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Fox 03-01-2010 03:17 PM

Attic joist cross beams & moving them
I wish I could make this a more simple question:

Can I move existing attic joist cross beams (If that's what they're called) by prying them up and adjusting them to another location, or do I need to nail down other supports first, then pry up the old ones?

Since I'm probably not explaining this properly, let me give you a little backstory:

This is a townhouse in Virginia. The joists are 2x4s, varying in how far apart they are from each other. (from 25 to 22 inches) There are 8 foot 2x4s running perpendicular across the top of the joists, nailed into each. I assume these are to keep them from shifting laterally. These "cross beams" as I'm calling them, run from either side of the attic, and are only in two places. (1/4th of the way from the front, and 1/4th of the way from the rear of the house)

I realize I'm extremely limited in what storage capabilities one can place into an attic with only 2x4 support. These are clearly only designed to support the attic itself, and nothing more. This said, I'm attempting to place plywood atop 2x4s standing on their sides, 16" OC, nailed into the joists with strongtie joiners. I originally purchased 4' pine (to reduce weight) 2x4s to run above the length of the few load bearing walls. Of course, these 8' 2x4s are running there already, perfectly in the way.

My question: Can I move these boards? Do I have to install nearby supports before doing so? Can I use these cross beams to hold my plywood, thereby serving two purposes, but killing two birds with one stone?

I may need to take a picture and upload it before this all makes sense. I can expound on how I'm attempting to provide a little additional storage in this townhouse (which very much needs it) by enabling the storage of light, high volume items that are not sensitive to high temperature extremes.

I'm terrified of placing too much weight into this attic or compromising the structural integrity. Given my knowledge of physics and the weight bearing capabilities of PINE 2x4s (although I'm having difficulty identifying the type of wood used in this attic as I'm not familiar enough with such matters to identify it by sight) I believe the crossbeam is simply to prevent the joists from shifting too much, but I've no way to knowing if it has any stress on it at the moment. I'm thinking I'll need to use those 4' 2x4s to lay down additional cross support before I could even think about pulling up the 8' ones, let alone trying to use them to hold the plywood.

I realize this is a bit complicated. If anyone would be kind enough to advise, I greatly appreciate it and am more than willing to clarify the situation!

Thank you!

Ron6519 03-01-2010 03:41 PM

Post some pictures to simplify the description. Also post pictures of the roof framing.

Fox 03-02-2010 06:20 AM

It really is necessary:

These aren't the best pictures, but it should work. Every thumbnail can be clicked on to take to a larger image, and then clicked on from that link to get the full sized image.

Below are the boards I'm talking about. Running from the bottom left, to the top center of the picture are the horizontal crossbeam 2x4s I was talking about. They're nailed into every joist.

This is the kind of strong-tie joiner I intend to use to place the 2x4s on end atop the joists at 16", placing the plywood atop them over LBWs:

The following two are shots of the trusses. I wish I'd a wide angle lens on this camera:

I can take additional shots from other angles if necessary. I'll be back at this property Friday.

Is anything I'm doing feasible?

Thank you very much! I know it's a rather convoluted project.

Scuba_Dave 03-02-2010 07:59 AM

Still not 100% sure what you are doing
Are you moving 2x4's on the floor to add insulation ?

Or you want to move the 2x4's that support the roof ?
2x4's to the roof are a truss system, you can't touch/modify those at all
An engineer would need to be involved to redesign the roof load/truss

Ron6519 03-02-2010 10:45 AM

With the truss system, you can't just add a floor and use it for storage. You would need to get someone in there( who's competent) to determine what you can do as far as adding loads to the floor.

tpolk 03-02-2010 01:18 PM

no messing with the truss- the 2x perpendicular to the truss and nailed on top are stiffbacks and serve a purpose- no playing with them. an engineer could give you what you need for light loading but i bet he will want to run from outside to outside bearing points but maybe not- gotta ask

Gary in WA 03-02-2010 08:42 PM

The horizontal "cross beams" are 2x4 rat runs as framers call them. These are installed at a set distance, as per truss design papers, to maintain the racking loads for the roof trusses. Contact a truss engineer for the specific answer as no one on this forum could help because of the measurements, loads, and other design qualities for your specific roof. Smaller trusses rely on the ceiling drywall for lateral shear loads, bigger ones, like yours, require more. I would not load the bottom chord any amount unless they were designed to do so:

Be safe, Gary

Fox 03-16-2010 04:15 PM

Sorry about the late reply. Another project has taken my time considerably.

Thank you all for the assistance.

GBR: Thanks for the name! I wasn't even sure what to call these things. They're most definitely "rat runs", as they're mid-span below the trusses. After some research I've found that they are necessary to prevent the rafters from twisting and shifting due to heat. (sometimes with added wood to form an L or U shape, then called a "strong-back")

I'll take some measurements and call in a contractor. The house certainly won't fall down if they were removed (and I've no intention of removing them) but it looks like I'll need to place my own, additional "rat runs" before shifting them, as a few inches left or right doesn't look like it will remove their ability to do their job.

I may actually wind up turning these rat-runs into strong-backs, adding another set (forming something like a railroad track) and placing plywood atop them for storage. (Low-mass/high-volume storage items)

Once I get all this figured out I'll contact a contractor, or construct a small animated demonstration to run by you guys to see if my idea sounds like it's feasible!

Thanks again for the comments and advice! It's nice to have some idea of what I'm talking about!


Fox 04-01-2010 10:36 AM

Just thought I'd update this, if anyone was wondering:

I spoke with a contractor after taking more pictures.

After I described the problem to him he told me that as this is a town house, the lateral movement isn't as likely, so the rat runs aren't as important as they would be in a single family home. His recommendation was to move them off the joists and place them on the trusses themselves. (I think he called them 'wind braces' or something like that?) Apparently that's how the code has been changed in this area since after this house was built.

I'm actually going to install secondary braces before trying to move these off the joists and onto the trusses, but at least they'll be out of the way of where I plan to place the plywood. (He did state that I can't be more than a few feet from the load bearing walls for fear of putting too much weight up there.)

Gary in WA 04-01-2010 09:07 PM

Just a note on placing plywood on the built-up framing. It will trap the moisture because of its low permeability, 0.70. This could trap and collect the moisture rising from the conditioned room below. Better to use solid wood boards with a gap between them.
Trusses usually span from exterior walls across the whole house, or a portion of. The load bearing interior walls may not be loaded at all, other than the floor loads below, unless a special roof configuration.

Be safe, Gary

Fox 04-01-2010 10:12 PM

Will the polystyrene foam I put between the joists cause any problems like the wood? I know it has absolutely no moisture permeability. I plan on lying it directly against the ceiling drywall.

Is there a way to tell if it is indeed a load bearing wall? I know the first wall with which I'm working has another wall below it, and a wall below that. The second has a wall below it, and while no wall below that, it has the main support I-beam (with pillar) in the basement.

Gary in WA 04-02-2010 08:48 PM

Hard to tell from here. In the attic picture, if a diagonal or vertical strut member in the truss design lands on a wall, it could be a bearing wall. Depends on the original engineering of the trusses. Most trusses are designed to bear on exterior walls, but not always. I would span from bearing wall to another with a joist strong enough the carry the proposed load, elevated on a block at each end tall enough to get your required R-value in insulation under it. Tie each end together with rim joists and lay some spaced boards for storage. Stay completely off the trusses.
The foam board will let moisture through it, unless 11" thick....then it is vapor impermeable- a vapor barrier, not a vapor retarder.

Be safe, Gary

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