But I believe that it will cost you so much more to have them dried or to try to buy dry timbers.
It really depend on a lot of things, which you should use.
Is the first floor area timber framed?
Will the second floor have exposed posts and timbers including rafters?
Or are you just using the 4x8s for joists for the appearance of a timber framed ceiling for the first floor?
If you are doing the last, then try for dry timbers as these size timbers green will shrink and this could upset installed sheet rock on the walls and floor boards on the second floor. Plus if you don't do the joinery right you could have problems.
Green hardwood cuts easier than dry. If you get green now and you don't store them properly then they could twist, bow or do other things.
When green timbers are cut and assembled shortly thereafter, then they are held in place by the joinery and have less chance to twist, bow and do other things, which could upset the assembly process.
I'm sure others will have some advice for you also.
Good luck with your project, and keep asking questions.
Jim

The current structure is a "conventional" 2x6 based frame.

The second story will basically be a 2x10 framed roof supported off of the new floor so mostly this is for appearance.

Is it the width of the timbers that is an issue here? Would 6x7 timbers be less likely to present problems?

If I can't get timbers which are already dry, how long will I need to put up green timbers before they are stable? I'm guessing from now till August won't really be long enough.

My thought was to install 6x8 beams along the top of the sill of the existing first story and then hang the 4x8 joists off of this using a half-lap joint. Reasonable?

I will certainly do so. Thanks for the feedback.

Chris:
If there is to be no living space on the second floor and if I understand you correctly and that the roof will sit on the ceiling of the first floor then the size of the ceiling joists aren't really that important.
If there is to be living space on the second floor then the first floor ceiling joists have to be designed for that second floor, floor load. And they are not ceiling joist but floor joists for the second floor.
I'm not qualified to suggest to you the correct size timber needed for the second situation. You should consult a timber frame designer who can properly design your 'new' roof, second floor arrangement.
The factors that will determine the size of the timbers are type of wood, span, spacing, load(s), and to some extent joinery methods. There are a lot of factors involved, and I probably missed some also.
Q: "If I can't get timbers which are already dry, how long will I need to put up green timbers before they are stable? I'm guessing from now till August won't really be long enough."
A: Air drying wood is figured on one inch in depth per year. So a 4x8 air dried would take at least four years. That's outside, stacked properly.
Q: "My thought was to install 6x8 beams along the top of the sill of the existing first story and then hang the 4x8 joists off of this using a half-lap joint. Reasonable?"
A: I'd have to assume you mean to install the 6x8 along the plate. Sills are at the bottom of the house were the walls met the foundation. Plates are at the top of the wall were the walls met the ceiling.
And then hang the new "ceiling" joists from the new plate timbers. Is this correct?
If this is the case and there is to be no living space on the second floor then this could work.
However your joist ends will have to be prepared properly to eliminate a potential point of fracture.
As I don't currently have a web page address where I can post you a picture to then import into this post, I'll email you directly a suggested sample joist end. A simple half lap joint could be a incorrect ceiling joist end. And it could crack at the ends.
Good luck, Jim

The second floor will actually be living space so the ceiling/floor joists need to be sufficient to support that load. The span is 15.5 feet from outside wall to outside wall and based on local guidelines for 2x8 floor joists, 4x8 oak beams at 32 IOC should be more than sufficient.

Yes, sorry, I did mean plate and not sill.

Thanks for the suggestion on the joist end. I look forward to that.

Chris,

I'm very concerned about the floor system you intend to fabricate. 15'-6" is a tremendous span for any floor system! Even 2x8's 12" OC would require a very high grade of lumber. I used the joist tables in the CABO code book for a quick check and SYP #1 is the only thing that would even make it in conventional framing. No grade of SPF even comes close.

2x10 conventional framing works much better, but you still need to remember the allowable deflection at 15'-6" is over 1/2". That's a "springy" floor unless you increase the depth of section sufficiently or use a much stiffer wood.

The other problem I would consider is using the same depth plates as joists. You can't fully house the joists unless you cut a "gain" on them, and even then you will only have a sliver of wood under the housing in the plate. The other problem is having to cut your soffit tenon halfway up the joist to hit the nuetral axis of the plate. You are going to be dealing with a lot of shear at the ends of these long span joists. I use "bearing members 2" deeper" as a rule of thumb.

I would strongly suggest hiring someone to correctly design your floor deck. It will be money well spent. Don't rely on local "code" for your guidlines. Remember what the sage John Abrahms told us: "Building to code means if it's built any worse it's illegal."

Concerned for your safety and QOL.

Rudy

Hi new to the forum. I am building a hybird cabin. I will be stick building the walls and using timbers for the second floor joist and all roof rafters. Help would be appreciated.

I do not have the time to properly air dry my lumber. I have the choice of either Yellow Pine or White Pine. Which do you recommend? It will be locally sawn with nominal dimensions.

Will WP span 14 feet using 4x8 or do I need to go to 6x8 for floor joists?

Some of the porch rafters which are 3/12 pitch will be 16' long. Will 4x8 work in WP or will they also sag? What if I gouge out the middle and slide a flitch plate inside and try to hide it?

I also have some porch beams that I will be running in pairs 16' out from the house to stone columns. Craftsman style. On top of these will sit 2x10s or 12s PT, 16" oc for the porch joist. Will WP span 16' with a 4x12 or am I crazy?

Thanks for the help.

Mr. Nut,

There is quite a range in design values (from good to terrible) for white and yellow pine depending on grade. The Timber Frame Joinery & Design Handbook, available here on the website, includes tables that will give you the numbers for each. You will need to be able to grade the timber yourself if you don't pay a grader to do it. For that you want to get a copy of a grading handbook and learn the grading rules.

The Northeastern Lumber Manufacturers Association (NELMA) handbook is good for white pine. You can get a copy for under $10 by calling them at 207-289-6901.

Once you determine the grade you need, you can see if it's available locally or not.

Good luck,

Rudy