How To Build A Soffit
Build a special enclosure called a soffit to hide ductwork and other obstructions
A soffit is basically a box framed with wood and covered with drywall that surrounds existing ductwork, piping, or beams. You'd usually see these in finished basements where various systems couldn't easily be moved. For example, the ductwork for my furnace stretches perpendicular to my home's floor joists, so the ductwork has to hang underneath. I built a soffit around the entire length of the ductwork and mounted some drywall. I also built another one around a beam that supports the floor above.
There are a few ways to do this which depend on what you've got in your basement. My ductwork stretched across 12 feet of open space, and then joined up with a wall I had previously built for another 15 feet. Here, I used two different types of formers that I'll explain shortly. A soffit former is basically a few pieces of 2x3's or 2x4's screwed together to form a U or L-shaped assembly. They're like the ribs that will make up the body of the soffit.In perfect form
To cover any ductwork that isn't near any wall, you would use a U-shaped former. You first screw a series of 2x3's or 2x4's to the floor joists along the entire length of the ductwork on both sides. We'll call these our support studs. This is where you'll mount each soffit former.
The tricky part with building a soffit is to get each former perfectly level. If they're not level, you'll notice bowing in the drywall at certain spots when finished. Since most houses today are built by professional contractors, the floor joists are more than likely perfectly level across the entire length of the floor, so we'll base the size of each former on that.
Furnace ductwork is installed in segments. Each segment is slightly ribbed to add strength while allowing for slight expansion and contraction when the furnace starts and stops. The middle of each segment will stick out more at the top and bottom than it will at the ends. Measure the distance from the bottom of the floor joist to the bottom of one of the ductwork segments at the middle. If you measure at one of the ductwork joints, you'll find the formers won't be tall enough when you come to install them near the middle of the ductwork segment.
Add about half an inch to your measurement to give yourself a little breathing space. My goal here was to have as much headroom when walking under the soffit as possible. However, if you'd prefer to minimize sound leakage in and out of the ductwork, you may want to wrap some soundproofing insulation around the entire length of the duct first, and then take your measurement while taking the extra thickness into consideration.
Use a stud and cut two pieces of wood to the exact length you just measured. Measure the width of the ductwork, then add the width of the two support studs plus the width of the two former pieces you just cut. Cut a stud to this length.
Screw this piece to the two former pieces to form a U-shaped former as shown below.Always on the level
When cutting your wood, I recommend you use a table or mitre saw instead of a handsaw. Each cut has to be precisely 90 degrees so when you screw the pieces together, you won't have to deal with slightly crooked formers all over the place. Crooked formers mean a crooked soffit. Fixing this is a real pain. Believe me, I know what I'm talking about.
With help from a friend, lift the former under the duct and slide it up so each side of the former hugs the duct on both sides. Push it all the way up so each end lines up flush with the outside side of each support stud. Screw one end of the former to the support stud. Line up the other end where it should be and check the form to make sure its level, then screw in that end. If you're alone to do the job, you can use bungie cords and a temporary screw partially driven into one end of the form to hold up one of the ends while you work on the other.
Build the next form in the same way. Mount it to the support studs 16 inches on center away from the first form. Make sure the form is level and check that both forms are level with each other. Then build and mount the next form 16 inches on center away from the last one, and so on. Be sure to use your level as you mount each form and make any adjustments. If your cuts are bang on, and you make sure each form is level relative to each other, your soffit will look really good once it's finished.
Don't think that you can get away with one or two formers that are slightly out of alignment. You'd be surprised how little your eye needs to spot a problem. Get it right now and you'll be alot happier later.
The reason we're mounting each form 16 inches on center from each other is because each sheet of drywall will be supported by four forms. Every fourth form will support the edges of two sheets of drywall. If you don't space out your formers properly, some of the drywall sheets won't line up with the formers and the edges will have nothing to rest on. This will result in cracking of the joint compound if you ever press on certain spots of the soffit.
You can get away with using fewer forms, but you must have a form supporting each edge drywall or you'll get severe cracking. You're not doing this to end up with an ugly home theater, so lets do it right.Up against the wall
If your ductwork happens to follow the side of a wall or any other structure, you'll only have access to a single side of your duct to mount your forms. In this situation, you'd simply build L-shaped formers. One end will hang from the floor joists, while the other end will be mounted to a support stud screwed to the wall or other structure.
Hopefully you have at least enough room between the ductwork and the wall to fit a 2x4 support stud. You have to screw the support stud to the wall exactly the right distance down from the bottom of the floor joist so the bottom of the support stud is level with the bottom of the former piece. If you can't fit a support stud, you can use a series of L brackets. The image shown here is a typical example found in many North American homes.
Measure the width of the ductwork from the wall to the edge of the other side of the duct plus the width of the support stud mounted to the floor joist plus the width of the former piece. Cut a stud to this length and screw one end to a former piece. Lift it up into position as you did with the U-shaped formers and screw one end into the floor joist support stud. Place the other end flush against the wall under the wall-mounted support stud and screw it in place. Don't forget to use your level.
If you have to use a combination of U and L-shaped forms, make sure you plan ahead with your measurements so you end up with a nice, uniform surface under your soffit.
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