some time ago i inquired about fastening some steel plate to the sides
of some 100+ year old, full dimension, rough cut oak floor joists to
stiffen them up. I've decided to sister them with like sized (2x8) wood
instead. These are the second story floor joists and they are 2x8 and
span 15.5 feet. I've got the ceiling out on the lower level and want to
stiffen them up before i go back with new ceiling. My question is, what
is the best way (and frequency) to attach them? Screws, nut and bolt,
nail, glue, etc?? It's a balloon style house and i'll be able to set
the new sisters up on the ledger board like the originals. My main
concern is pulling them together in such a fashion as to best create a
remove the "not" from my address to email
If possible to have enough open time, I'd use some adhesive too. A
bead of construction adhesive along the length, then nail in place.
Give the wood is rough cut and old, adhesive may not do all that well
I've seen a few methods. Driving a pair of 16d nails every 12",
alternatively, a pair of screws about the same distance, and bolts
through. Good reason to buy a nail gun that will fit in the space
I'd take a look here too
I would think the joists are 2' apart. I suppose you know to put the
crown, of the sisters, up. Lift the old joists, a bit, by jacking,
before attaching the sisters, if the lifting doesn't interfer with
anything. X bracing between the joists would hurt, either, if
In order of my preference, with construction adhesive: 1) nut and
bolt, 2) lag bolts, 3) nails... staggered top and bottom every
Thanks for the reply. they are somewheres in the 16" range oddly
enough. (my wall studs range from 12" to 20". LOL! ) Working with this
old house (1871) has certainly been enlightening.....
I was wondering about the nut and bolt thing. If drilling all those
holes would weaken more than helping. I do have a right angle drill and
can do that.
remove the "not" from my address to email
As long as you keep the bolts in the middle third of the joist depth,
the holes won't matter.
If you decide to use double rows of the SDS 1/4" screws keep them at
least 2" from the top & bottom of the joist.
A couple comments & questions......
What are the actual dimensions of the original oak joists?
If they are 1-7/8 x 7-7/8 (or are they larger?) and the dimensions of
the sistering joists are 1.5" x 7.25",
the sistering process will only add 62% to the floor stiffness.
The old oak will probably not be very accepting of nails. :(
This is unfortunate since nailing is a great way to a create a "no
Bolts are a less than optimum way to sister joists....too much slop in
To get the sister joists to accept some of the dead load & to preload
I would suggest jacking up each old joist slightly and holding that
position while attaching the sisters.
My preferred method of sistering the joists is to use Simpson 1/4" x
3" SDS Drive Screws
A single row of screws at 12" o/c should be adequate since each screw
is good for ~200lbs+.
Double rows staggered at 12" o/c would be great but over kill.
I suggest match drilling both joists at 3/16", see how that works for
oak joists to avoid splitting.
These screws should pull the joists together nicely.
Construction adhesive and nails work for me,
as long as the joists are straight and come together.
Simple and I'd say about as effective as anything
else for the application. The point to joining them
together is to prevent them from bowing laterally
and you don't have major force in that direction.
Nails are very good for shear transfer (no slop in holes like bolts)
but that 100 year old oak may not accept them readily.
Depending on the sister joist geometry, the SDS screws may or may not
need to take much shear load.
It all depends on the load path. If the sister joists bear up against
the floor sheathing and the sister joists bear against the ledger
(with or without blocks), then the screws will carry very little
If the sister joists only bear on the ledger & not against the floor
sheathing, the SDS screws will wind up taking load out of the original
joist and into the sister joist.
Getting the sister joist to bear cleanly & continuously against the
floor sheathing will be difficult.
The SDS screws (or adhesive) will do the important job of getting the
old & new joist to work as two springs in parallel.
Lateral support of the new joist is a secondary task for the screws,
composite action is the primary.
when this old house sisters beams they always use a single bead in a
continious lazy S fashion.
using a massive amount of adhesive may not help.
they showed a previosly sistered beam that someone used circle
it didnt adhere, the circles trapped air and prevented adhesion.
in any case use carriage bolts to hold it alltogether..... if its a
bad issue add a steel plate between the sisters for extra stability
Bolts require a hole tat least as large as the bolt shank, this makes
a sloppy fit.
Lags (SDS) screws need a pilot hole (screw root diameter or smaller),
makes for excellent shear transfer..... no slop.
A good nail gun should be able to handle it, but it would need to be tested.
I have seen shipping pallets made of some incredibly hard lumber and nailed
together. The nails will break rather than pull out.
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