Improving Old House Structure: Reducing Floor Bounce And Sag  With A Suspended Partition

By Bruce W. Maki, Editor

During our old house bathroom remodel project, we were faced with a structural problem: the second floor was not stiff enough. It would bounce badly whenever somebody would take a step. When the house was built in the early 1900's, the builders used 2x8 floor joists to span 16 feet. On the first floor the used 2x10 joists (which current codes require for a span of 16 feet). The 2x8 joists may be strong enough to prevent collapse (they measure 1-3/4" by 7-5/8", a little bigger than today's 1-1/2 x 7-1/4 standard size) but the floor shaking was downright annoying.

Furthermore, the homeowner wanted to install marble floor tiles. Rigid tiles such as stone or ceramic require a rigid floor structure. So we knew we had to do everything we could to strengthen and stiffen the floor.

Our approach had two parts: sister the floor joists with additional framing lumber, and reduce the joists span by supporting the floor near the middle. See our article on sistering floor joists for the first part of this approach.

To support the floor in the middle of it's span, we could have built a beam underneath the joists, since we were simultaneously remodeling the bedroom directly below. But such a beam would have been hard to hide. But the attic above the bathroom had plenty of space, and the large room was going to be divided by a wall. (The project is actually two bathrooms.)  Normally a dividing wall (usually called a partition) would add to the weight on the floor, causing further sag.  I reasoned that I could devise a way to transfer part of the floor's load onto the partition, and then let that load hang from the ceiling joists. But the ceiling joist are only 2x6, although they don't span the full 16 feet width of the room because of the sloping ceilings on the second story. My experience crawling around the attic told me that the ceiling joists were just strong enough to carry the weight of the ceiling plaster, and not any additional weight.

So I had to somehow transfer the load to the walls at the edge of the bath room. I decided to employ a beam of double 2x10's, that would bear upon the outside gable-end wall, and on an interior wall. And the 2x10 beam would "sandwich" the wall studs to securely hold them without needing fancy metal brackets.

By doing this we would take the the load of the floor and pass the forces onto existing wall studs in a way that spread the load out over as many studs as possible. Overall, the house has no extra load (except for the marble floor tiles, about 700 pounds), but some studs have slightly less load and some have more. I felt that it would be a safe bet.

Now, I admit that my mechanical engineering education gives me an advantage over a lot of people. But this is not rocket science. Some simple and sound principles are at work here:

• Do no harm. Only add to the structure, do not take away material from the structure.
• Envision the "load path", or what framing members will be carrying the weight of the floor. The load has to be carried all the way to the foundation, always bearing on appropriate framing materials.
• Calculate the loads involved to see if there are any large and unreasonable loads applied to any individual framing members. (See notes at the end of article)

A View From The Outside: Location of the 2x10 plate attached to the wall studs, in the attic. Location of the new wall. Note the rectangular attic vent at the peak of the gable. It is visible in photos from inside the attic, such as the next shot...

A view in the attic, looking toward the gable-end wall. The pale board is the 2x10 plate that we attached to the existing wall studs. We cleared away the cellulose insulation and removed the ceiling lath where the new wall would go. We kept a stock of planks in the attic for crawling on, to avoid crashing through. (We'll leave that to Tim Taylor.)

For almost all of the framing here we used 3" premium deck screws. (Never rely on drywall screws, they are too brittle and their shear strength is not adequate for load-bearing framing.)

The other supporting end. The pale lumber here is a pair of new 2x6's used to sandwich the ceiling joist directly above the old interior wall. That wall is visible through the hole. These 2x6's run all the way to the ends of the original ceiling joist, and were attached with 3" deck screws.

One advantage to using deck screws is the reduced amount of debris that falls from the walls and ceiling. The wood lath on the ceiling allowed quite a bit of dirt and cellulose insulation to fall through, and hammering only amplified the problem.

Another view of the new 2x6's. One board is higher than the other because of the some backer boards used to hold the wood lath. Note how the new boards extend all the way to the end of the old joist. This beefing-up is necessary so the load from the new 2x10 beam is spread out across as many studs as possible.

A few weeks prior to this we:

• Made a reinforced concrete pier in the basement, directly underneath the middle of this wall.
• Jacked up the main-floor joists directly beneath this wall, using a 12 ton bottle jack. We raised the joists about 1 inch in the middle of their span, which removed about half of the floor's slope.
• Added a supporting column in the basement to help hold up this interior wall. This is part of the "load path" that carries the load all the way to the foundation.

The double 2x10 beam, anchored at only one end. We had to let one end float free until all the studs were installed between them.

The methods used to anchor the 2x10's to the wall. The board on the left was installed first, using a joist hanger. The second board was too close to allow a joist hanger (there was no room to get a nail or screw into the diagonal "toe-nail" holes), so we attached a pair of heavy-gauge angle brackets to the board, and then screwed it to the wall with Simpson Strong-Tie screws, which are meant for attaching framing anchors.

The studs were inserted between the 2x10's, right at each ceiling joist. Each stud was screwed to a ceiling joist and the 2x10's were screwed to the stud.

Another view of a stud-to-joist-to-beam connection.

Of course, getting to this stage meant that the studs had to penetrate the floor deck in the bathroom. The holes were cut with a reciprocating saw, and the studs were merely left dangling (although they were carefully aligned to be plumb). Later we attached the lower end of the studs to the second-floor joists at the same time that we installed the joist sisters.

The bathroom has a ceiling that is just over 8 feet high. We used 10 foot 2x4's for studs and their length was perfect, providing plenty of overlap at each end.

Notice how the gable-end 2x10 is a bit lower than the beam? I assumed that all the ceiling joists were 2x6, but the joist against the wall, (which is really just a cleat) was only a 2x4. Consequently, that board sat a bit lower than expected. In hindsight, the extra money for a 2x12 board would have been well spent.

The new "suspension studs" that support the middle of the floor span. There were four such studs. One stud was omitted to allow for a narrow connecting door between the two bath rooms. The total length of the new wall was just under 8 feet.

The studs simply dangled through holes cut in the floor. They will be attached later.

Later, while sistering the floor joists, we connected the hanging studs to the second-floor frame. We used 5/16" by 6 inch long hex bolts, with very large washers to prevent crushing of the wood fibers.

Some Minor Details:

Since all the walls were going to be covered with drywall, we needed to provide nailing surfaces to support the wallboard.

The original bathroom remodel job, done about 10 years ago (the one that converted a bedroom into two bath rooms), had simply fastened a stud wall against the old and crumbling plaster.

We attached blocking at the corners, so the new drywall panels would have a surface to attach to. Attaching blocking in this fashion is almost impossible with nails (because you can't hold the board in behind the stud and nail into it) but is a snap with deck screws. We just used a Quick-Grip clamp to hold the block in place and zipped in a few screws..

A surprise. Later, we removed the lath on the outside walls, and discovered that on the gable-end wall, the original carpenters had scabbed together 2x4's to reach the highest points. Since gable-end walls are normally not load-bearing, this was once considered an acceptable practice. We screwed the adjoining studs together, and also added numerous pieces of blocking to cross-brace the studs.

This house uses a technique called "balloon" framing, where the studs run continuous from the first floor deck all the way to the rafters. This method was the norm until the early 20th century, when over-logging caused long, clear lumber to become scarce and expensive. Today, almost all houses are framed with the familiar "platform" method, where the first floor walls are erected, then the second floor deck is built, then the second floor walls are erected while using the second floor for a work platform.

Copyright © 2000, 2005 HammerZone.com

Written May 14, 2000
Revised January 11, 2005