Screws for joist sistering

[JStructsteel]

I will be doing some joist sistering for a friend this weekend to help strengthen her floor.

Anyone ever specify SPAX screws? I have many #14x3" screws, was going to use them along with construction adhesive to reinforce the floor.

Any issues with SPAX screws?

 

[dik]

I use the screws/nails for holding the parts together so the adhesive works...

Dik

 

[JStructsteel]

Dik, good point, I think thats more than likely the way I will go too. These screws are pretty heavy duty, will add some to the mix, but i could reduce the number of them.

 

[oldestguy]

I'd save the screws and use a nailer. That way, for the same effort you can get a lot more improved structurally and there is no problem with splitting. Have a couple of clamps and away you go. Forget the glue also.

 

[dik]

nails are good... and as OG noted, likely, the best way to go... I'd still use glue.

Dik

 

[JStructsteel]

Well I have the screws, so going to use them. I will test a few in some of the old joists, and pre drill if need be.

I feel the glue is a big part of the system, especially for shear. Im not sure why its being suggested to be neglected.

I guess I should say, im just trying to stiffen the floor, not really and capacity. Some tile has cracked on floor above.

 

[dik]

We built my parents home using lumber salvaged from an old lumber yard... that was 50 years back and the Doug Fir lumber was 50 years old at the time... 20' lengths without a knot... so well seasoned that you couldn't pull a nail out of it or drive one into it... had to use pneumatic nailers... Really tough stuff.

A posting seems to have gone missing... if the members are the same size and grade, and loaded together, there is little shear between them (on the glue line)... but, I still like adhesive so they both act together.

Dik

 

[emmgjld]

Not replying for OG but I also tend to not use glue in that proper application (thin film), is often not consistently found in my inspections. The use of hand nailing, screwing (predrilled for hard, brittle lumber), bolting or power nailing usually depends upon, access, condition of the lumber and prior experience of the assemblier.

 

[dik]

emmgjld: If I'm not 'sniffing' it, I like to use glue to hold things together... just one of my personal quirks.

Dik

 

[JoshPlumSE]

I don't see much point in the glue if it's connecting the sistered joists together. Where I really like to use it is for connecting the joist to the floor sheathing. Just for serviceability issues (noise reduction and such). So, if that's what you're doing, I'm all for it.

 

[BridgeSmith]

If the aim is to increase the stiffness of the floor, I would suggest adding full-length joists to the system, and don't worry about the attachment the to the existing joists. If that isn't feasible, and reduced clearance underneath can be accommodated, increasing the depth will be more effective. Even adding a 2X flat as a bottom flange for the joists will increase the moment of inertia quite a bit, perhaps more than the sister joists. Since the aim is increased stiffness, I would definitively recommend a generous coating of PVA wood glue (like the plain old yellow Elmer's, not construction adhesive) along with the screws.

 

[Jerehmy]

If you're doing full length sisters and the members are the same size you don't need many fasteners between the members. Glue seems unnecessary.

If you're doing scabs and the members are the same size, you'll need more fasteners and you may use glue if you'd like.

If you're scabbing or sistering with members of different sizes, it depends. If you're looking to create a composite member, then you'll need a lot of fasteners and glue will help a lot.

I mean you could literally scab a 10ft long 2x10 to a 14ft long 2x10 residential floor joist with just (2) #12x3" at the ends a (3) at the center to meet strength requirements. I'd obviously do more than this since the configuration isn't efficient, but it just goes to show. When the members are composite is when you need a a lot of fasteners and possibly glue.

 

[JStructsteel]

These will not be bearing at the ends, more than likely just a few feet short.
I doubt with the old joist sizes I can get exact size match without riping them, so will go one size less, i.e. a 2x8 on old 2x10. Screw and glue, sort of a belts and suspenders thought.

When the floor was originally installed it was screwed and glued down and that helped stiffen alot, but not quite enough for the tile, the grout is coming out in places. Unfortunately the home owner is a bit over weight, so that adds to the issue.

I forgot to add there is no blocking either, so that will be installed and help things too hopefully.

 

[Jerehmy]

Just adding some images showing what I was saying. A 14ft 2x10 scabbed with a 10ft 2x10 at midspan. Only fastened at midspan and ends.

https://files.engineering.com/getfi...b-b095-4ba6-b413-ddf8a58fae5d&file=Moment.JPG

https://files.engineering.com/getfi...a-4621-94b0-2927a7dc206b&file=Joist_Shear.JPG

https://files.engineering.com/getfi...97-42f1-99c3-ca05b90f5894&file=Scab_Shear.JPG

 

[Compositepro]

Screws would be more effective at clamping the joint than nails. Usually the adhesive needs the pressure to spread and fill the gap.

 

[Deadblow]

The glue will be absolutely necessary to prevent squeaks from the joists rubbing together. I recommend you use polyurethane construction adhesive like Loctite PL Premium. That stuff is awesome! I would even glue to the subfloor or stay far enough away from it with the sistered member so the subfloor doesn't rub the new member and squeak.

EIT

 

[MotorCity]

Seems like there a two camps: those who insist on the glue and those who despise glue.

Put me in the "despise glue" category......save it for the arts and crafts. I just don't get the warm fuzzy feeling about smothering two surfaces with goop and crossing your fingers. I'll take the nice hefty mechanical connections any day.

Kidding aside, I know glue/adhesive has its place. Just amused by the strong arguments for/against it.

 

[emmgjld]

My post above is not meant to disparage adhesives. Being a part time woodworker, I truly appreciate what a proper glue joint can accomplish. Just consider an LVL!
Unfortunately, I have also seen a lot of product, usually applied too thick, trying to depend on power-nailing or screws to 'smooth it out' and not achieving much more than a dry joint. I tend to count on the adhesive as being an extra in the design margin, unless I am dealing with a cabinetmaker's mentality.

 

[Jerehmy]

MotorCity

Yes, glue definite has it's place, I'm just not a big fan. I AM a big fan of adhesive anchors, so I guess that makes me a hypocrite?

There's also places where glue is prohibited, such as when fastening sheathing for shearwalls (SDPWS 4.3.6.3.1).

 

[dhengr]

Jrisebo:
I assume you are actually trying to strengthen and stiffen the floor system, under the tile flooring. Look at the floor sheathing deflection on the existing 16" o/c joists, how much of a contribution is that causing to the overall problem? If that’s significant then you probably want your new sister joist to be fight up under the sheathing to reduce the sheathing span a bit. Alternatively, if the existing joists are the primary problem, and you can get full length 2x8's in there, I’d use them, and have them bearing on the sill plates at each end. You do have to use some engineering judgement and experience about screws vs. nails and their shear values, spacing, etc., but as mentioned above, they do apply some clamping strength while the glue dries. Use a good brand of structural adhesive, not some no-name bargain brand. In either case, an important detail for making this fix work, is that you jack the existing joist up .25 - .5" (to be determined) over the middle third, then fix the new joist to the old, and finally remove the jack. Otherwise, the new joists will not start contributing until some significant added load is applied and all the connections are really brought into play. Obviously, this requires some judgement too, you do not want to be lifting the existing joists off their bearings, or cracking plaster or more tile. But, you do want to partially unload the existing joist and then apply the sister joist, so the new joists are truly sharing the whole load.