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Old 03-20-2010, 01:10 PM   #1
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floor framing question


Hi all.

I am in the process of dismantling a trailer that one side of my house was built around.
I've taken out a section about 18 feet long from one end.
Now I have to build new floors, walls, and ceiling.

The trailer was not actually attached to the house, it was basically sitting inside it with walls built around it.

Demolition was no problem but now I have issues with the construction.
I have a book on framing, but the section on floors shows that the floor is built with the joists first laid on top of the sill plate,and then walls on top.
Because the original structure had no floor, my configuration is (from the crawlspace) a cinderblock wall sticking about two feet out of the ground, sill plates on top of that, and then walls built directly onto that.

According to the book, if there were no walls, I'd have set up the header joist and then nailed 3 nails through the header into the joists. I can't do that because (I assume) I will have to attach my header joists (and the whole outer frame) to the wall studs before the joists are put in. A colleague suggested joist hangers to solve the issue. But seeing all the nails used in the book (the 3 above, plus nails toenailed into each joist and the sill plate), I am concerned that if all the joists are just supended and not actually attached to anything, it won't be sturdy enough to hold up the floor and everything on it (this is the kitchen/dining area).

So the questions are:

1) if I bolt all the pieces of the outer frame (all 12x8) to the wall studs (maybe with 4 1/2 inch bolts x 2) on the outermost studs and then put a couple of nails through these pieces at each of the in-between studs, will that be strong enough to hold this section of floor (about 12.5 feet x 18.5 feet)?

2) if so, will it be enough just to use joist hangers for all of the inner joists on both ends, and if not what should I do?

3) any other tips for strengthening this frame?

if it isn't obvious,this is the first one I've done.

Thanks

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Old 03-21-2010, 03:34 PM   #2
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floor framing question


Don't quite get a clear picture from your description about hanging what you call the header joist, but lag bolting it can provide sufficient support; I just can't offer suggestions due to unclear picture on my end. As for hanging the floor joists, using the appropriate size joist hangers will be plenty strong enough to support the floor; use the proper hanger nails and fill every hole with a fastener.

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Old 03-22-2010, 03:19 AM   #3
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OK. Thanks.I just wasn't sure, since the book I have shows five nails going through every joist (3 on the end and 1 toenailed on each side at the end).To be clear on the header joists (maybe they're called end joists), I'm talking about the two 2x8s that will be bolted to either opposite wall and on which the joists will be hanged.The dimensions of the floor are roughly 13' x 19'.So, I'm thinking that I first bolt a 13' 2x8 onto the studs of the shorter wall with 2 4" or so 3/8 lag bolts on each of the outermost 2 studs, and then two nails for each stud in between.Then I butt 2x8s against these perpendicularly along the two adjacent walls and attach them in the same way, then I hang a joist hanger every 16" and hang a joist spanning these two longer dimension walls. When I get to the end of the first two header joists, i'll put up two more and hang double joists at the junctions. does that sound right?additionally, the sill plate sticks out about two inches past the wall studs, creating a bit of a ledge to build on, so I guess the structure will be a little more stable than if the frame were just "hanging" off the wall studs.Eventually, I'm going to run this new floor frame across the whole house, about 70 feet.
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Old 03-22-2010, 10:05 AM   #4
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Still not clear about the double joist you describe, but

It sounds like the new header joist/band joist will be sitting on the protruding sill plate, which should be bearing on some type of foundation wall. Unless the sill is wider than the supporting foundation it will be fully supported, thus the floor load will be directly transferred to the foundation, and not relying on the shear strength of the lags/nails. If this is so, there is really no purpose for the lags. If the sill plate is cantilevered past the supporting foundation, you will need to use lags at each stud, as they will be the sole support for the entire floor.

You have to run the floor joists so they are spanning the short (13') dimension; 2 x 8's cannot span 19'. Use t & g plywood for the subfloor, block between the joists to keep them from trying to twist and to help spread the load, and use plenty of heavy duty subfloor construction adhesive (not cheap general purpose adhesive) to glue the plywood down. I would also screw the plywood down, to head of a problem with future nail sqeaks, as the 2 x 8's will have a little more movement than ideal.

Be sure to use proper depth hangers , not 2 x 4 or 2 x 6 .

Last edited by troubleseeker; 03-22-2010 at 10:15 AM.
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Old 03-22-2010, 07:38 PM   #5
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What I meant by double joist was, with the boards running end-to-end along the long dimension(19ft), i'll have to pick a length and cut them off. I assumed where these boards abutted, there should be a joist flush on the end of each, two joists side-by-side in a double hanger.

On the sill, it looks like 2x6. with the 2x4 studs flush along the back, it looks like about 2 inches of ledge past the 2x4s, and falls short of the end of the foundation wall (no cantilever). So I guess, like you said, the weight will all be transferred to the foundation. so what would be the correct way to attach the outer frame, 3 nails through to each stud?

Also, since you mentioned it, I checked some span tables I found on the internet, and it looks like I should switch to 2x10s instead. Even if it's overkill, I'll have a lot more peace of mind. You probably saved me some future grief there. Thanks.

On subfloor, my book says that 3/4" is the standard for hardwood, but 5/8 should be used for tile floor that will require an underlayment. I plan to make this floor tile on one half and hardwood on the other. Should I put in half and half on the plywood or use the same plywood for all an level it otherwise? (objective being to have the floor level across).

Thanks again.

Last edited by wengang1; 03-22-2010 at 07:56 PM.
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Old 03-25-2010, 11:33 AM   #6
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Quote:
Originally Posted by wengang1 View Post
What I meant by double joist was, with the boards running end-to-end along the long dimension(19ft), i'll have to pick a length and cut them off. I assumed where these boards abutted, there should be a joist flush on the end of each, two joists side-by-side in a double hanger.

On the sill, it looks like 2x6. with the 2x4 studs flush along the back, it looks like about 2 inches of ledge past the 2x4s, and falls short of the end of the foundation wall (no cantilever). So I guess, like you said, the weight will all be transferred to the foundation. so what would be the correct way to attach the outer frame, 3 nails through to each stud?

Also, since you mentioned it, I checked some span tables I found on the internet, and it looks like I should switch to 2x10s instead. Even if it's overkill, I'll have a lot more peace of mind. You probably saved me some future grief there. Thanks.

On subfloor, my book says that 3/4" is the standard for hardwood, but 5/8 should be used for tile floor that will require an underlayment. I plan to make this floor tile on one half and hardwood on the other. Should I put in half and half on the plywood or use the same plywood for all an level it otherwise? (objective being to have the floor level across).

Thanks again.
No need for a double joist at the splice, just have it fall on a stud. I generally try to land on a stud that falls in between the new floor joist layout, and then after the floor joists are in place nail a block over the splice, fitting it between the joists.

If the additional height is not an issue, definately go with the deeper joists. I did not suggest this as I though you were working to match some existing fixed floor height. Four nails into each stud is pretty typical for a 2 x 10.

Definately use the 3/4" subfloor, the biggest enemy for tile is a weak subfloor that flexes, causing grout or tile cracks. If it is important to have a flush transition between the wood and tile , you can add an additional layer of subfloor under the hardwood portion. The thickness will depend on the type and thickness of hardwood you plan on using.

Last edited by troubleseeker; 03-25-2010 at 11:36 AM.
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Old 04-18-2010, 01:10 PM   #7
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Hey.
Sorry for the delayed response. I'm doing the work myself and every step is taking a lot longer than I thought it would.
I have a new problem with this framing job.
I finally got the whole thing cleared out, even the steel support frame for the trailer and the cinderblock pillars. NOw that I can see clearly across the whole house, I see why the other side of the house has such low floors.
The guy built the other side (the mirror image of the side I'm building) didn't frame the floor in the conventional way. He actually built the wall that divides the two sides of the house so that the wall studs were flush with the sill plate and foundation wall (foundation wall is 6", sill plate is a 2x6, so on my side the sill plate sticks out 2 inches past the wall studs and is flush with the foundation wall. Then on his side, he put up a 2x8 as the header and nailed it against the wall studs and the sill plate, not hanging down over the cinderblock, but lined up along the bottom with the bottom of the sill plate. And for the part that I don't really understand, he then nailed a 2x4 along the bottom of the 2x8 (so that there are now two boards "hanging in the air" with nothing under them. Finally, using 2x8 joists and joist hangers, he cut the dimensions of the 2x4 out of the bottom corner of the joists so that they sit on the 2x4 (treating it like a ledge I guess) and he only put nails in the bottom two holes of each hanger (since there is only air behind the holes above those). It really doesn't look all that sturdy. I'm wondering if it even meets code.
But my problem is that the way he did that makes the two floors uneven by several inches if I do it the conventional way. The main thing is that I'm using 2x10s and I was planning to rest them on the sill plate. If I want this floor to be the same height as the other, I have to account for the 2 inches difference in the wood width and the inch and a half of the sill plate. I'm attaching my own artist rendering of the situation for clarity.
basically just not sure how to get the joists flush and what the hangers will be attached to.
Gary
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Old 04-19-2010, 12:09 AM   #8
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Your problem is the ledger 2x8 nailed or lagged onto the studs. This connection requires engineering. Then the added 2x4 ledger, and the giant notch in the 2x8: http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf

Be safe, Gary
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Old 04-19-2010, 09:36 AM   #9
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I'd never seen that before. I did know that 1/3 was the maximum notching depth (and these are nearly half). I don't see why he even added the 2x4 to start with. If the 2x8 was lagged onto the studs, it seems like he could have directly attached the hangers to that and save all the notching time. Also, when I bought the house, the inspector said everything looked great.

My main problem is how to get the other floor level with that one using 2x10s.
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Old 04-19-2010, 08:30 PM   #10
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OK> a picture is worth a thousand words.
Here is a much clearer picture of it, drawn to scale.
I've come up with 2 options.
The right side is the finished floor, and the left side is the floor I'm working on.
Neither option gets it level with the other side, but both come within 1/2 to 3/4 inch.
I used 1/2 plywood to space the rim joist, and in the 2nd pic, I'm using 1" plywood to make up some of the difference.
In the first option, going with a 2x8, I have less than 1/2 inch hanging over the cinder block, and I can put nails in every hole of the hanger and into wood. in the second pic, I would only be able to nail through the top 6 holes of the hangers, but not the bottom 2. also of the 4 toe nail slots in the hanger, in option 2, the bottom 2 couldn't have nails.
Thoughts?

GaRY





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