Using 2x4's For Joists - Building & Construction - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Building & Construction

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 05-02-2012, 11:34 AM   #1
Newbie
 
Join Date: May 2012
Posts: 1
Rewards Points: 10
Default

using 2x4's for joists


Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.

Advertisement

tyrral is offline   Reply With Quote
Old 05-02-2012, 11:54 AM   #2
Member
 
Join Date: Sep 2009
Location: California
Posts: 170
Rewards Points: 184
Default

using 2x4's for joists


Quote:
Originally Posted by tyrral View Post
Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.
The problem you'll have with trying to use 2x4's as floor joists is that most span tables don't include 2x4's. This means that it will be hard for you to convince your planning department to approve your plans without some sort of proof that the 2x4's are adequate.

I've used 2x4's for floor joists before, but I had to prove that they were adequate for the design loads.

I suggest that you try to use a standard size floor joist and then refer to the span tables in your local building code. If using a standard size floor joist creates a problem with your finished floor elevation, look into notching the joist at the supports (read your local building code for notching requirements).

Finally, if you can't get a standard size joist (2x6 or larger) to work, you'll have to find a way to prove to your building department will accept 2x4's. You might try this, but I doubt they'll accept an online calculator as proof.

http://www.awc.org/calculators/span/...rcalcstyle.asp

Advertisement

loftezy is offline   Reply With Quote
Old 05-02-2012, 12:23 PM   #3
Ole Wood Worker

 
BigJim's Avatar
 
Join Date: Sep 2008
Location: Lookout Valley, Chattanooga, Tennessee
Posts: 7,872
Rewards Points: 2,836
Blog Entries: 1
Default

using 2x4's for joists


Quote:
Originally Posted by tyrral View Post
Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.
I don't know what cross beams are, do you want to use the 2X4s as ceiling joists or floor joists? 2X4s for either in a 4' span will work and carry the load but if in a house they may not pass inspection.
__________________
New members: Please consider adding your location to your profile.

If you aim at nothing, you will hit it every time!

Jim
BigJim is online now   Reply With Quote
Old 05-02-2012, 01:27 PM   #4
Member
 
Join Date: Apr 2012
Location: UK
Posts: 713
Rewards Points: 500
Default

using 2x4's for joists


What is your Code's stipulation for domestic floor loading per sq.ft.?
tony.g is offline   Reply With Quote
Old 05-02-2012, 04:57 PM   #5
Member
 
Join Date: Feb 2010
Location: MI's Western UP
Posts: 599
Rewards Points: 500
Default

using 2x4's for joists


It should be OK, but I'd go closer than 16" OC. 2x4s are cheap.

another option might be to put plywood below the joist as well as above. This would make the plywood the stressed member and the 2x4s would be acting more like the webbing of an I beam.
forresth is offline   Reply With Quote
Old 05-02-2012, 05:34 PM   #6
Member
 
Join Date: Jan 2012
Location: Pacific North west
Posts: 1,311
Rewards Points: 626
Default

using 2x4's for joists


you don't use 2x4's for floor joists.
Nailbags is offline   Reply With Quote
Old 05-02-2012, 05:39 PM   #7
Member
 
concretemasonry's Avatar
 
Join Date: Oct 2006
Location: Minneapolis/St. Paul, Minnesota - Latitude 45.057 Longitude -93.074
Posts: 3,828
Rewards Points: 2,144
Default

using 2x4's for joists


When you add floor joists you are increasing the deflection and "bounce" that is not reflected in the typical tables that only address strength and not comfort or livability. It will deflect more than you expect since you are adding the deflections of each layer.

Not specifically covered by the minimal codes, but sounds like a mistake waiting to be discovered.

Dick
concretemasonry is offline   Reply With Quote
Old 05-02-2012, 05:44 PM   #8
Framing Contractor
 
Join Date: Apr 2006
Location: Caldwell, NJ
Posts: 1,758
Rewards Points: 1,000
Default

using 2x4's for joists


Quote:
Originally Posted by tyrral
Can I use 2x4,s for joists on 16" centers if the span is 4'1"? The joists will sit on top of the cross beams.
The answer.is no. Anyone here who tells you that.you can has no idea what they are talking about and giving you useless and dangerous information.

All you have to do is call your building department and ask them.
__________________
Joe Carola
Joe Carola is offline   Reply With Quote
Old 05-02-2012, 06:24 PM   #9
Member
 
Join Date: Apr 2012
Location: UK
Posts: 713
Rewards Points: 500
Default

using 2x4's for joists


On a 50" span, assuming live load 40 and dead load 10 psf, 2x4s would be ok @ 16.25"c/s.
The bending stress would be below 350psi, which is very low. Deflection, shear and bearing would all be well within normally accepted limits - see attached figs. for stress.
Attached Thumbnails
using 2x4's for joists-scan0049.jpg  
tony.g is offline   Reply With Quote
Old 05-02-2012, 06:34 PM   #10
Member
 
Join Date: Mar 2011
Posts: 99
Rewards Points: 75
Default

using 2x4's for joists


Quote:
Originally Posted by concretemasonry View Post

Not specifically covered by the minimal codes, but sounds like a mistake waiting to be discovered.

Dick

Jeez, it was just a question, you don't have to call him a dick about it.
rubberhead is offline   Reply With Quote
Old 05-02-2012, 06:43 PM   #11
Member
 
concretemasonry's Avatar
 
Join Date: Oct 2006
Location: Minneapolis/St. Paul, Minnesota - Latitude 45.057 Longitude -93.074
Posts: 3,828
Rewards Points: 2,144
Default

using 2x4's for joists


The responder (Dick) was a registered engineer with over 30 years experience and is aware of the practicality of designing a suitable structure and just not being a nit-picker and being precise with calculations based on assumptions and minimums.

Dick
concretemasonry is offline   Reply With Quote
Old 05-02-2012, 07:03 PM   #12
Civil Engineer
 
Join Date: Mar 2009
Location: Boston
Posts: 4,309
Rewards Points: 2,196
Default

using 2x4's for joists


To Tony.g, your computations are incorrect. Specifically, in the last line where you compute the bending stress, you use the formula Stress = M/I, which is incorrect. The correct formula is Stress = My/I, where M is the moment, y is the distance from the centroid of the beam to the edge, and I is the moment of inertia. If you want to get rid of the y, you can use Stress = M/S, where S is the section modulus, however that is not what you did.

The correct answer for maximum stress should be approximately 570 psi. Still allowable for most any species of lumber, but substantially higher than what you presented.
Daniel Holzman is offline   Reply With Quote
Old 05-02-2012, 07:09 PM   #13
Civil Engineer
 
Join Date: Mar 2009
Location: Boston
Posts: 4,309
Rewards Points: 2,196
Default

using 2x4's for joists


Check that, I see you used the correct formula, however a 2x4 is not 2 inches wide and 4 inches deep, it is typically 1.5 inches wide and 3.5 inches deep, so the correct maximum stress is about 570 psi as I noted, however it appears the error was due to incorrect dimensions of the joist. Unless you think the OPS has access to full size lumber, pretty unlikely.
Daniel Holzman is offline   Reply With Quote
Old 05-02-2012, 07:28 PM   #14
Member
 
GBrackins's Avatar
 
Join Date: Apr 2012
Location: Fairhaven, Massachusetts
Posts: 2,877
Rewards Points: 2,032
Default

using 2x4's for joists


for ceiling joists using 2x4 http://awc.org/pdf/STJR_2012.pdf

see page 11 and 12
__________________
Gary

"You get what you pay for, and sometimes free costs more!"
GBrackins is offline   Reply With Quote
Old 05-03-2012, 06:01 AM   #15
Member
 
Join Date: Apr 2012
Location: UK
Posts: 713
Rewards Points: 500
Default

using 2x4's for joists


Quote:
Originally Posted by Daniel Holzman View Post
To Tony.g, your computations are incorrect. Specifically, in the last line where you compute the bending stress, you use the formula Stress = M/I, which is incorrect.
Hi Daniel,
I think my writing may have been at fault as what you referred to as 'I' was in fact 'Z', though I think you spotted that later.
As a matter of interest, something I came across quite recently in steel construction is that in the US, you use 'S' for elastic mod. and 'Z' for plastic mod. Confusingly, here and in Aus, its the other way round!
On your second post, you noted the timber section-size as being 1/2" less in width and depth. Again as a matter of interest, is it really that much? Timber sizes here are typically between 3/16" and 1/4" less than stated size, but they let us do calcs based on the stock size(!)
If your finished size is 1/2" less, I agree it would have an effect on the stress, and probably deflection as well.

cheers.

Advertisement

tony.g is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
stacking 2x4s or sistering joists? dbeers02 Building & Construction 7 03-12-2012 12:46 PM
Building a 2nd Ceiling using 2x4's and Drywall cwk36 Remodeling 1 10-27-2011 09:58 PM
Sistering 2x4s for 2nd Ceiling cdogg36 Building & Construction 1 06-13-2011 01:30 PM
Build 10 foot wall with 5 foot 2x4's Gandalf Carpentry 22 12-08-2009 10:50 PM
Replace rotted 2x4's in cement patio KennS Building & Construction 3 06-19-2008 12:41 AM




Top of Page | View New Posts