joist sistering question


F

franz fripplfrappl

To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both
sides.

Glue and screw
or
Glue and nail?

It'd be easiest to use the pneumatic nailer but would screwing be
stronger?

Also, what spacing for either screws or nails?
 
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Z

Zephyr

To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both
sides.

Glue and screw
or
Glue and nail?

It'd be easiest to use the pneumatic nailer but would screwing be
stronger?

Also, what spacing for either screws or nails?


Why dont' you just put in another 2x8 instead of some ply?
less screwing / nailing.
 
D

Don Phillipson

To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both
sides.

Glue and screw
or
Glue and nail?
Why either? I put bolts through, so as to
enable tightening equally from either side.
 
R

RickH

To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both
sides.

Glue and screw
or
Glue and nail?

It'd be easiest to use the pneumatic nailer but would screwing be
stronger?

Also, what spacing for either screws or nails?
jack the centers up a little to crown the old joist before you sister
anything, and the job will come out better and stronger. I'd just go
with another 2x8 that way you are not limited to the 8 foot span of
plywood.
 
R

RickH

good 2x8's are difficult to find

--

=================================================
Franz Fripplfrappl- Hide quoted text -

- Show quoted text -
Go to a real lumber yard and get some douglas fir if you want strong,
you're right to stay away from the "prime" grade "whitewood" at HD,
that stuff is soft, does not have a long grain, sure it looks perfect
and straight but it's better for walls than joists. Some fir, even if
slightly twisted makes a better joist than "whitewood", main thing is
that it be straight along the small sides because the twist will be
flattened out after you bolt it, nail it, glue it, etc anyway. Lay
the small edge on concrete if it rocks or gaps too much, then pick a
different board, but a slight crown is not necessarily bad either.
 
J

J. Clarke

RickH said:
Go to a real lumber yard and get some douglas fir if you want
strong,
you're right to stay away from the "prime" grade "whitewood" at HD,
that stuff is soft, does not have a long grain, sure it looks
perfect
and straight but it's better for walls than joists.
I don't know what "whitewood" is but my local Home Depot has
structural grade Douglas fir, same as the local "real lumber yard".
 
F

franz fripplfrappl

Go to a real lumber yard and get some douglas fir if you want strong,
you're right to stay away from the "prime" grade "whitewood" at HD, that
stuff is soft, does not have a long grain, sure it looks perfect and
straight but it's better for walls than joists. Some fir, even if
slightly twisted makes a better joist than "whitewood", main thing is
that it be straight along the small sides because the twist will be
flattened out after you bolt it, nail it, glue it, etc anyway. Lay the
small edge on concrete if it rocks or gaps too much, then pick a
different board, but a slight crown is not necessarily bad either.

Good tips. Thanks.

I've seen some very bad boards at the big box stores: warped, checked,
split, open knotholes, etc.
 
B

Bart

Nails have better shear but screws better pullout.

Pull-out is not an issue for this application. Plus, the glue will do
a great
job of keeping the ply against the joist. Spare yourself the wasted
time.
Glue, Clamp, Nail.
That's what I've done to 'cap' trusses in 100yr old house and garage.
Plan to do the same with the roof joists to remove some sagging.
 
J

J. Clarke

Bart said:
Pull-out is not an issue for this application. Plus, the glue will
do
a great
job of keeping the ply against the joist. Spare yourself the wasted
time.
Glue, Clamp, Nail.
That's what I've done to 'cap' trusses in 100yr old house and
garage.
Plan to do the same with the roof joists to remove some sagging.
FWIW with an impact driver Spax screws go in just as fast as nails.
 
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A

aemeijers

franz said:
To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both
sides.

Glue and screw
or
Glue and nail?

It'd be easiest to use the pneumatic nailer but would screwing be
stronger?

Also, what spacing for either screws or nails?
Insufficient data- what problem are you trying to solve? Is joist
damaged, is floor bouncy or sagging, are you planning to add a grand
piano and a waterbed above, or what? What you are describing is
gusseting, not sistering, and is usually only used on a damaged joist
that would be a PITA to switch out or sister, which means setting
another real joist right beside it, going all the way out to where the
ends of the joist rest on the sill plate, crossbeams, or whatever.

Gusseting to stiffen a joist is usually done with metal plates, by the
way, through-bolted through the joist. If the joist is sagging, you have
to jack things back into square before you start. If it is just a bouncy
floor, adding or replacing the 'X' braces between the joists may be all
that is needed. Traditional cure for a too-long span is to add a beam at
the midpoint, held up by screw columns, or grafted into the structure of
the basement or lower-floor walls.
 
B

BobK207

I'm going to be doing the same thing to some 16' spans of 2x8 rough cut oak
joists.  I'm using 1/4 X 5 flat bar and 3/8 nuts and bolts.  Right now, i'm
in the process of jacking them up slowly from underneath to take the belly
out of the floor.  After I go about 1/2" past level, (about another month)
we'll bolt the steel plates to the side of every other joist and we'll have
a solid and level floor once again.

s

Steve-

You state:rough cut oak joists. I'm using 1/4 X 5 flat bar and 3/8 nuts and
bolts. <<<<

Do I understand correctly that the 1/4" x 5" flat plate will be added
to the side of the oak 2x8? If so, depending on how it is placed, a
plate added to one side will only increase the 2x8 by about 50%. Two
plates nearly double. If you really want to boost the joist
stiffness the, plate needs to be deeper (closer to the 8" dimension of
the oak). Through bolting, for all the extra work compared to short
lags from each side (Simpson drive screws), doesn't really buy you
all that much.

cheers
Bob
 
B

BobK207

To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both
sides.

Glue and screw
or
Glue and nail?

It'd be easiest to use the pneumatic nailer but would screwing be
stronger?

Also, what spacing for either screws or nails?
Why are you wanting to "beef up" these joists? What you want to
accomplish drives the retrofit design.

cheers
Bob
 
B

BobK207

Thanks Bob, for the input.  My dad ( a lifelong boilermaker ) and I kind of
farmer engineered the idea to stiffen up and straighten a bellied upper
floor.  I already have the cribbing and jacks in place slowly 'bending'the
joists back the other way.  I'll consider putting a wider plate on.  As it
is, i'm spending about $500 to do every other joist.  Perhaps i should go to
a 6 or 7" plate eh?  It'll just be my wife and I upstairs, one side is her
sewing room and the other side is our office.  We already have it aboutas
loaded as it will be.  I hate the bounce though.  This house is 1877 and was
built to the specs of the day for balloon construction.  I see why they
don't allow 2x8 to go over 12' nowadays. <G>.  I already re-did the entire
downstairs floor and put a support in the middle as well as 2x10's in the
middle. (the heavy traffic and stairs area).

steve




<You state:
 >>>>>I'm going to be doing the same thing to some 16' spans of 2x8


cheers
Bob

The depth on the plate gives you the most bang for your buck. I'd
go thinner & deeper rather than shallow & thicker,

The stiffness of the side plates (& beams for that matter) go as the
depth cubed but only linear with thickness thus a 7.75 deep piece is
about 3.7 x stiffer than the 5". Will a full 8" piece fit or would
it have to be trimmed? :(

Are we talking about stock "flats" or are they going to shear it out
of plate? If its going to be sheared oyu can get any depth you want.
Sheared plate is fine, don't need any fancy CR flats here. Yeah, the
CR flats will be staight, flat & true but sheared out of HR plate
should be a lot cheaper. Plus the oak joist in the weak direction is
WAY stiffer than the plate, so the screws will pull it fla, if
necessary. Just start screwing mid span & work symmetrically to the
ends to avoid a bulge.

I'd have the shop punch holes for the Simpson drive screws (the short
ones), for oak you'd have to pre-drill. I'm thinking two rows 12" o/c
staggered is probably enough. Six inches o/c is a little crazy.
Maybe 8"? The screws are there to keep the plate for buckling out
of plane. With the joists jacked, you'll be sure to get the plate to
take load.

I'm not a huge fan of asymmetric installation but I would consider
doing every joist but only a single side plate to keep the material
cost close to the same .............but now all the joists are nearly
2.5x stiffer than the oak alone. You'll be getting performance more
like 2x10. :)

maybe someone else in the ng can take a look at my concept & make sure
I didn't make a gross error.

cheers
Bob
 
W

Wayne Whitney

I'd have the shop punch holes for the Simpson drive screws (the short
ones), for oak you'd have to pre-drill.
The Simspson SDS 1/4 x 2 seem like a good choice. BTW, the holes for
the screws need to be a hair over 1/4", I had some holes punched at
1/4" exactly and had to ream them out a little to allow the screws to
pass easily.
I'm thinking two rows 12" o/c staggered is probably enough. Six
inches o/c is a little crazy. Maybe 8"? The screws are there to
keep the plate for buckling out of plane.
So the screws are there are to transfer the shear load to allow
unitary action and then as you say to keep the plate from buckling out
of plane. How does one design the fastener pattern for each of these
purposes? Obviously for the buckling, the plate thickness is
important. So the fastener pattern will determine the answer to
another question raised, what is the minimum plate thickness?
maybe someone else in the ng can take a look at my concept & make sure
I didn't make a gross error.
Looks good to me: E_steel / E_oak is about 18, and 2" / (1/4") = 8, so
a full height steel side member is 2.25 times as stiff.

Cheers, Wayne
 
B

BobK207

The depth on the plate gives you the most bang for your buck.   I'd
go  thinner & deeper rather than shallow & thicker,

The stiffness of the side plates (& beams for that matter) go as the
depth cubed but only linear with thickness thus a 7.75 deep piece is
about 3.7 x stiffer than the 5".   Will a full 8" piece fit or would
it have to be trimmed? :(

Are we talking about stock "flats"  or are they going to shear it out
of plate? If its going to be sheared oyu can get any depth you want.
Sheared plate is fine, don't need any fancy CR flats here.  Yeah, the
CR flats will be staight, flat & true but sheared out of HR plate
should be a lot cheaper.  Plus the oak joist in the weak direction is
WAY stiffer than the plate, so the screws will pull it fla, if
necessary.    Just start screwing mid span & work symmetrically to the
ends to avoid a bulge.

I'd have the shop punch holes for the Simpson drive screws (the short
ones), for oak you'd have to pre-drill.  I'm thinking two rows 12" o/c
staggered is probably enough.  Six inches o/c is a little crazy.
Maybe 8"?    The screws are there to keep the plate for buckling out
of plane.   With the joists jacked, you'll be sure to get the plate to
take load.

I'm not a huge fan of asymmetric installation but I would consider
doing every joist but only a single side plate to keep the material
cost close to the same .............but now all the joists are nearly
2.5x stiffer than the oak alone.  You'll be getting performance more
like 2x10.  :)

maybe someone else in the ng can take a look at my concept & make sure
I didn't make a gross error.

cheers
Bob


More good info here, thanks. So you think i could go how thin? 3/16" even
1/8" and still do the job? Oddly enough, the old rough cut joists are
almost a full 2" thick, but they are only about 7-3/8" wide like a modern
one. I do have a friend in the steel fab business that could shear the
pieces of plate and punch them.

steve
I've done this sort of thing in various forms of the years.

And the question always comes up "how thin can we go?"

Well, thinner is cheaper but at a certain point it would not be much
better than paint.

I have a machine shop buddy as well & he keeps the "aerospace
engineer" in check. He may not be able to "run the numbers" but I've
never seen anything he's built fail & nothing he build is wasteful of
material. Working together we've come up with some really cool,
cheap & workable designs. I guess in 30+ years working metal he
learned some stuff. :) And his limit for this sort of thing is 1/4"

I could give a long winded (typical, I guess) answer about thickness
but the short answer is:

I'm ok with 1/4" but thinner (3/16 or less) makes me nervous.


Long answer:

The 1/4" added to a single side more than doubles the joist stifness,
so thicker really isn't needed.
Thinner runs the risk of the plate not acting as a unit with the
joist>>>>>>that means more screws.

The out of plane (sideways) buckling of the system is driven by the
oak joist. The joist in the weak sideways direction is ~30x stiffer
than the steel plate. That's why we need to screw the plate to the
joist. If we went thinner the sheet would be even more likely buckle
& curve away for the joist. it would have very little sideways
stiffness of its own (~40% of the 1/4" plate) so you'd need more
screws

How many more, I'm not sure (I'd swag it at ~2x) & I'm really not sure
how to calc it.......time for a lunch with some of my former students
to get their input. :)

Going from 1/4 to 3/16 saves ~25% material cost but IMO drives the
risk of the retrofit not working, way up.

BTW the span on these joists is ~16', what is the spacing?

The reason for this question is......are you sure the bounce is coming
from the joists? If the joists are more than 16"oc the subfloor
could be the problem.

IMO 1/4" is still real "plate" material......3/16 drops into the
category of sheet.

Steve, Wayne is correct about oversizing the hole for the SDS. I'd go
only about .010 but check the proposed hole size with some screws.

cheers
Bob
 
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B

BobK207

The Simspson SDS 1/4 x 2 seem like a good choice.  BTW, the holes for
the screws need to be a hair over 1/4", I had some holes punched at
1/4" exactly and had to ream them out a little to allow the screws to
pass easily.


So the screws are there are to transfer the shear load to allow
unitary action and then as you say to keep the plate from buckling out
of plane.  How does one design the fastener pattern for each of these
purposes?  Obviously for the buckling, the plate thickness is
important.  So the fastener pattern will determine the answer to
another question raised, what is the minimum plate thickness?


Looks good to me: E_steel / E_oak is about 18, and 2" / (1/4") = 8, so
a full height steel side member is 2.25 times as stiff.

Cheers, Wayne
Wayne-

You've got a couple of things going on depending on how the plate
isadded & whether is bears on the joist support.

Like I said in my post to Steve, I'm really not sure how I'd calc the
screw pattern but two rows 12"oc staggered seemed reasonable for 1/4"
plate. Since the plate is full depth we're not making a "composite"
beam like a T-beam or other assymetric shape, the fastener schedule
need not be as dense as for those types of apps.

So I'm thinking all we need to consdier is the overall load path.
Load goes into the "joist & steel plate system" over the entire span.
If the plate doesn't bear on the joist support (wall top plate?) then
we need to take the collected load out of the plate & back into the
joist at the end of the plate. And this would require a heavy screw
pattern If it does bear, we need to provide a bearing plate so that
side plate doesn't cut into the top plate.

Here's where I really like to bounce this concept of someone who can
poke holes in my ideas. I'm not totally sure what the screws pattern
needs to look like near the ends IF the steel plate doesn't bear on
the joist support. If it does, I pretty sure that the 2 rows- 12"oc
staggered is fine.

What I'm say is that if the steel side plate completely mimics the
joist then the screw pattern is less important. If side plate doesn't
span & bear as the joist does....then the screw pattern is more
critical.

cheers
Bob
 

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