Sizing floor joists, and thickness and type of plywood sheeting?

[plowhog]

I'm building a mezzanine in my barn for more storage. I need some help sizing the floor joists and choosing plywood sheeting thickness.

The span of the joists is 14' 6". One side of the joists will sit on top of the "top plate" of the outer wall of the barn, and the other side will sit on top of newly constructed 6x6 posts and 6x6 beams.

The age of the wood on the outer wall is unknown, but could be 100 years old. It looks in mostly decent shape, vertical 2x4 and 2x6 supporting 4x4 horizontal top plate. I will probably sister some additional 2x4 vertical supports along that wall. This outer wall might limit how much load my mezzanine should carry-- I just don't know.

I'm thinking of either 2x8 joists, or 2x10 joists, 16" on center. From what I have researched, 2x8 would be the minimum. One tricky part is the joists sitting on the top plate of the outer wall of the barn. The rafters and roof material join at about a 45 degree angle, so "some" of the top corner of the joists will have to be trimmed to allow the joist to slide all the way onto the top plate. With 2x8 I would have to cut less material than 2x10, since less of it would interfere with the rafters and roofing. The added height of the 2x10 would require more cutting trimming. I do not want any center supports

Cost isn't really an issue. The price difference between 2x8 and 2x10 is not that great. I'm more concerned about the impact of having to cut off the top corners of the joists so I can slide them fully onto the top plate of the outer wall. Does that compromise strength? Is a 2x10 any stronger if *more* of the corner has to be cut?

Also, the plywood sheeting? What kind of sheeting is recommended for a mostly but not perfectly water tight barn? I don't get much moisture in there-- but there are occasional drips here and there through the roof. Also what plywood thickness? 5/8? 23/32 or 3/4? I know there are different grades of plywood and I want to choose the proper one for my use.

Regarding weight of contents on the floor, this area will be for general purpose storage. It isn't forklift accessible, so nothing crazy heavy. And the heavy lumber I store will be below the mezzanine, not on it.

 

[Snobdds]

Wood gains it strength in framing lumber by the height due to having a higher compressive strength. You generally want the most height in framing lumber you can before it becomes a burden to fit.

All that said...floor joists should be at least a 2x10 for that span. If you can get away with a 10 inch TGI, do that.

 

[quicksandfarmer]

I'm thinking a ledger board bolted to the studs of the existing wall and joists supported by joist hangers. That way you can put the joists wherever you want.

There really isn't a category for "mostly but not perfectly water tight." Construction is assumed to be either wet or not wet. If it's wet, treated lumber would be recommended.

 

[caver]

I wouldn't rule out I-joists.

 

[duffer]

I-joists will work, but won't take the load a 2x8 or 2x10 would. If you're going to put any significant weight on it, go with solid 2x's.

I'd also use ¾" decking on 16"centers. 5/8 & ½" will sag and cup over time.

 

[Alan W.]

x[emoji[emoji6]]s on [emoji6]” is a no go.
You are going to be using it as storage so it can get heavy over time.
SYP x[emoji[emoji6][emoji6]]s on ” centers. I would then place a plate against the wall on the end that will have to be cut at an angle for clearance. Thus will better support that end.

 

[Alan W.]

Apparently my phone and the app are on drugs. I dont know why my previous post came out that way.

 

[rScotty]

SNIP...

Cost isn't really an issue. The price difference between 2x8 and 2x10 is not that great. I'm more concerned about the impact of having to cut off the top corners of the joists so I can slide them fully onto the top plate of the outer wall. Does that compromise strength? Is a 2x10 any stronger if *more* of the corner has to be cut?

Yes, the center of the 2x10 span still has more strength to resist bowing. You could also adde a rim joist - which some call an "interior ledger board" - to the top plate which would give more suppor for the ends of the 2x10s.
My bias is to overbuild when I can do so and stay close to the budget.

Also, the plywood sheeting? What kind of sheeting is recommended for a mostly but not perfectly water tight barn? I don't get much moisture in there-- but there are occasional drips here and there through the roof. Also what plywood thickness? 5/8? 23/32 or 3/4? I know there are different grades of plywood and I want to choose the proper one for my use.

3/4" is more stable. You will want the stability of the 3/4" for the floor. Generally the more plys that the plywood has, the better quality ..... and higher price.

Regarding weight of contents on the floor, this area will be for general purpose storage. It isn't forklift accessible, so nothing crazy heavy. And the heavy lumber I store will be below the mezzanine, not on it.

Your plan sounds reasonable to me.