct lies in between the joists with the top duct being flush with the top of your joists? Can the duct be removed? We have to keep the register closed anyways because it gets too cold/hot for that small of a space. And, if we can remove it, any suggestions how to maybe cut it, pinch the edges and seal it off from the inside of the house without going underneath and cutting into the underbelly?
Since this involves sub floor repair, I didn't use the HVAC forum. Please correct me if that was wrong. Also, this is my first post. I read details are needed, but was this too much?
Thanks for any advice!

Bethany

[Greg]

It's your call as to removing that short section of duct. As you say bathrooms tend to get very warm due the the size. One thing to consider if you live in an area that has cold weather is that the ducts also keep the pipes warm in freezing weather.

If you remove the duct make sure you have a good seal on the main duct so the other registers are not effected.

Greg

[stevieb]

I say if there is more than 1 vent in the bathroom disconnect this one, Close the opening at the main duct with sheet metal and screws, You may have to cut the subfloor back some or even worse case work under the home.
God only knows what they are thinking when they cut these vents. My bathroom vent must be stepped on to get in the bathroom door. I have replaced the vent grill several times.

[JD]

I have run into this problem many times, but I have always found that there was about 1/2" clearance between the duct and the floor. Getting creative with tongue & groove sturdi-floor plywood helps in some cases, but it sounds like that would not help you. I have also used 5/8" plywood approx 12" wide, split between the two sections of subfloor, from joist to joist over the ductwork. I will laminate the bottom of that plywood with Seal-Tite window flashing to insulate the wood from the duct. I use Seal-Tite for lot's of things. Good Stuff. There is usually just enough room to add small pieces of plywood as blocking on the sides of the duct, to support my 12" splint. Everything gets glued and stapled, using PL polyurethane adhesive and 2" - 16ga construction staples. You may be able to use screws, but it is very tight in there. On the splint itself, lot's of glue and screws. 2" o.c. in each direction is not too many screws for plywood on plywood. I will use as thick of a splint as I can, and screws that match the thickness of the splint and subfloor, being careful to not bury the screws any more than absolutely necessary, to be sure the screws don't penetrate the duct.

Again, there has always been enough clearance for me to shove that splint in there, so your situation may be different.

[bethanyc]

Thanks everyone for the help! I am going to try to tackle it today. Unfortunately, I can only work weekends and getting my hubby to do home improvement projects (with or without me) is laughable. He'll do dishes, laundry, mow the lawn, pick up the kids....just don't ask him to use a hammer!

I am going to try to leave the duct, but at least I know it can be removed. It dawned on me during dinner last night (Random time I know) that the reason the other vent openings in the home have collars down to the duct work is because they are from the main trunk line and are running under the joists. The one I am working on is between the joists because it crossed over from the trunk and so they must have an angled piece of duct as a connector or they crimped the duct or something of that nature because this opening was literally attached to the bottom or the sub floor. The difference in height of this duct and the others is the height of the joist! Instead of using a nice short collar of some sort, they just connected the duct to the sub floor!

Mobile home isn't bad living once you can straighten out all the things the manufacturer didn't think of (or care) to do.

Thanks again! I'll let everyone know of the outcome, though I am only doing a little each weekend. It may be a while for the real outcome! lol