Sistering Floor Joists
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
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.
||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
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
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
|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 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
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.
||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
|The finished job, after removing the hydraulic jack and
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.
Back To Top Of Page
- Basic Carpentry Tools
- 12 Ton Hydraulic Jack
- Pneumatic Nailer (Optional)
- Drills and Driver Bits
- 1/2" Wrenches
- Lumber, 2x8x12'
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