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New construction of second floor, would 2x8' be okay to use instread of 2x12'?

14K views 26 replies 8 participants last post by  wesshe69 
#1 ·
We have bought a high school gym and we are making it our home. It has great bones. We are going to build a second floor and was thinking that it would be ok to use 2x8 floor joist instead of 2x12 floor joist, What do you think?
 
#12 ·
We have bought a high school gym and we are making it our home. It has great bones. We are going to build a second floor and was thinking that it would be ok to use 2x8 floor joist instead of 2x12 floor joist, What do you think?

We will be adding walls...

Span will be 14 ft to 14 1/2ft and it will have 2 bedrooms 3/4 bath and a sitting area.
So you have a big open area that is most likely on a slab and you will be framing walls on top of this slab around 14'-14' 6" supporting a second floor. What do you intend to do about footings underneath these walls that will be supporting the second floor?
 
#11 ·
2x8's are going to be very bouncy....and that is if you do it 12" OC.

If your worried about the cost difference between 2x8 vs 2x12....then you should not be doing the project.

I'm doing a 2-story addition right now....the upstairs is 2x12 12" OC....the original house is 2x8 12" OC...BIG difference....

Additionally, I used 1 1/8" T&G upstairs.....I've had several people comment that it almost feels like they are walking on concrete.....yea...it's that firm...

Do all the work yourself and save a lot of money....

If you want to see what mine looks like....click on the link in my signature....
 
#15 ·
Okay, so you do not need a permit. You still need to decide the basis of design. There are only five ways I know of to determine the basis of design for a structure. Here they are, not in any particular order:

1. Build to code. If there is a particular code for your area, you use that. If there is no code for your area, you select a reasonable code such as the International Residential Code, and go by the requirements of that code.

2. Build according to architectural and engineering plans. This requires you to hire an architect and engineer. This may be necessary if you are building a structure that is beyond standard code, for example you are building an underground house and code does not address such a structure. Also, if you go this method, you can design using materials not discussed in code, so you have more flexibility than strict code design.

3. Build by copying another structure. You examine another structure that is similar to the one you propose, and attempt to duplicate the construction technique.

4. Build by experience. Many contractors are familiar with construction practice based on their personal experience, so they build that way. No code, no engineering calculations, this is kind of a variant on technique 3. The problem with this approach is that any flaws in the design are repeated on the next project.

5. Build by guesswork. This is the technique of last resort. You have no idea how to build based on code, calculations, experience, and there is no similar structure to copy, so you just guess. Very dangerous approach.

My suggestion to you is to decide on which code you would like to base your design on. I suggest the International Residential Code, latest edition. There are tables in the code that should allow your contractor to select an appropriate sized joist for your application. If for some reason you want to deviate from code, or code is not applicable to your specific situation, I suggest hiring an architect to help you through the design process. I have seen may structures built by methods 3, 4 and 5, I do not recommend those approaches.
 
#6 ·
We have had contractor tell us that he would use 2x12 even at the 14ft span, then We have had another contractor tell us that 2x8 would be good with 14ft span. We dont mind going with 2x12 if that is what is needed however if 2x8 is going to the same as a 2x12, why spend the extra money....We feel sure that we will be going with the 2x10. What is the rule of thumb for the size of the floor joist for the span?
 
#7 ·
We live in an unincorporated town...We dont have to have a building inspector. We are not changing the existing structure, it is the shell. We will be adding walls...The only permit that is need in the town and state is electrical. We have been talking with several building contractors and their advise differ.
 
#9 ·
JOE, We live in an unincorporated town...We dont have to have a building inspector. We are not changing the existing structure, it is the shell. We will be adding walls...The only permit that is need in the town and state is electrical. We have been talking with several building contractors and their advise differ.
 
#23 ·
Thank you DM...Just because our town doesnt required building permits, and inspections doesnt mean that we are going to do that. We are consulting Building contractors and listening to what they say and I have to add that there are alot of contradictions maybe because we live in an unincorporated town. We feel this way...if something goes wrong and a contractor is involved in the building process, if we have not done our homework ultimately we will be responsible. We must make good sound choices and how do we do that...is doing our homework...RESEARCH. This will ensure our home will be safe, and sound...right? Thank you again DM, your advise was helpful.
 
#27 · (Edited)
We will be building two low bearing walls and adding to an existing low bearing wall. The first one will be in the middle of the gym, the 2nd LBW will be 14' from it, and 14' from it we will be adding to an existing LWB. (This is where the stairs will be placed). The first wall (in the middle) will be on top of a 2x12 floor joist running the same direction and there after 2' centers. The second floor joist will be running from one LBW to the other LBW. We believe that we are going to triple the 2x12s to get the support that we need on the LBW.
 
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