Q: After being ignored at Home Depot, I am coming to you for help. I am turning a dining room into a bedroom, and I want to build an interior wall with a door.

I found the prehung doors OK. But I am a bit confused. The width of the jamb on these doors is 4 1/2 inches. I figure I will center the 2-by-4 framing in the middle of the jamb. That should leave me with 1/4 inch remaining on each side of the door jamb. I would then use 1/4-inch drywall on each side, which would make it flush with the jamb. Correct?

This should be a fairly basic weekend project, but I can’t get past visualizing the installation of the door. Can you help? Also, what is the best way to soundproof the new wall? Can you please correct me if I am off track?

A: We applaud your willingness to tackle this project yourself. It’s projects like these that both of us cut our remodeling teeth on.

Indeed, you are off track. We know the terminology can get a little confusing, so we’ll try to give you a step-by-step guide for installing the new partition wall. One word of caution: Your time estimate may be a bit optimistic. Taking the project from soup to nuts will probably take more than a weekend, but not much more.

Confusion about the thickness of the wall stems from the assumption that a 2-by-4 actually measures 2 inches by 4 inches. That was the dimension of the board in its rough-sawn state. After milling, a nominal 2-by-4 framing member actually measures 1 1/2 inches by 3 1/2 inches. The 4 1/2-inch-wide jamb on the pre-hung door is made to accommodate 1/2-inch drywall on each face for a total wall thickness of 4 1/2 inches. So no 1/4-inch drywall for you.

Now, as to framing the wall: You’ll need a stud every 16 inches and top and bottom plates that run the length of the wall. Since it doesn’t bear any weight, you can get by with a single top plate for this type of retrofit. Realize, though, that if you were framing the house from scratch, two top plates would be used with the top one being notched into the top plate of the intersecting walls to stabilize the partition wall.

In addition, you’ll need framing material for the door opening. Figure one stud for each running foot of wall. A 12-foot wall requires approximately 12 studs, but get a couple extra to account for mistakes. You can always take them back or keep them around for your next project.

The first thing to do is decide exactly where the wall will be placed. Remove the baseboard from the walls that the new wall will intersect. Do it gently so you can reuse the base. Then, with a chalk line, snap a line on the floor. If the floor is carpeted, cut the carpet along the line and fold it back to expose the subfloor. There’ll be some carpet repair to do at the end.

You may need to install some blocking in the ceiling and in the walls to anchor the partition wall. The top plate should be nailed either to the ceiling joists or to 2-by-4 blocking that bridges the ceiling joists. The studs on each end of the new wall should be nailed to the top and bottom plates of the intersecting walls and to a block that you have installed in the framing halfway up the wall.

To determine if you need to install blocking in the ceiling, look at the direction of the ceiling joists. If the attic is accessible, take a quick look from up top. That will tell you the direction they run. Ceiling joists tend to run the same direction as the rafters, though not always. If the joists are perpendicular to the new wall, no blocking is required. Just nail the top plate into the ceiling joists. If not, it may be time for a little surgery.

Try to install the blocking from the attic. This will save you from having to patch the ceiling. If that’s not possible, you will have to remove enough drywall to expose the bay between two ceiling joists. Toe nail these 2-by-4 blocks into the ceiling joists every 4 feet. That will give you something solid to nail the top plate to. Then patch the holes with drywall. Don’t tape and texture at this time. Do the same thing on both intersecting walls.

Next, frame the wall. Using a level, from the chalk line you have snapped on the floor, draw plumb lines from floor to ceiling on each side wall. Then, snap a line between the two walls.

Now lay out the wall. Place the top and bottom plate side by side on the floor. Begin at one end and measure 15 1/4 inches and draw a line across both plates. Mark an “X” on the side of the line away from the wall. This mark will ensure that the center of the stud falls on the 16-inch mark. Thus, the studs will be 16 inches on center. That allows the edge of the drywall to bear on a stud with enough room for the adjoining sheet to also attach.

From the first line you drew, mark lines every 16 inches to the end of the wall. Locate the door. It’s helpful to place it next to a stud.

The door opening should be framed 2 inches wider than the width of the door. For example, a 30-inch door takes a 32-inch framed opening. The framed height is also 2 inches higher than the height of the door.

Studs are nailed to the floor plate and top plate, cripple studs measuring 80 1/2 inches are nailed to the studs and a header is nailed to the top of the cripple studs and to the studs. When the bottom plate is removed, the frame opening should be square, plumb and measure 82 inches high by 32 inches wide. The measurements assume a standard 80-inch-high door.

Some folks nail the wall together on the floor and tilt it up into place. But in a confined space like a finished room, this can be difficult. With jobs like yours, we’ve found it easier to nail the wall together stick by stick. In either case, don’t cut out the bottom plate for the door until the wall is built and you’re ready to install the door.

To build the wall piece by piece, start by enlisting a friend or two to help hold the top plate to the ceiling and nail it to the joist (or blocking). Nail the bottom plate to the subfloor. Then nail the studs in place, leaving room for the door opening. Finally, frame the door opening. A doubled 2-by-4 works for the header. When the wall is completely framed and the bottom plate is secured, cut out the bottom plate from the door opening.

Put drywall on both sides of the wall. We like screws for attaching the drywall. Nails tend to work their way out of the studs and make ugly dimples. Tape and apply whatever texture you like. This is the time to tape and texture the patches created when the blocking was installed. Add baseboard, door casing and paint — and you have two rooms where there was once one.

For a little sound suppression, install nonfaced batt insulation in the cavity. Make sure it’s not compressed.

By the way, we’re disappointed at the lack of help you got at the home center. It’s been our experience that these huge retailers tend to hire sales folk who are ex-tradesmen and generally know their stuff. Perhaps you just caught someone on a bad day.

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