New Floor Joists 1

[jonrichard1]

My attic is getting all new floor joists. Currently, the joists are 2x6x12 - but we want to put a bathroom/closet down, therefore I'm installing 2x8x12 (to meet standards).

I plan on sistering the new joists to the old joists. Can anyone tell me the correct procedure? Bolt size, number of bolts, placement etc...

Much appreciated....
Jon

 

[jrmack]

A 2x8 will not span 12 feet. More like 2x10@16"c/c. A bathroom with tile floors will require two layers of plywood, especially if you use tiles larger than 10".

I am not sure why you want to use bolts. The existing 2x6 are not contributing to anything. Just forget that they are there. I would be more concerned about nailing of the deck and glueing of the deck.

I would recommend that you hire a local structural engineer to advise you on what is best, otherwise something may not be done quite right.

 

[SlideRuleEra]

jonrichard1 - Are the joists 12 feet long or 12 inches on center? You will need both values (length & spacing) to make the calculations.

 

[jonrichard1]

The joists are twelve feet long. An engineer told us this is what we needed to do, however he did not mention the proper size lag screws, washers, etc.

In one section of the attic two of the 2x8x12 will overlap -this in order to span the length of the house.

 

[RVSWA]

How's it going…
Have you considered the difficulty of draining your plumbing, or fitting a "P" trap in a 7.25" depth? The 2x10 suggestion has that advantage. Use construction adhesive to address any squeaks that may develop between the two joists. Two rows of 8d nails at 12" o/c should transfer the necessary force to engage the existing 2x6's, nails are cheap. I can't stress the importance of consulting a Licensed Engineer in your area. There are many considerations that may not be immediately obvious.

 

[dicksewerrat]

Deflection will be the primary item to look at. Tile floors and plumbing fixtures do not like to move . Especially during use.

 

[jonrichard1]

The engineer I consulted said 2x8 was the minimum and the 2x6's that were in place would not do. Because the bathroom will be relatively close to the stack, hot/cold pipes, etc, 2x8's should do the trick. Per his recommendation, I went ahead and bought/cut all the necessary joists.

Essentially, I have about $400 worth of lumber in my yard ready to go. After reading these posts, my head hurts.

Given his on site recommendation, I'm going to go ahead and place the 2x8 joists (once the electricians are done replacing the knob/tube).

I'll let you know how it goes.....God willing.

I read a site that indicated lag screws and glue - was that incorrect.

Thanks everyone-
Jon

FYI: There is no bathtub going into this bathroom. Simply a shower and two pedistal sinks.

 

[Striker]

In most jurisdictions, the floor load is 40# PSF live load and 15#PSF dead load, but you could effectively reduce the dead load by 5# if you DO NOT attach the 2X8 to the 2X6, otherwise the assembly acts together and must meet the requirements. Typically, if I had to specify 2X8 in this instance, I would say use 2X8 SYP#2 @ 12" o/c, though I would prefer 2X10 SYP#2 @ 16" o/c.
If your local code allows the reduction of floor loads to 30# (many do) then 2X10 SYP#2 @ 24" o/c will just make it. In any instance, you must use 3/4" subflooring preferably glued to each member with either a high VOC PVA glue or something like Liquid Nail.
As far as attaching the new joists to the old...Don't ..let them act independently, Also, it might be a good idea to add a bearing block (A 2x4 laid flat on top of walls) under each new joist. This will prevent the flex from the floor from affecting the ceiling downstairs. Otherwise it could cause the ceiling to crack when pressure is applied to the ceiling gypsum or plaster.
And a little more to help you out... if the somewhat bad news is not enough.... For making your turn with the toilet drain, you should seek out a 90 degree street elbow and a closet flange to fit on the male end of the street elbow, You just will be able to make a legal turn with the toilet plumbing drain in 7 1/4" plus the floor decking. If you can't find that, then a regular elbow with a MALE closet flange will get you just about as close.
As far as doubling up on the floor decking, it should not be needed IF you use a standard tile underlayment board such as HardiBoard or WonderBoard. These come in 1/4" 3/8" and 1/2" thicknesses. If you are not using a ceramic, then you should be able to apply your sheet flooring directly over a properly prepared plywood floor.
Also, insulate between the floor and ceiling to soften the sound transfer from upstairs and to mask any pitiful singing that might emmanate from a happy camper using your new shower.

Good luck on your project and please DO NOT try to save money on the floor joists, in the long run, you will not be happy with the deflection and vibration when someone walks across the floor.

 

[keithsines]

2 x 8's will work fine for the loads, especially if they are sistered to the 2 x 6's and glued and nailed as previously suggested. There is an old rule of thumb that will work in 99% of all ordinary house loads (30-40 lbs. psf) with a 16" o.c. spacing. Multiply 1 1/2 times the height of the joist material you want to use and it will give you the maximum span when using spf lumber. For example: 1 1/2 x 8 = 12. Therefore, a 2 x 8 will span 12 ft. As a former Cheif Building Inspector for the county I used that rule of thumb all the time. In bathrooms, to allow for plumbing, particular drain lines, a 2 x 10 would be better for the fall (1/4" to the foot) required for proper drainage.

 

[LPPE]

I'd say nail the 2x6 and 2x8's together. If you're worried about a single 2x8 not working, then 'sistering' a 2x6 to it will certainly improve its capacity. I dont know what your nail spacing should be, but you should use 14d to 16d nails and, we have a saying around here, "Nail the sh!t out of it."

 

[Striker]

I have had a chance to review the data for your project a little more in depth this morning and here is what I have come up with.

Using a single 2X8 SYP#2 @ 16" o/c will make the 12' span with a 7% margin with joist member deflection from bending being the controlling factor. This will make the floor a bit springy, and if the bottom of the joist comes into contact with the ceiling material, it could cause the gypsum to crack. on the ceiling below.

If you glue and nail the 2X8 to the side of the 2X6, presuming the 2X6 bottom chords are 24" o/c you still only have a margin of 9% with bending being the controlling factor. You will also be relegated to the possibility of the ceiling cracking below, but it will work just fine. Most engineers like to see 10% but 9% is pretty close.

A 2X8 SPF #2 @16" o/c by itself is undersized for the span by 13%. Once again, bending being the problem. At 24" o/c 2X8 SPF #2 and a 2X6 fully glued and nailed misses the mark by 5%.

Since you already have the joists and they are already cut, presuming they are SPF, I would decrease the joist spacing to 16" o/c it would mean adding 1 joist for every 4' of floor length. A small investment for peace of mind. This is presuming that you do not attach the 2X8 and 2X6 together, otherwise the floor load will need to be increased to account for the ceiling assembly.

These calculations are based on a typical of 40#LL and 10#DL. If you are able to reduce the loads to 30#LL and 10#DL your margin increases to 32% over for 2X8 SPF #2 @ 16" o/c, which is well within what you are wanting to do.

Once again, good luck..

 

[jonrichard1]

These posts are making me feel a lot better. However, I am unclear as to weather I should sister the joists to the existing ones, or let them stand on their own. Some say yes, others say no.

If I chooose NOT to attach the new joists to the old (drumroll please) how do I effectively nail/screw them into place. To answer this question I will consult an engineer via phone, but I would appreciate your thoughts as well.

Much appreciated,
Jon

 

[Striker]

You should weigh the pro's and con's and make your own decision basically, depending upon what you are comfortable with. Here is how I see it..

nail to existing

pros
easier to install
increased strength by combining members
less measuring involved

cons
potential ceiling cracks from joist bending
increased floor loading
nailing the joists together

do not nail

pros
can place joist away from existing fixtures and wires
ceiling not affected
reduced floor loading
do not need to nail joists together

cons
need increased number of joists
horizontal blocking required on joist ends
bearing blocks needed on each end
more measuring involved

There may be more, but you will need to come up with them since you are most familiar with your own situation.

Now, if you do decide to nail them together, I personally would recommend one of two options.
a) Two rows of 12d nails along each member 8" o/c end to end
b) staggered 12d nails top to bottom 12" o/c with a structural grade PVA glue (high VOC wood glue) along entire member for a 100% coverage.