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Old 03-11-2012, 09:20 PM   #1
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stacking 2x4s or sistering joists?


I bought a 1920's four corner style house. It is 2 stories plus a huge attic.
The ceiling joists on the second floor are 2x4s. I have taken down all the plaster and lathe and am replacing it with sheetrock.
My question is, the attic is huge and I would to like to finish it off into some living space. The "floor space" in the attic is around 500-550 square foot so I would gain a pretty good amount of area.
The roof peak is @ 12 feet to the peak from the top of the floor in the attic.

It has 2x4s for the second floor ceiling. I know this is not strong enough for a floor. Can I double stack another 2x4 on top of the current 2x4s either running with or crossways across and attach them together mechanically with hardware ?

I thought if i stacked them, then ran 1/2x7 inch plywood strips beside them the full length and anchored them together, this would be strong enough to add 3/4 subfloor over that. So it would would be 2 stacked 2x4s with a half inch thick piece of plywood covering the sides of the boards and everything anchored together.
The other way would be running another set of 2x4 joists standing on end across the current joists at a 90 degree angle like a pole barn roof is made. The floor would still be 7 plus inches thick not including the subfloor.
There are several support walls holding up the current ceiling joists and I have also added a few when I framed in some closet spaces in the bedrooms.
Adding sister joists would require I rewire the second floor since the wiring runs through drilled holes the joists.
Any thoughts?

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Old 03-11-2012, 09:25 PM   #2
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stacking 2x4s or sistering joists?


Not even close to being the required strength to carry a floor.
http://www.awc.org/calculators/span/...rcalcstyle.asp
I'm surprized you do not already have seams cracking and nail pops.

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Old 03-11-2012, 09:40 PM   #3
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stacking 2x4s or sistering joists?


Any suggestions that would make it strong enough? The house has dimension lumber in it. The ceiling joists are not part of a truss. The rafters are cut and set on the exterior wall plates.
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Old 03-12-2012, 01:52 AM   #4
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stacking 2x4s or sistering joists?


You need a structural engineer to determine how to strengthen the floor (if it is possible without removing the roof).
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Old 03-12-2012, 07:59 AM   #5
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stacking 2x4s or sistering joists?


Quote:
Originally Posted by dbeers02
Any suggestions that would make it strong enough? The house has dimension lumber in it. The ceiling joists are not part of a truss. The rafters are cut and set on the exterior wall plates.
Yes, do it the right way and higher an architect it engineer to spec out what size floor joists you will need. You have no choice but to use floor joists. You will need plans permits and inspections anyway whether you or an architect or engineer draw them. Are you planning on getting permits?

Nothing you say will work with your 2x4s. Your not re-inventing the wheel here.

The wires will have to be disconnected and fed back through the new joists.
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Old 03-12-2012, 08:44 AM   #6
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stacking 2x4s or sistering joists?


You did not mention the clear span of the floor joists, that would be a useful figure to begin the process of analyzing alternatives. Generally the size of floor joists is taken from tables, which are typically found in the local code book (much of the U.S. runs off the International Code Book). Your local building inspector will likely be very knowledgeable about which code you run off, and can direct you to the section where the minimum allowable joist size is specified.

Whatever it is, I can assure you that it is considerably larger than 2x4. You are transitioning from unloaded space to floor loading, which is typically 30 or 40 psf, depending on the use type assigned (again, the local building inspector will tell you which use type is required).

I have seen cases where existing dimensional lumber was added on to, but this was typically done in situations where there was no feasible alternative. I do not perform structural design over the internet, but I am pretty comfortable saying in this case that there is no cost effective technique for using 2x4's to work as floor joists as part of an augmented system, so in all probability you will need to install properly sized joists, either dimensional lumber or I joists. You can certainly leave the 2x4's in place, they will add little usable strength, but you may want to salvage the lumber for other uses. Of course the wire and any other transverse elements will need to be relocated, part of the job.
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Old 03-12-2012, 12:01 PM   #7
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stacking 2x4s or sistering joists?


Some important things to point out, I believe the OP is asking if he can take an existing 2x4 ceiling joist and add another 2x4 on top of it and pretend it is like a 2x8. If he is asking can he do this, the answer is no.

Not enough information is provided to determine whether a 2x8 is adequate, but if a 2x8 floor joists was adequate, it would be better to sister a 2x8 joist to the side of the existing 2x4.

2x4 and 2x6 lumber is usually a wood species which is of inferior wood species, locally for me S-P-F is common, because usually it is being used in structural applications like studs in a wall where it doesn't need the strength as much.. In a wall, the stud as a vertical member will tend to be used in compression and the load will tend to be spread across several members, top or bottom plates tend to also be in compression and aren't acting to react in beam loading situations anything but a distributed load.

Larger cross-sections more often will be of a stronger species, such as I locally find 2x8, 2x10 and 2x12 I can purchase at big box stores are hem fir or douglas fir. These tend to be used in floor joists, headers, beams and such where they will be oriented horizontally and carry loads between points where it is supported.

In reality, your existing ceiling joists are probably aged and dried to where they are stronger than a 2x4 you could add to the top. I have a 1917 bungalow type house with a half second floor, and I have 2x4 rafters and ceiling joists... On the rafters, I was doing something like what you describe by adding 2x4's, but my purpose was not structural - it was to increase the rafter cavity depth for insulation and ventilation baffles. The old wood was actually sometimes hard enough that my pneumatic framing nailer didn't drive the nail all the way.

The point is that really, the new 2x4 couldn't add much stiffness, and the floor stiffness of your proposed construction would depend on the old 2x4's which you already recognize are not adequate for use as a floor.

A continuous cross section of a 2x8 is stiff and strong as it is because if you think of it's 8" nominal dimension as if it was made of layers, the strength comes from the layers being rigidly constrained to eachother.. Taken to the extreme, think of the rigidity of a 2" thick stack of sheets of paper, each paper can slide relative to the one next to it, so it isn't very rigid. Still, even if your new softer 2x4 was constrained to the older harder 2x4, it wouldn't contribute much because a load between supports tries to stretch the structural member at the bottom and compress it at the top, so the softer 2x4 would still compress... The difference in material strength would place strain at whatever means is trying to bond the 2 boards at the middle.

Another point which I don't think I understand well enough to go into full detail, but lumber is cut from a tree in a particular way. It is rated based on standards about number of knots, and the board is cut in a manner that the heartwood is at the center rather than biased towards one side or the other... This is stuff that is done a particular way because people that are smarter on the subject than me figured out that it was the best way to do it... So by example, a 2x6 and a 2x12 ripped in half are not the same thing.
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Old 03-12-2012, 12:46 PM   #8
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stacking 2x4s or sistering joists?


Thank you all for all the good information. I will contact a engineer and have him come look at what I have. I would rather be safe than sorry.

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