15.5 feet. That's crazy.
I did some beefing up. I first jack each joist, then add stiffening. I hand
nailed, and bolted, and screwed. Did not PL Premium. Good thing to do. My
first floor joists are 2x10 12 foot long. I found stiffening works with
only a 5/8 inch sheathing cut from 4x8 sheet. I was not really worried
about stiffening, mostly trying to get rid of the sag. Best done over a
long time period with the jacks. I didn't need to go from end to end on the
joists, just middle.
When I hand nailed, I first drilled holes. Also with bolts. I had to try
and pull together warped boards. Nailing is otherwise sufficient, but add
glue. Check above joists. You may need some shims.
Steve, Simpson has a line of screws meant for what you are doing. If
those joists are as old as you say, they may be too hard for this and
might require predrilling. I would use at least a 400 grade
construction adhesive. You will need a system of strings or laser(s) to
monitor what is going on. I would think you want to get to "in plane"
and then actually crown each joist a like amount, perhaps 3/4 to 1".
Keep the whole world singing . . .
remove the seven
Working on a similar situation I used 3/4" plywood 8" nominal width
and sistered both sides of the old joists. Opted for a slow cure epoxy
for the adhesive, as only a few of the joists suffered from old
plumbing cut outs. The cut outs were filled with new wood as needed
and the whole clamped until final cure, no other fasteners used as
they would only be for show.
Since you are dealing with old oak, this might be your best option.
Also, rough cut lumber is best secured with epoxies for maximum
adhesion as the resin tolerates considerable unevenness with no loss
Joe's suggested fix is a very good one.
I "top" sistered my living room ceiling / attic floor joists,
orignally full sized 2x4's that I "glued into" 2x8's using a slow cure
epoxy paste (Sikadur 35?).
I just mushed them together and let them cure, no clamping. I tested
a trial sample and the "glued up" 2x8 failed at a bottom chord knot
before the glue budged.
The Sikadur 35 is a heavy paste that allows a bond line as thick as
If you have oak up there I would be thinking about a way to display it
and not put up a ceiling . I cant imagine the oak beams you described
not being strong enough for the job. They have worked for 100 years
they will be doing the job in 100 more. Don't even think about nails.
"too much span for 7-1/2" of wood"
Depending on how much bounce you want to remove, you might consider
using an engineered wood product.
Steve's situation is an issue of stiffness not strength.
Too much bounce means he needs to add stiffness and that relates
mostly to joist depth.
Per the joist calculator mentioned recently in another thread, a 15'
span needs a 2x10 of decent timber or a 2x12 of lesser material.
If my calcs are correct, Steve's original joists sistered with new
2x8's will be stiffer than a 2x10 by 13% but t only be 55% as stiff as
How can you measure stiffness without testing the plank?
Do you use an accepted norm?
This stuff is beyond me, except I can see how glue would work well
I always clamp glue, but never used epoxy.
I use a lot of drywall screws and lag bolts to slap stuff together.
My brother was a general and I listen to him for some things.
He told me once that 2 nails can be better than 6 screws for resisting
shear in some circumstances.
Don't remember what we were talking about, maybe roof hurricane
Of course stiffness can be determined by testing but stiffness can
also be calc's based on the joist geometry & material.
Especially in Steve's situation where he's interested in boosting
An idea of the existing stiffness can be calc'd as well as for the
proposed sister. Added 50% might help, adding 100% or more would be
WIthout an idea of where his floor is compared to "idea" he might be
over killing or way under shooting.
Take a look at this joist calculator. You can "play" with the joist
size, timber species & timber grade and see how the allowable span
Drywall screws should only be used for drywall and temporary, rather
than permanent loads.
" He told me once that 2 nails can be better than 6 screws for
resisting shear in some circumstances."
that's a pretty general statement, I know for a fact that a single
1/4" SDS will take the place of a handful of nails.
His statement tends to be more correct for small screws but IMO screws
(SS) are better in a lot of locations (exterior exposure & esp open
patio covers) since over time they will not rust and encourage rot or
lose strength. Galvanized timber connector nails will last a long
time, SS ones even longer but #10 SS screws in the same application
will last a long time plus provide better withdrawl resistance.
I know nails are shear fasteners and better than screws in shear but
over the years I've seen those shorty connector nails rust out / rot
out in patio covers to the point that I changed my tune.....
what good is a screw that's rust away & help rot out the wood?
IMO better to drill out the hanger nail hole and use a larger dia SS
screw (larger root diameter) than stick with nails that I know will
cause the failure of the entire system.
I always clamp (or screw) when using traditional glues.
When I top sistered my ceiling joists there was no easy way to clamp
so I used epoxy paste that was forgiving of joint gaps.
I pretested a sample made with rough timber, dirty just brushed off.
The sample failed at 175 psi shear when a knot in the bottom member
Based on my crappy sample, I knew that my sistered joists would
perform better since I wire brushed both faces of the glue joint and
none of the bottom chord members had large flaws. Plus, based on my
calcs, my shear demand was WAY lower than 175 psi more like less than
Reason I asked is I put 2 extra ceiling joists in my garage when I
bought this place.
How it was built didn't seem right. 6' centers in some places.
Probably built in 1959.
Those joists are 21' 2x6 and from what I can figure basically just tie
the top sills together and square.
Good place to hang lights and the door opener too.
Anyway, I've seen too many garages fall out of square.
Besides that I wanted to put plywood up there as a storage platform.
When I ordered the joists from HD I said pine, because I thought
that's what the old ones were.
They said they only had Douglas fir that long, so that's what I got.
Precut the angles, jigsawed a hole in the fascia, and slid them onto
the plates. Nailed them in. Patched the fascia hole.
A couple years later I put up alum gutters, soffit/fascia.
Put a few 4x8 x1/2 plywood up there and started storing stuff, nothing
Boxes of Christmas decorations and the like.
Garden tools in the winter, snow shovels in the summer, etc.
Then I told my kid to get rid of the 4 tires he was keeping in there
because the alloy rims were "special," or put them up on the platform.
I was tired of them getting in my way.
He did that. 2 stacks of 2, alongside each other.
A couple years later I was chatting with my 80-year-old neighbor in
the garage and he pointed out one of my new joists was sagging.
He's retired building trades and has the eye for it, but when he
pointed it out I saw it.
Tires came down and it sprung back up.
6" won't take much load over 21'.
I learned that quick.
But I'm still tripping over or moving those tires.
I started using them to put together 8' workbenches at least 30 years
and they're good for that. You can knock them down if you have to,
though I never did any of 6 or so I built.
No real load on the screws. Solid as the day they were built,
Fasteners are a science in themselves. I wouldn't do anything
structural without expert advice on fasteners.
Vic, those "joists" you describe from the 1959 garage are not
They were not designed or installed to resist bending loads.
They are "tension ties" to resist the spreading force generated by the
To span 21' as a joist you would need at least a 2x10.
I stand by my comments on drywall screws. They are easily the most
mis-used fastener in America.
They are not the appropriate fastener for anything other than
drywall. Hacks use them to hang kitchen cabinets.
Bob, just wondering your if your calcs take into account the existing
wood is 100+ year old oak.Usually these timbers were made from
heartwood. It is surprising to me it is not rigid enough already. I
would look to see if it is in fact the joist that are giving when he
walks on them and how they are giving. The problem could be that the
flooring is not strong enough to properly distribute the weight from
joist to joist. Lack of a proper subfloor could cause this which is a
common condition in old homes. Just something else to think about.
oh, believe me, when you walk across the west one third of the room and
the china hutch on the east wall jingles, it's the joists moving. <G>
And no 7.5" of wood of any vintage or species is good for a 15.5' span.
I redid the lower level with 2x10's and put a support beam down the
center of THAT span. Of course it wasn't a problem to do, since the
joists are only 8" off the dirt!
remove the "not" from my address to email
The wood is not the problem. The joist depth & span are the issues.
Steve made no mention of the subfloor construction and, yes,
the subfloor construction can influence overall floor behavior but
it's pretty much a secondary effect.
Steve could "box out" his wimpy joists with a lots of blocking, lots
of glue and a structural skin on the bottom surface of the joists to
create composite action floor. This would increase the floor stiff
and get adjacent joists to share load. It would be a LOT of work and
still probably not work as well as sistering especially if Steve used
A number of year ago I worked on a floor vibration problem. It
involved a kitchen and living space over a 25' garage. Unfortunately,
the joists were spec'd as 12" floor trusses. They skimped on the depth
to stay within a city overall building height limitation. The condos
were three story.
The 12" floor trusses were too flexible, should have been at least
14". A real PITA to fix.
just for informational purposes, the flooring upstairs is 1x6 yellow
pine T&G 90 degrees to the joists. Same age as. The ceiling downstairs
will be 1x4 pine T&G (painted) so i won't get any benefit from that
either. I'm mainly just not wanting the china cabinet upstairs to sound
off (as bad, i know i won't eliminate it entirely) every time we walk
across the bedroom or sewing area.
remove the "not" from my address to email
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