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Worker-B 08-19-2009 10:24 AM

How much weight will a 2x4 joist attic hold
My house was built in '92 and I'm in the process of updating my upstairs bath, and I need to run another 12/2 line up from the basement to my attic, but upon looking in my attic, the attic has only 2x4 joists and is covered with cellulosic fiber insulation, and my question is; I need to get into the attic and put in two juction boxes and move several 12/2 and 14/2 lines, but I'm not sure if it will hold my weight.

Does anyone know if an attic built with only 2x4 joists will hold a person weighting roughly 250 lbs, and unfortuantely I need to go all the way accross the attic to get to the bathroom, so is there a safe way to move accross the attic?

jogr 08-19-2009 10:54 AM

It should hold. It would be best (and more comfortable) to lay some plywood down across several joists. If the insulation is deeper than the joists you want to be sure that the plywood isn't transferring your weight to the drywall ceiling through the compressed insulation.

You might be able to do most of the work from below using snakes. Are you sure you need a new line from the basement and can't tap off an existing line?

Scuba_Dave 08-19-2009 10:56 AM

How far do the 2x4's span?
How far apart are they? 16" 24" ?
I keep wood in the attic at my MIL's
--strips of plywood or 1x10/12 etc that I can move around to walk/crawl on

Termite 08-19-2009 11:00 AM

The crucial piece of information we don't have here is the span of the ceiling joists. 2x4's are normally not used for ceiling joists though. 2x6 is pretty much the standard minimum. If the house has roof trusses a 2x4 bottom chord is very likely.

Longer spans would cause more deflection. Will they fail or break under your weight? Nope. Not a concern in 99.9% of cases. My concern is that your weight could be enough to cause deflection in the joists that they weren't meant for, which could translate to some slight cracking of the ceiling's drywall joints.

Taking a couple pieces of plywood with you into the attic will certainly help distribute your weight across more than one joist at a time. Take two pieces though...Kind of hard to do it with one!

Termite 08-19-2009 11:01 AM


Originally Posted by Scuba_Dave (Post 316479)
How far do the 2x4's span?
How far apart are they? 16" 24" ?

Dave beat me to it while I was typing! :laughing:

zircon 08-19-2009 11:18 AM

Stay close to the walls
In addition to the plywood, I would try to avoid the middle of the span.

Worker-B 08-19-2009 11:25 AM

I appreciate all the information, and I'm not sure of the 2x4 span as there was at least 6-8" of cellulosic fiber insulation covering the entire ceiling and joists, but now that I know it should hold my weight I will use a rake to pull back the cellulosic fiber insulation and will lay plywood across the joists to move around. As for the electrical issue, I was under the assumption that I need 12/2 for the bathroom GFI, and unfortunately the person who originally wired my house used one 14/2 line for the entire bathroom, lights, fan, GFI and hallway outlets and one upstairs bedroom lights and outlet, so I wanted to make sure that I brought it up to code since I have gutted the bathroom and have the walls down to the studs.

Scuba_Dave 08-19-2009 12:18 PM

My MIL has 2x4's & I never worried about it
I'm only about 170ish fully clothed with a bunch of change in my pocket
But I never heard the slightest creak etc

KC - you had more info tho - :thumbsup:

Gary in WA 08-19-2009 02:07 PM

Not plywood, use a 2x6x8', or a lot. Plywood will deflect a lot more than a 2x6, especially at 24" on center, which your trusses probably are. Screw them down, unless they interfere with the insulation.

Dave, carry a 90# bag of concrete mix up there and think about it.

Ask the electrical question in electrical., aisle #3.
Be safe, G

chuck1010101 09-08-2013 01:20 AM

How much weight will a 2x4 joist attic hold
Worker-B: Having worked a few years in a truss manufacturing plant down in FL, I think I can help you a little bit (I hope). First off, almost ALL (unless it is going to be a vaulted ceiling or exposed beams) have "webbing" made from 2" X 4" top chords, and the bottom chord of each truss. And it's those 2" X 4";s that make up the strength in the truss. If your house has that "W" shaped webbing, then you could put a whole platoon of 250 soldiers on the bottom 2" X 4's and it would NOT break, because, like I said, the design of the truss, with those middle web pieces in either direction cause either more compression strength, or more tensile strength, depending on which "chord" you are standing on. Then, in the olden days, the ppl would cut little pieces of say, 3/8"-1/2" plywood, then nail then down on both sides at any point where two boards within the truss meets. Nowadays, they take these various sizes and shaped metal "plates" that have literally dozens, if not hundreds of those plates. The truss builder takes his "rocket hammer" which weighs between 20-24 ounces, and slams down on ONE corner of the steel plate. The whole truss is then run through a "press" which drives the tiny "nails" in the metal plates, which are places on both sides of the truss at every junction where 2 boards meet. Then, do as others have said and get you some boards, (preferably at least 2" by 8" IMHO,or rip down some 3/4" plywood, in, say 16" to 24" widths and those will be your "knee boards in which to crawl around in the attic without falling through the ceiling. HTH, Chuck

SeniorSitizen 09-08-2013 06:54 AM

To prevent acting like a tight wire walker in an attic, lay 1x12 fir down to walk on. It's been holding heavy weights in attics with 24" OC joists for decades and gives a platform to stand on so you can concentrate on the job at hand rather than concentrating on keeping your balance.

danpik 09-08-2013 08:19 AM

I suspect that this being a 4 year old thread the original poster has long since finished this project. Wondering if there is some way to lock threads older than, say, 1 year so they don't get dredged up and confuse everyone

chuck1010101 09-08-2013 05:42 PM

Re: How much weight will a 2x4 joist attic hold
I noticed that date right AFTER I hit the send button on my 2 cents worth on the subject. Sorry about that. Chuck

oh'mike 09-08-2013 05:57 PM

Dredging up old threads happens every once and a while----

Often something good comes from it---these older threads contain great information that still helps visitors and members learn and solve current problems.

Plus it's fun to chuckle at a member that answers a four year old thread:laughing:

chuck1010101 09-08-2013 08:41 PM

Re: How much weight will a 2x4 joist attic hold
That's true. Just like myself. I'm brand new, but I learned a few things from that post myself. And it seems like a great forum. Now to figure out how to start a new thread, because we're here in "tornado alley" in Alabama and I need some help with some questions I have about building my own storm shelter besides trading in my #2 shovel for a back hoe LOL.. Kinda hard to do, since the hole is halfway finished, and sits behind the house and lodged between a bunch of woods at a place that I couldn't get a backhoe in there (unless I hired a dozer for several days first LOL). I'll start those questions o a new thread though. Chuck

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