20 x30 1 1/2 story loft question?

Started by deabob11, October 10, 2007, 10:45:45 AM

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deabob11

When i built my 30' walls i started on wall from the east side and the other wall from the west side.   So what happened is that when i stand one stud and look across teh room to the other stud it is about 6" off to the left.  I cut a pocket for a 2x6 at about 8' on these walls to permenantly support the loft floor.  I am about to put up the loft floor joists (the 2x12x20's that span the this width).  The problem is that when i run the 2x12 across the room that it is about 4.5" away from the stud on the other side wall.  So the jiosts will rest on the 2x6 between the studs.  Got it?  So now i am not sure what is safe to do.
I have 10' 2x6 walls 16"oc with a 2x6 running all 30' in the pocket at 8'.  
Options
1.  Fill the space with 3 2x6 scraps cut to 4" and nailed to the wall  stud to fill the space, then attach the jiost to the 2x6's.
2.  Buy a simpson holder that will attach the joist to the 2x6 in the pocket, the joist will be between the studs. about 5" from one stud and 9" from the other stud.
3. tear down the wall and start over... Sheathing is on... i want to aviod this, but it might be the best option.
4.  Add 23 more studs where they need to be starting from the east, so this wall would have a stud about 16"oc from the east and the west  about 46 total.
5.  just toenail the jiost to the 2x6 between the studs.
thanks for any help!  We go back to work on Thursday to get that floor in and start on the roof.  
Right now i am leaning towards option number 2.  

glenn-k

Do you have a double top plate it is resting on?


deabob11

the top of the wall has a double plate, that is for the rafters (i think that will be ok).
my main concern is the floor joist for the loft.  The joists (2x12x20) will be resting on the 2x6 that is cut into the 10' wall 8' up.  The joists will be resting towards the middle on that 2x6 and not right next to a stud.  

MountainDon

#3
deabob,

If the loft joists are sitting on the 2x6 ledger they should be supported okay. I gather that your main concern is whether or not the joists will be held in place rigidly enough, as in will there be a tendency for the joists to tip, or rotate out of plumb. ??

I would think that once the flooring is installed that will prevent any twisting moment.  :-/  I don't know that a bracket would be of any greater help.  Adding a bunch of cut off 2x6's like in choice #1 would rob you of some insulation value.

Will the joists be exposed from below? Installing blocking near the outside wall would help keep the joists correctly positioned.

Just thinking out loud.

deabob11

my thought is that the floor joist may bow, crack, whatever to the 2x6 because it will be in the middle instead of next to the stud.


glenn-k

I would just block the sides in and not worry about it.  You could shoot a full width block in behind it if you wanted to, for extra bearing area.  16" centers , I'm guessing the load would be only about 600 lbs or less for each side which should be well within the bearing capacity of the wood - near 2 inches bearing even without being double or next to the stud.

Note that I just make semi-educated guesses - so John, PEG or somebody else may have better advice -- just seems to me that it should still work even if not ideal.

MountainDon

#6
The AWC joist and rafter calculator indicates that a minimum bearing surface of about 3/4" is required. (Varies with wood species and loads) If you have the joists lapping over the full thickness of the 2x6 ledger that should be ample. I doubt it matters strength-wise if the joist sits on the ledger right next to a stud or not. But like Glenn said, that's a semi-educated guesstimation.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Douglas+Fir-Larch+%28North%29&size=2x12&grade=Select+Structural&member=Floor+Joists&deflectionlimit=L%2F360&spacing=16&wet=No&incised=No&liveload=40&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span

jraabe

#7
I just did a little beam calculation... assuming 666 lbs on each floor joist (probably higher than it will ever see), that ledger could easily carry that load at the midpoint of the span between two studs. In fact even a 2x4 ledger would be a strong enough beam. A let-in 2x6 will be fine as a beam/header.

That said you will probably still want to do some sort of blocking between the joists to stabilize them and provide nailing. That will also help spread the load into the walls and sheathing.

So you should be fine, but remember, the more you can connect the joists and the walls, the better that wall will be at absorbing the outward thrust of the rafters.

glenn-k

QuoteSo you should be fine, but remember, the more you can connect the joists and the walls, the better that wall will be at absorbing the outward thrust of the rafters.

There you go -- that's why John gets the big bucks. :)

He knows a lot more stuff than we do. :D


deabob11

Thanks, i plan on putting the loft joist in the middle of the ledger and attaching it with a simpson rafter tie.  ;D
There will be about 5" on one side and about 9" on the other.  What kind of blocking are you talking about on the side?
a 2x12 or a 2x6, or (2) 2x6 on each side or will the rafter tie be enough?
Thanks!

Hmmm... i used the above calculator link for the 2x12x20 floor joists.  Do i need a center support if i used the #2 boards?

MountainDon

Quote... I used the above calculator link for the 2x12x20 floor joists.  Do I need a center support if I used the #2 boards?
I don't have the plans, do they specify species and grade?   I didn't run the joists thru the calculator before. Now I see that if it was for a regular first or second story floor then it would appear that even Doug Fir select structural is right at the edge of being okay at 16" O.C. Perhaps for a loft floor the assumption is that the loads will not be as great?? I dunno myself.  :-/

Better wait on John for definitive answer.

Blocking would usually be same size lumber. or X-shaped pieces. The rafter tie itself won't give as much in twisting resistance that blocking would offer. They are good at limiting lifting forces. Will you have enough room to swing the hammer for the nails that go into the joist?

deabob11

at 30psf they are ok, the calculator started at 40 psf.  When i tried to order the wood they said #2 was right at the edge of the span limit the room is 19'3" from ledger to ledger.  These boards were used for both the loft and the main floor of the cabin.  Our neighbor said that it looked like the bottom floor needed some kind of center support?  We dont plan on having much in the cabin.

MountainDon

When it comes to floor joists it's always best not to push the envelope and go with maximums as you may find the floor has a bit too much spring to it. And you may not.   :-/  I have no idea if lofts are calculated the same way, same loads.

deabob11

thanks again for the help.
I would like  6'5" of headroom in the loft and want to go with the lowest roof pitch possible.  
If I do an 9/12 instead of a 12/12 roof pitch, how far down from from the top of my ridge beam do i need to put the bottom of the 2x6 collar ties if i make this change to the roof?  
Otherwise, what is the lowest roof pitch i can have to leave 6'5" from the floor of the loft?
I have 15" from the loft floor to the top of the double top plate.
I will be using 2x6 rafters.  I bought long enough for 12/12, but would like to make it a little less scary to roof!
Thanks,


jraabe

#14
The floor framing is called out as either 11-7/8" I-joists or select structural 2x12 @ 16" o/c. The select structural isn't as critical in the loft where the normal loading would be 30psf at the interior areas with 5' or more ceiling height.

Personally, I would frame the lower floor with I-joists and use the best 2x12's I could get for the loft. But scores of folks have used all 2x12s without complaint.

deabob11

the framing is done and... when i ordered the lumber i didn't look at the plans (or materials list) and just ordered wood from home depot they sold me #2 for the span.  This whole cabin is #2 2x12.  It seems to be doing fine so far.  There is a little bounce in the floors.