Stiffening 2x8 floor joists

[ishiboo]

I have a 2x8 floor which is quite bouncy (13'6" span, old wood), which I'd like to stiffen some. With the way things are laid out, it would be difficult to sister a second set of full-length 2x8s.

I have a ton of 1/2" OSB, I'm thinking about ripping some down to 7" tall pieces (to account for the plaster sticking up above the lath) and installing it on a tight nailing/glue schedule, on either one or both sides of the center 4-5 joists.

From my armchair engineer standpoint (actually I went to school for EE/ME, but never completed), this would stiffen the joists quite a bit... am I wrong?

There is also modern-style blocking, which I would have to remove and replace... but instead of just one set down the middle I think I would do 3 sets distributed evenly.

Anything expensive/fancy is out of the picture, ie sistering steel plate... there is no noticeable sag and I want to keep the budget low on this, would just like to help the bounce if there's an easy way of doing so.

TIA!

 

[6768rogues]

Your idea would help to stiffen the floor. Bridging is important to stiffening a floor, so your idea of adding additional bridging will also help. You could try two layers of OSB with the joints staggered. Also, you could use a jack or jack post to lift the center of the span a little before doing your work so that it is prestressed when it returns to its normal load.

 

[StingRay]

It will probably help a little but if you remember back to your engineering it's the material at the extents that does the work. Think about an I joist the middle or web is OSB and the flanges are dimensional lumber. Reallly you'd be adding very little of a what isn't the best material at the edges where compression and tension are happening.

 

[ducati]

Run x bracing down the center of your joist, this will make it so the load on your floor is spread out more than one joist. The red is say a 1"x3" the black is your joists

 

[ishiboo]

It will probably help a little but if you remember back to your engineering it's the material at the extents that does the work. Think about an I joist the middle or web is OSB and the flanges are dimensional lumber. Reallly you'd be adding very little of a what isn't the best material at the edges where compression and tension are happening.

Definitely it is the material at the extents, specifically the bottom which has a lot of tension. My thought is the OSB increases overall stiffness there as well.

The other option would be to sister 2x4s at the bottom of each with the same securing method, I think the OSB will be stronger though.

 

[tomsteve]

not to sound bad or anything, but without spending some money, it would just be a band aid and need to be dealt with at some point. whats the spacing on the floor joists. ripping OSB down and using that probably wont work too good unless your plywood is as long as the span. sistering more 2 by 8 isnt a good idea either as you need more depth than width for structural strength. basically, i dont believe there is an "easy" way to do this. i have had to do this in a remodel and sistered LVL's to the existing floor joists. if you want to try something, i would say go with solid blocking between the joists. split the span into 3 and run it at the three locations from end to end.

 

[ibedayank]

 

[red]

Is there any way you can sister 2X material to the existing 2x8? If you can sister even 2x6 against the 2x8 with glue and some nails, would be stronger than osb.

Here's what the pro's have to say-
http://www.finehomebuilding.com/PDF/Free/021184090.pdf

 

[mikester]

I have the same kind of situation in my 111 years old house. Making it worse is the way they put a foundation under the house in the late 60s. The joists are notched at the plate so when you look at it theres about a 2x4s width supporting the outer joist at the foundation. I thought about how to fix this. The only thing I could come up with is framing a whole new wall under the joists up against the blocks. So far it isnt a big problem so I might put it off as long as I can ! I sistered all the joistd under my kitchen before I had tile put in. It was a huge amount of work. Hardest part was taking the romex off and putting it back up.

 

[gnxtc2]

This what I did to my floor. My situation was somewhat different than yours but it significantly stiffened the floor. My span is 35'.

I ripped down 3/4" plywood the width of the 2x8s. Nailed/glued the plywood to the 2x8. I took another 2x8 and nailed/glued it on top of the plywood. I then took 1/2" carriage bolts and tighened everything together.

On the top, I screwed down full sheet of 3/4 plywood centered on the seam. On the edges, I ripped sheets of 3/4 plywood length wise (2'x8'). I took the ripped plywood and screwed the sheets alongside the full sheets staggering the seams.

I know some will be against this.

This is before. The previous owner put 3/4 plwood with VCT already glued on. I junked the plywood. There was 3/4"x3" stradling the seam.

This is after:

Billy T.
[email protected]

 

[ishiboo]

not to sound bad or anything, but without spending some money, it would just be a band aid and need to be dealt with at some point. whats the spacing on the floor joists. ripping OSB down and using that probably wont work too good unless your plywood is as long as the span. sistering more 2 by 8 isnt a good idea either as you need more depth than width for structural strength. basically, i dont believe there is an "easy" way to do this. i have had to do this in a remodel and sistered LVL's to the existing floor joists. if you want to try something, i would say go with solid blocking between the joists. split the span into 3 and run it at the three locations from end to end.

I think you have your structural engineering mixed up Of course sistering works, and is the "easy" way of doing this. Every joist has a certain amount of deflection under a certain amount of load. By sistering, assuming the new lumber is the same as the old number, you would have half the deflection for the same load.

Yes, the joists should have been deeper, but that's a difference between new and old homes. Sistering works and is well documented. I'm just wondering how close of a benefit OSB provides.

I'd guess for overall tension/compression, it does quite well, as that's what's used in wood I-beams. The purpose of the flanges at the top and bottom is for keeping the OSB straight, which the joist already would do, so it can bear the load, and to provide a place to nail.

 

[ishiboo]

Is there any way you can sister 2X material to the existing 2x8? If you can sister even 2x6 against the 2x8 with glue and some nails, would be stronger than osb.

Here's what the pro's have to say-
http://www.finehomebuilding.com/PDF/Free/021184090.pdf

Thanks for the post. That gives my method as the 3rd choice, with the second being sistering another joist (not practical) and the third being adding a beam underneath (also not practical). It also gives the addition of blocking (bridging) as the final option, which I was also going to do... hope the two will give a good end result

I'll also install the new subfloor (3/4" T&G OSB) with construction adhesive... I picked up a couple cases of 28oz tubes on sale for $2.29 a tube... cheap!

 

[RPH]

The discussion prevouis on this matter had 2 x 4 glued to the bottom of the joists. The math was presented to support the method. The glue was critical in the process. It was a 2 part epoxy type. The 2x was laid flat on the bottom and did not have to go end to end. I have the link back to the original forum on this. I will try to locate and post.

 

[RPH]

Found it!
http://www.johnbridge.com/vbulletin/showthread.php?t=54371&page=
Good discussion on how to stiffen the floor.

 

[ishiboo]

Found it!
http://www.johnbridge.com/vbulletin/showthread.php?t=54371&page=
Good discussion on how to stiffen the floor.

Yup, well aware of that one thanks I need to do this without interfering with the ceiling below it, and preferably without loosing headroom above.

 

[Nick M]

Use Acro jacks and a plank, bridging across joists to bring them dead level if they are already not. Rip 1/2" (7/16") osb the width of the joist. Then glue (PL) and hold up tight against subfloor. Screw (lots) to the sides of the joists, 2 layers each side overlapping osb joints. That would be a stiff joist. No loss of headroom.

Edit: nevermind. This was your original plan. Do it, make sure to keep the OSB tight against the subfloor and lots of glue and screws. Either two layers on one or both sides, or at least scab the joints of one layer of OSB a few feet. This will greatly stiffen those joists. Blocking with also help greatly.

 

[diggerrick]

I think these guys know what they're talking about:

http://www.thisoldhouse.com/toh/asktoh/question/0,,1089153,00.html

 

[Nick M]

I think these guys know what they're talking about:

http://www.thisoldhouse.com/toh/asktoh/question/0,,1089153,00.html

There you go! Get the table saw out and have at it!

 

[ishiboo]

My plan exactly! Thanks for the validation folks. Don't want to waste my time if it won't work well or if there is a far better solution.

One thing I'm wondering is, in the This Old House case, why Tom didn't recommend running a beam perpendicular to the joists and supporting it from the ground? That'd completely take care of his issue...

 

[srmofo]

Believe it or not when I installed my laminant flooring 2 years ago, I put down a sheet of 1/4 OSB over the existing floor. It was piece together from old hardwood and plywood and was unlevel in several spots. I wanted everything to be as flat as possible. I put 1 tube of adhesive per sheet and a screw every foot.

I bet it reduced the bounciness by half. It was an unintended bonus. My house was built in 1904 and my joist are old and cut at the ends also. I still plan to sister some ply, and put new blocking in but theres so many other things on the list I just havent got around to it yet.

 

[ddawg16]

FYI.....OSB is one of the woods of choice to use in California for shear walls. The stuff has excelent tension and compression characteristics.....it's a good choice to use.

As suggested earlier....if you have any sag....I would jack it up a fraction....then attach the OSB...but leave the jack on long enough for the glue to dry or your efforts are wasted.

Addionally, since one OSB sheet is going to be longer than the other....stagger the edges....one side the the joist...put the seam towards one end....other side...other end.

The TOH said to use nails....the guys I have talked to about it said screws or bolts. The shear forces are going to be handled by the glue....as long as that bond does not break....your not depending on the hardware that attaches it to have shear strenght....so the key is to make sure what ever you use has good holding strength....I think I would us screws....with maybe bolts on the ends of the OSB planks.

 

[diggerrick]

I just did one in one of our rentals a couple months ago, but with a matching 2x8 glued & screwed. I cut it about 1/4" short (left 1/8" on each end), and tied each end into the beams with the pre-made L brackets that are basically half of a joist hanger, but without the bottom L. I don't know if it makes a difference, but I staggered the screws from each side

 

[Neighbor]

One of the best ways to increase deflection strength in a header is to sandwich a piece of sheet metal. You can get a roll of 6 inch or 8 inch sheet steel at the big box stores. Attach it with a few nails to existing floor joists and add the playwwod over top of that. The steel is very reistant to flexing along it's vertical surface.

 

[mobetta]

note TOH article uses 3/4 plywood. i think we all know plywood and osb are very different, even though the are very much the same. I would consider using 3/4 cdx, even though you already have the osb.

nails would be way easier if you have a framing nailer.

 

[ishiboo]

Just an update - I ripped a ton of 7" high 1/2" OSB today, it's getting late but I glued and nailed two to two joists... already a MAJOR improvement. You don't notice ANY deflection. With the opposite sides nailed and the blocking reinstalled, I think it'll be perfect.

Thanks all for the suggestions and tips.

I agree, plywood would probably have better tensile strength, however this span would be rated for the same size OSB I-beam so I think the 1/2" OSB will be acceptable.

When I get finished I'll post pics and let everyone know how it went. My original plan was to do the whole joist, however I started with the 8' pieces in the middle and it seems to do the job just fine. I don't think I'll cut end pieces and attach.

 

[Justanoldguy]

Run x bracing down the center of your joist, this will make it so the load on your floor is spread out more than one joist. The red is say a 1"x3" the black is your joists

This is the way it was alway done here in NZ.
But use 2 x 3 or 2 x 2 for it.
Or just use 2 x 8 solid blocking between joists.
Either way gives a huge amount of extra stability.

 

[Justanoldguy]

Google "herringbone strutting" for more info.

 

[diggerrick]

Just an update - I ripped a ton of 7" high 1/2" OSB today, it's getting late but I glued and nailed two to two joists... already a MAJOR improvement. You don't notice ANY deflection. With the opposite sides nailed and the blocking reinstalled, I think it'll be perfect.

Thanks all for the suggestions and tips.

I agree, plywood would probably have better tensile strength, however this span would be rated for the same size OSB I-beam so I think the 1/2" OSB will be acceptable.

When I get finished I'll post pics and let everyone know how it went. My original plan was to do the whole joist, however I started with the 8' pieces in the middle and it seems to do the job just fine. I don't think I'll cut end pieces and attach.

For a simple span with uniform loading and/or a concentrated load at mid span a beam of uniform cross section (and corresponding section modulus), the maximum moment and maximum stress are at the midpoint of the span. The less than 3ft you have on either end of your 8' OSB stiffeners aren't seeing nearly the stress as the middle part you stiffened. I would only run them out to the ends if the joist was cracked (the one I just repaired was). If you were going to do that, you should have staggered the splices from side to side. It would also help to tie the stiffeners into the beams at the ends if you extended them.

 

[djmartins]

OSB or ply (1/2" ply preferred) sandwiched between two 2x8 adds a LOT of strength to a beam!
That's the way I would go, use 8 penny nails 4" apart at 1" from top and bottom, staggered so the nails don't line up top to bottom.
If the nails are 4" long you clench the protruding ends over so the nails can't pull out.

 

[BLUBAYOU]

what about using ring shank nails instead?