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russ_mc 02-24-2012 02:05 PM

Ceiling Joist Size
 
I have an unusual situation in a rec room of a house we just purchased. This rec room has 3 beams spanning a 20 x 20 room that are supporting the 2x6 ceiling joists (drywall, no attic storage) and the roof above. The ceiling joists are on top of the beams and the beams protrude into the room - dividing the ceiling into 4 parts. (the beams are resting on the outer walls and the ceiling joists are on top of the beams - not how I would have done it). The bottom of the beams are 8' from the floor. So, to make a flat ceiling and a better look, we are going to frame up a new drywall ceiling wherein the joists are attached with hangers to the bottom of the beams. The beams are only 4' apart (from each other and the side walls), so I can hang 2x4's to support the drywall ceiling. (the original ceiling will be left in place)

My problem is that I have to work the new ceiling framing around a fireplace that is 10 feet across and against one wall - protruding about 6" from the wall. The front of the fireplace is parallel to the beams. I could attach a board to the fireplace brick and hang the new ceiling joists to that board, but I prefer not to.

Instead, I plan on framing around the fireplace and I am trying to figure out the best approach. The beam to which the ceiling joists will be attached is 4' from the chimney and the outer wall. My thoughts are to run one 2x8 on either side of the chimney - hanging these on the outer wall and the beam (4' span). Then I run a 2x8 in front of the chimney and hang it on these two 2x8's. I will call this the chimney "header". The span of this 2x8 will be 10'. This chimney header will be the support for the 2x4's that will span from the beam to the chimney (2x4's - 16oc).

Is this sufficient? If not, what table do I look at to determine if this is sufficient? I think my chimney header is inadequate, but it sure would be nice to see some table or some way of determining this.

thanks......russ

joecaption 02-24-2012 02:43 PM

Without at least some pictures were all going to be guessing here. Even then it's just guesses. Someone really should be on site that knows what there doing before you start this job.
My first clue was you saying you were going to try and use 2 X 4 for ceiling joist. Unless your only running about 6' there not going to work.
http://www.awc.org/calculators/span/...rcalcstyle.asp

titanoman 02-24-2012 02:43 PM

How about a drawing?
If I follow you correctly, your plan is a good one.

russ_mc 02-24-2012 03:37 PM

Picture
 
1 Attachment(s)
The span for the 2x4's is only 4' between beams - plenty for supporting 1/2" drywall ceiling.

Sorry about the crude picture, but it is what it is. I am omitting all of the joists except those around the fireplace.

If you guys agree that 2x8's are sufficient, tell me how do you know? I'm looking for some calculation or table that will guide me the next time. Make me wiser.

Thanks.........russ

russ_mc 02-24-2012 03:49 PM

Forgot something
 
I forgot something. There is no "live load" on this ceiling - the ceiling is actually suspended below another ceiling. (The original ceiling joists are on top of the beams. The new ceiling joists will be attached using hangers to the bottom of the beam. You cannot physically get to the area between the ceilings (about 1 1/2' separation). So that is the reason for the 2x4 joists instead of the usual 2x6's or larger.

thanks..........russ

titanoman 02-24-2012 03:50 PM

...............................^

Daniel Holzman 02-24-2012 04:01 PM

I am not going to size the beams or joists for you, but I will tell you how it is done, so you can "become wiser". All residential structural elements subject to bending, whether they be beams, joists, or rafters, are sized this way.

1. Determine all of the loads on the elements. This is often a combination of code mandated distributed live load and actual point loads applied to the element. Determination of loads always requires tracing all loads downward from above the element, and accounting for unusual loads such as seismic (earthquake), and wind.

2. Compute the maximum moment on the structural element due to the imposed loads.

3. Determine the maximum allowable stress in the element. This often comes from tables based on the species and grade of the lumber, or the type of engineered lumber.

4. Compute maximum fiber bending stress imposed on the element by the loads. Compare to the allowable bending stress. If the maximum stress exceeds the allowable stress, you need a stronger element.

5. Compute the maximum deflection of the element. Check the deflection ratio (D/L) to see if it is within code allowance.

6. If the element is subject to a combination of bending and compression or tension, check it for combined stress.

Now you know how to do it. Fortunately many common residential structural elements are sized from tables. These almost never give you the most efficient size, but they are allowed by code, so where applicable you can size joists, beams and rafters from span tables.

russ_mc 02-24-2012 04:06 PM

Thanks
 
Many thanks for your swift reply.

I have been crusing the span tables for several days now, but I fail to see how to apply these tables and come up with needing to double-up 2x8s (I do think that you are right - those 2x8's are taking at least 1/2 the load of the ceiling between the chimney and the beam - makes sense to double them up) . I am well aware on how to size headers over door and window openings and how to size standard rafters/floor joists/etc using these tables, but this just bugs me. Any more guidance on where to look or read would be immensely appreciated or even rules of thumb - I am always learning.

Thanks again.


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