Sistered floor joists increase structural strength and stiffness. Bounce-Be-Gone:

Sistering Floor Joists

In This Article:

After the floor is raised slightly, new lumber is placed beside the original floor joists and nailed in place.

Related Articles:
Skill Level: 3-4 (Moderate to Advanced) Time Taken: 12 Man-Hours

By , Editor


What Is Joist "Sistering"?

Sistering a joist simply means attaching more material to the side of the joist. This can mean a new joist of the same size and length is screwed or nailed firmly to the old joist, or it can be a smaller structural member. Sistering could also involve sandwiching the old joist with new material on both sides. Typically this involves framing lumber, but it could involve engineered lumber, structural steel or formed steel joists that are made of heavy gauge sheet metal. 

There is a fundamental problem with sistering joists: while a certain size of lumber may physically fit in beside the old joist, getting the new board into position usually poses serious problems, because of walls, ceilings, and floors that were added after the original joists were dropped in place. In the house we worked on, there was ample room for new 16 foot long 2x8 joists, but the only way to install them would have been to cut holes in walls or remove the sub-floor of the room above.

So often joist sistering involves using multiple pieces of lumber that are not as long as the original joist.  This approach can greatly improve the strength and stiffness of the joists. But the placement of the joints between boards can make a difference.  Joints should be placed as far from the middle of the span as possible. In this article, one alternative would be to use 14 foot long 2x8 lumber, placing the board so that neither end quite reaches the ends of the original joist.


Project Background:

The one hundred year old house in this article had very springy floors. The house consists of four sections, each 16 feet wide. The first floor was framed with 2x10 joists. It has a minor problem with deflection. But the second floor was built with only 2x8 floor joists. (Today this would not meet building codes.) The floor did not seem to be in any danger of collapsing, but was very "springy". When the homeowner's 70 pound dog walked across the room, the floor would shake considerably. With the intent of preparing the room for floor tile, we decided to spend a few hours and a few bucks to reduce the flexing of the floor structure.


Floor joists before adding extra structure. The ceiling just after the wood lath had been removed.

We had already done some wiring changes, so we had to disconnect a few wires that penetrated the joists we were going to double-up. 


This house is a little different than usual. The room above is being remodeled at the same time, and a partition was built to divide that room.

A normal partition would simply mount between the floor and ceiling, but we decided to try something different. We used long 2x4 studs to connect the second floor with a beam in the attic, so the second floor literally hangs from a sturdy structure above. View that article.

This partition runs perpendicular to the floor joists and divides the 16' joist span into two spans of about 10' and 6'.  If the partition ran parallel to the floor joists, we would have to rely strictly on joist sistering to improve the stiffness.

Several weeks prior to this project we had removed the plaster from the walls and installed new insulation. We could still access the space at the ends of the joist, to allow our sisters to reach the ends of the original joists.


Reaching the ends is not a serious requirement, however, just a small benefit. Without getting into a long technical discussion, let me say that the most benefit comes from adding material to the middle of the joist span, and that the ends of the joist experience mostly shear stress, which is less of a concern than the tensile stress on the bottom-most fibers of wood at the middle of the span.


We simply hoisted the joists into place and popped one nail at each end. The air nailer really sped things up.

Note the Quick-Grip clamp used to hold the board up.

The two sister-joists met right under the stud, which had been dangling from above, attached in the attic to a beam.

We tacked the board in place.


In a few minutes we had 10 pieces of lumber held loosely in place ( we did 5 joists).


A view looking up, showing the ends of the sister-joists and the studs that will support them.


Since the floor in this house sagged as well as felt flimsy, we decided to lift the floor a fraction of an inch before we permanently fastened the sisters to the old joists. 

The Heavy Artillery:

We placed a 4x6 beam under the middle of the joist span (not under the hanging partition) and with the aid of a 12 ton hydraulic jack, we lifted the floor about 1/4".

This is an article in itself. View that article.


With the floor held up by the hydraulic jack and lally columns, we used some bolts and BIG washers to clamp the hanging studs to the joists.
  • The bolts are 5/16" x 6" long.
  • The washers are 5/16", 7/16" and 7/8". 
  • We used two of each washer per bolt.


The washers were arranged in ascending size. They need to be kept concentric before they are tightened.


We used one or two Quick-Grip clamps to hold the stud in place while we tightened the nut and bolt. In hindsight, 3/8" bolts would have been better because there would have been less risk of breaking a bolt while tightening.

Because of the huge washers, we experienced no crushing of the wood fibers.

Because the allowable bearing pressure on wood is so much less than steel (about 400 to 600 pounds per square inch versus tens of thousands of PSI) the force of the bolt needs to be spread out over a large area. If smaller washers are used, the wood fibers become crushed long before the bolt is tight enough to clamp the pieces together. Simply using washers the same size as the bolt is never adequate when bolting wood together.


After using the bolts to sandwich the sisters between the stud and the old joist, we proceeded to permanently attach the sisters to the old joists.  We used our nail gun, of course, with 3-1/4" nails.  If anybody has to do this by hand, we'll gladly send them a sympathy card.  Nailing like this is just plain old work! It's even a bit of a chore with the gun.

After the nailing, we drove 3" deck screws (not drywall screws) from the old joist into the new wood, just for kicks. We pre-drilled the holes to prevent splitting the old, hard wood. (Plus it's hard to drive screws through that old lumber.) This operation took two people about half an hour.

We installed solid blocking at the middle of the 10' span...
...except around the wiring, where we used cross-bracing. These were attached with 3" deck screws.


Connection between supporting wall and beefed-up floor joists. We drove deck screws to connect the stud to the joists, for extra protection. (We have seen wood dry out and shrink, causing bolts to loosen)
The finished job, after removing the hydraulic jack and supporting columns.


What a difference a hundred dollars in lumber makes.

The floor is much more stiff than before. I would estimate that 80% to 90% of the bounce is gone.  I suspect that the majority of the improvement is from the hanging-partition method of supporting the floor. Getting this kind of improvement with sistering alone might require either double-sistering, or using a sister that was one or two sizes larger that the original (i.e. using 2x10's or 2x12's instead of just 2x8's).

Whatever the case, there are some simple concepts here:

  • As long as we only add material to the original structure, there should be no harm caused by our work.
  • The added material adds a small amount of weight to the load-bearing walls of the house. Normally this is not a problem.
  • In our area, building permits are not required when supplementing an existing structure, as long as no load-bearing members are altered. This is considered a repair, which in our area does not require a permit. Check with your local building department.



Tools Used:

  • Basic Carpentry Tools
  • 12 Ton Hydraulic Jack
  • Pneumatic Nailer (Optional)
  • Drills and Driver Bits
  • 1/2" Wrenches

Materials Used:

  • Lumber, 2x8x12'
  • 2x8x10'
  • Nails


Back To Top Of Page 

Before You Hurt Yourself,  Read our Disclaimer.

Search Page

Home  What's New  Project Archives  H.I. World

 Rants  Contact Us




Copyright © 2000, 2005

Written April 6, 2000
Revised January 11, 2005