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-   -   sistering 2x6 ceiling joists (http://www.diychatroom.com/f19/sistering-2x6-ceiling-joists-9819/)

nelsonC 07-11-2007 03:55 PM

sistering 2x6 ceiling joists
 
Hello,

I am in the process of renovating a 1930s cape cod style house.

We intend to put a small bedroom in the attic. The joist/studs in this house are all 24"oc, old pine, hard as nails. The ceiling joists are 2x6 with a 10' span.

It has been suggested that we reinforce these by sistering them with new lumber using a strong adhesive and bolts/nails. It was also suggested that we don't need to rest these joists on the beams at either end, because what we're trying to achieve is rigidity, and the 3" that the old joists rest on each beam with is not in danger or sheering or breaking. Is this a good idea? It certainly would make things easier (ie no cutting of blocking, no levelling of new wood that is sure to be of narrower dimension).

Did I mention that the ceilings will be finished 1/2" plywood?

There is no clearance to increase the dimensions of the lumber, so we're going to have to continue to use 2x6 pine. So, once they're sistered, we're looking at 4x6 pine (theoretically) spanning 10 feet, 24"oc.

Any problems here?

Thanks! :)

John

troubleseeker 07-11-2007 09:27 PM

I don't see why you would have to double 2 x 6's for your ten foot span, unless you need to sister to flatten out the ceiling framing.

nelsonC 07-12-2007 07:35 AM

thanks for the reply.

actually, I was mistaken. it's more like an 11 foot span.

either way, the reason I need to sister them is because 2x6s 24"oc = a pretty bouncy floor. I thought 2x8s were standard joist material?

I think that our plans to put a bedroom in the attic necessitate strengthening the existing joists.

am I wrong here?

Jeekinz 07-12-2007 12:17 PM

Sister 2x8's and install another 2x8 between the joist bays. Making them 12" oc. A little extra wood and alot sturdier floor. Glue and screw the subfloor.

nelsonC 07-12-2007 01:17 PM

thanks Jeekinz, but the existing joists are 2x6. I plan to sister those with 2x6s, otherwise I'd have to either rip up the upstairs floor or lower the downstairs beam.

all I'm really looking for here are two pieces of info:

1. do the sistered joists need to rest on the beams or can they stop just short?

2. should that be enough to support an attic bedroom (sistered joists yield a total joist dimension of 3.5"X6" spanning 11 feet, 24"oc)?

thanks!

warnerww 07-12-2007 02:16 PM

Nelson I guess the question is can you get the sistered joists onto the beam. If you can this would obviously be better. If you can't sistering will help the bouncing but will not provide more strength at the beams. Will it hold either way I would think so. Is one way better than the other? Answer defiantly yes.

Jeekinz 07-12-2007 02:21 PM

They cannot stop short. Can you install 2x6 joists at 12" oc, too? You're going to use 3/4 subfloor material, correct?

If I absolutely had to use 2x6's, I would glue and screw sisters to the exsisting joists, add 2x6's 12"oc with hangers, and screw down a 3/4" sub floor.

nelsonC 07-12-2007 02:24 PM

Yeah, the subfloor will be 3/4" ply.

I came across this recently:

Reaching the ends is not a serious requirement, however, just a small benefit. Without getting into a long technical discussion, let me say that the most benefit comes from adding material to the middle of the joist span, and that the ends of the joist experience mostly shear stress, which is less of a concern than the tensile stress on the bottom-most fibers of wood at the middle of the span.

Also, what type/size screws would you use?

Thanks,

John

warnerww 07-12-2007 07:25 PM

Quote:

Originally Posted by nelsonC (Post 52720)
Yeah, the subfloor will be 3/4" ply.

I came across this recently:

Reaching the ends is not a serious requirement, however, just a small benefit. Without getting into a long technical discussion, let me say that the most benefit comes from adding material to the middle of the joist span, and that the ends of the joist experience mostly shear stress, which is less of a concern than the tensile stress on the bottom-most fibers of wood at the middle of the span.

Also, what type/size screws would you use?

Thanks,

John

Hence both will probably work but one is better. I would not say it is only a small benefit. Maybe in theory it could be. However we are talking about wood. You may think all the boards are strong and in great shape but one can never be sure. I have seen boards I thought were in good shape not hold. If just one board shears at the beam then you go from a 24" center to 48" center.
Anyway I would use liquid nails adhesive and bolt completely through with 3/8" bolts with washers on both sides every 2 feet.

troubleseeker 07-12-2007 09:58 PM

Quote:

Originally Posted by nelsonC (Post 52648)
thanks for the reply.

actually, I was mistaken. it's more like an 11 foot span.

either way, the reason I need to sister them is because 2x6s 24"oc = a pretty bouncy floor. I thought 2x8s were standard joist material?

I think that our plans to put a bedroom in the attic necessitate strengthening the existing joists.

am I wrong here?

I also misunderstood the situation. I thought you were sistering ceiling joists to hang a new ceiling on, not looking to strenghten existing ceiling joists into floor joists. The 2 x 6's are definately undersize for floor joists, but since you said you cannot go any deeeper size materials, I believe you would get better results by adding an additional joist (for 12" spacing) than by sistering the 24" spacing. You will need to remove any existing blocking at the wall plates to allow you to set the new joists on the walls. A couple of rows of blocking betwen the joists will also help by providing more contact areas to help spread the load over a wider area. Use tongue and groove plywood for the sub floor, a generous bead of heavy duty sub floor adhesive, and screw the floor down with 2 1/4 " screws rather than nail it. The screws will be much more resistent than nails to working loose and causing future squeeks.

crecore 07-13-2007 06:08 AM

For the extra $40 or whatever I would also go with Advantech tongue and groove subflooring (polyester based glue under higher compression). It's a lot stiffer and will help get rid of bounce. Use a poly based const adhesive and screw.

AtlanticWBConst. 07-13-2007 06:13 AM

Quote:

Originally Posted by crecore (Post 52775)
For the extra $40 or whatever I would also go with Advantech tongue and groove subflooring (polyester based glue under higher compression). It's a lot stiffer and will help get rid of bounce. Use a poly based const adhesive and screw.

It's all we ever use for floor sheathing ....

Link: http://www.huberwood.com/main.aspx?p...antechflooring

Drakeheating 11-16-2008 09:17 AM

I actually have that problem now. I am trying to strengthen an existing ceiling that has 2x6's 16" oc that span 13'. One end is fastened to the house with a ledger board and joist hangers and the other end rest on two 2x6 cross members with about a 2' overhang (16' 2x6's) I would love to be able to use 2x8's but need all the heard room possible. I have been told that the existing roof structure would be fine hanging 5/8" sheet rock from it but am concerned having 3 little girls playing in this room. I don't want to take any chances. Would I be ok to sister 2x6's to each existing 2x6 and be able to handle the weight of the snow and the new sheet rock with the given span?

pcampbell 11-20-2008 06:35 PM

How can one determine how much a 2x6 on 16 will support? It is a cape code style home, but the staircase runs Parallel, not perpendicular to the front of the house. Around the staircase they only span ~9 feet each, but beyond the staircase, they span the full 18 feet + the width of the staircase = 21 feet??? This is an attic, with a full staircase in a New Jersey ~1950 built house. I am not sure what type of wood this is. Just trying to pick up some general info for down the road...to see what would be involved in turning this into a BR. Or even a BR+Bath. It could be interesting or not, to know that our main floor has 2x8s on 16".

Marvin Gardens 11-20-2008 08:56 PM

Quote:

Originally Posted by Drakeheating (Post 185771)
I actually have that problem now. I am trying to strengthen an existing ceiling that has 2x6's 16" oc that span 13'. One end is fastened to the house with a ledger board and joist hangers and the other end rest on two 2x6 cross members with about a 2' overhang (16' 2x6's) I would love to be able to use 2x8's but need all the heard room possible. I have been told that the existing roof structure would be fine hanging 5/8" sheet rock from it but am concerned having 3 little girls playing in this room. I don't want to take any chances. Would I be ok to sister 2x6's to each existing 2x6 and be able to handle the weight of the snow and the new sheet rock with the given span?

You're out of luck. Can't span 13' with a 2x6 and expect it to stay up with weight on it.

Gonna have to go with 2x8's structural or 2x10 #2 or better.

Then there is the issue of the double 2x6's that the joists are attached to. I would find out what kind or span that is before I do anything.


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