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Old 11-18-2008, 09:12 PM   #1
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adding joist under subfloor


I have 48" OC joists and I need to get into the crawlspace to add 16" OC 2x8 joist perpendicularly between them.

The reason is that the current floor deflects too much for a tile install.. material does not cost too much but it is a good insurance for the tile installation!

So I have selected Simpson joist hanger LUS28 and also 10D nails that is needed. I have a palm nailer which is supposed to really speed up the installation.

What is the best way to install? I will have a helper cut the wood to length as I go. I am thinking drill 2 holes from below, so we can locate the joist and fasten the subfloor down from above (not sure to fastern now or after I install the bracket?). Meanwhile I would put liquid nails on the edge, push up against the plywood and install the brackets to the existing joists?

It's not like regular deck or new construction where you install the brackets and just drop the joist in from above, I want mine snug against the bottom of the plywood before I install the bracket, so what is the best way that you would do this?

Thanks in advance.

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Old 11-18-2008, 10:24 PM   #2
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adding joist under subfloor


Is it a basement or a crawl space?

If a basement, you could mount the hanger at one end, insert the joist, swing it up at the other end and jack it up with a jack post. insert the 2nd hanger.

Crawl space, use a scissor jack mounted on a support.

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Old 11-18-2008, 10:32 PM   #3
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adding joist under subfloor


Your best bet for a solid installation is to use the liquid nails as you mentioned and screw down from above into the new 2x8 before the liquid nail dries. this will pull the 2x8 up tight to the subfloor and the liquid nail will help prevent squeaks from forming later. You may need to hold the 2x8 from below while someone else screws down from above. Then install the Simpson hangers from below. Make sure all nail holes in hanger are filled.
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Old 11-18-2008, 10:43 PM   #4
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adding joist under subfloor


Yes, it's a crawlspace, my concern is will I be able to get the simpson bracket nice and snug when I nail it in, since the bracket came spread open like a V shape I have one chance to rough it in place using those hooks on the bracket.

But I guess after all the nails are done, the bracket pretty much hold the wood in space even if the bottom tray of the bracket is not 100% tight against the wood, Looking at the diagrams, the 3" shear nails will hold it in space pretty good. It isn't load bearing job or something, just enough to stop an adult foot's weight from flexing the plywood.


Thanks
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Old 11-18-2008, 11:30 PM   #5
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adding joist under subfloor


This has to be installed ON the center beam and ON the outside sill plate. How will you get it turned under there?? Trimming one side on both ends seems to ME the only way. Where would you hang joist hangers on?? Unless you can get the subfloor to flex alot to get it turned once up there flat,,specially if its the same exact size as old ones. Wouldnt be ALOT harder to take up subfloor and do it from top. 48" is a LONG span!!!
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Old 11-19-2008, 07:17 AM   #6
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adding joist under subfloor


If you have joists 48" on center, the first question should be: What is the size of those joists and what is the span? It sounds like you wish to add headers on 16" centers between the existing joists. Is so, you will only stiffen part of the floor system in one direction only. Depending on the joist size and length, you may still have significant flex in the joists and floor in the opposite direction.
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Old 11-19-2008, 07:39 AM   #7
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adding joist under subfloor


Quote:
Originally Posted by Maintenance 6 View Post
If you have joists 48" on center, the first question should be: What is the size of those joists and what is the span? It sounds like you wish to add headers on 16" centers between the existing joists. Is so, you will only stiffen part of the floor system in one direction only. Depending on the joist size and length, you may still have significant flex in the joists and floor in the opposite direction.
I agree with Maintenance. After you fill in the joists to get them to 16" on center, you might need to add support perpendicular to the joists to split any span over 12' feet.
There's also a good chance the original joists(and floor) have sagged and the new joists will need to be jacked into place. This could cause some issues with the walls, causing cracks.
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Old 11-19-2008, 11:32 AM   #8
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adding joist under subfloor


The existing joists are 4x8 and supported by piers every few feet. Plywood is 1-1/8". the new 2x8 16" OC will be added between these, perpendicular direction. I forgot the mention the scale of the project
Here is a crude drawing of my flooring area.






It's a small area starting from the outside wall, only 9 ft out so I don't think there are too much sag in this area?

Also I have been looking for screw jacks that you can screw in place and leave under there. They are blue in color not too expensive, I was told ace hardware sells them. Do you guys know where else might sell it?

Last edited by DIYGST; 11-19-2008 at 12:00 PM.
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Old 11-19-2008, 12:28 PM   #9
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adding joist under subfloor


You have a series of joists that are double 2x8s on a 9' span. I think adding your 48" headers on 16" centers will work to stiffen inside the joist cavities, but I would add a beam across the center of the joists to stiffen the 9' span, or a column, mid-span under each joist with a solid footer to bear on. I didn't look at any tables to check, but without stiffening the joists, I doubt that you reach the deflection limits to put down a lasting tile floor.
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Old 11-19-2008, 12:35 PM   #10
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adding joist under subfloor


Thanks Maintenace 6
I see the piers are kind of far apart.

Now any idea where I can get those screw jacks, what are they called?
I don't want to jack up the joist then drop it onto the new beam, although that is cheaper I am afraid I might damage something.

Also would 12x12 concrete paver work as a footer? How far do I dig in the dirt to get a firm base?

OK.. I found this site to give some useful info about using adjustable columns, interesting read though because the post I described might not be for permanant use. But it does list some suitable for use supports

http://www.octoberhome.com/articles/...ustcolumn.html

Last edited by DIYGST; 11-19-2008 at 12:59 PM.
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Old 11-19-2008, 10:52 PM   #11
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adding joist under subfloor


I found some sag in the plywood between a couple of the 4' spans.
Is it OK to use a jack directly on the plywood to raise the sag, while I install the joists in place? There is not much point to install the new joist while it can see-saw about the sag from the bottom, the contact is minimal.

Another question, are the plywood subfloor cut around the jack studs that support walls? Because plywoods are put in after the vertical studs, am I right? So applying lift to the plywood directly about 1/4" or so shouldn't affect the walls at all?
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Old 11-19-2008, 11:01 PM   #12
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adding joist under subfloor


Quote:
Originally Posted by DIYGST View Post
I found some sag in the plywood between a couple of the 4' spans.
Is it OK to use a jack directly on the plywood to raise the sag, while I install the joists in place? There is not much point to install the new joist while it can see-saw about the sag from the bottom, the contact is minimal.

Another question, are the plywood subfloor cut around the jack studs that support walls? Because plywoods are put in after the vertical studs, am I right? So applying lift to the plywood directly about 1/4" or so shouldn't affect the walls at all?
In box framing the wall framing sits on top of the sub-floor. Normally!
In my house, the guy that built it in 1948 nailed the walls to the rim and floor joists, then installed t&g fir for a sub-floor.
I've never seen this done before!
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Old 11-19-2008, 11:13 PM   #13
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adding joist under subfloor


I see Wildie I am assuming the THICK 1"+ plywood, the day they put them in didn't droop in the middle, so if I jack it back up flat now, with a helper from above checking with a 48" stiff beam level across the span to let me know when to stop, I shouldn't break anything?

Thanks..
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Old 11-19-2008, 11:47 PM   #14
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adding joist under subfloor


You can go to a junk yard and get scissor jacks from junk cars. You could just leave them in place. Not the most professional fix but it would work. Hope this might help, Dorf Dude
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Old 11-20-2008, 06:24 AM   #15
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adding joist under subfloor


I would use a 2' piece of 2x4 and jack the plywood up with that. I wouldn't jack directly against the plywood. You might punch into a void and ruin the plywood. You'll want to be sure the plywood is nailed down well along the edges before you start. Otherwise you are liely to pop it loose from the existing joists. Remember it didn't sag in a day and now has somewhat of a set to it. Go slow until you see how the subfloor reacts to being jacked. The subfloor should run under the wall plates. From the picture it looks like there are some sort of piers in place already. I assume that you are adding more to shorten your usupported spans. I really wouldn't use pavers as a base. You are not adding a great deal of weight to these piers and I would assume that you don't have any frost worries in the crawl space, but most pavers are only 2" thick. Maybe some of the precast deck footings that I see at the box store. Set them on undisturbed earth with a short 4x4 post. Since you don't have many joists, I'd just put one mid-span of each joist. Basically you are then just turning each joist into a beam of sorts with short joists tied into them on hangers.


Last edited by Maintenance 6; 11-20-2008 at 06:31 AM.
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