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xinull
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# Posted: 21 Sep 2020 12:46am
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I'm planning a 20' wide cabin on 3 girders. I haven't looked for 20' lumber as some of you have suggested, but getting 20' lumber to my land would almost be impossible. I'm still struggling finding a way to get 16' lumber there  So 20' lumber is out of the question.
One choice would be to offset the left and right joist so they overlap each other over the middle skid. But that would not line up with my studs and my rafters anymore.
It's important for me for all the studs and rafters to all line up with my joist for structural integrity. I'm cantilevering 9" on each side, plus i'm doing the walls and roof 24" OC. Floor will be 12" OC so everything lines up
So the only way i see possible is to join 2 floor joist , like in the picture i'm attaching, end-to-end and do what i've read as scabbing them (not demonstrated in the picture).
I think i've read Plywood is the better choice to scab them...is that correct?
Should i be putting/attaching plywood on both side of the joist?
And how long should the plywood overlap each joist? I've read 2 feet on each side?
I'm open to other suggestions that would keep the cost to a minimum and still keep everything structurally sound.
cheers
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spencerin
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# Posted: 21 Sep 2020 12:56am
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No engineer here, but not sure your wall studs need to match up with your floor joists. Floor joists aren't supposed to carry the weight of the walls and roof - that force is supposed to go straight into the foundation. I've seen some cabins successfully built with the overlap method you described.
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ICC
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# Posted: 21 Sep 2020 01:27am - Edited by: ICC
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1. The wall studs hardly ever line up exactly over a floor joist. For example, it is not uncommon to use 16" OC wall studs in conjunction with 24" OC floor joists, or vice versa. Floor trusses are sometimes spaced at 19.2" OC. and used with either 24 or 16" OC wall framing. Corners will throw off the most careful planning to try and match wall stud to floor joist.
The sub floor sheathing ties the floor assembly together. Then the walls are assembled laying on the floor and tilted up with the bottom wall plate nailed to the rim joists and floor joist ends.
The wall top plates are doubled with corner overlaps. Then exterior wall sheathing covers everything from top plate to bottom plate. If you build on a wood framed floor {not a slab} then buy the 9 foot or 10 foot long OSB panels so the exterior sheathing can also overlap the rim joists.
2. No need to scab the floor joists in the middle. We usually lap the juncture over the middle beam if using a center beam or do a simple butt joint. That center joist joint is blocked using solid same depth material. By the time the subfloor sheathing is nailed down that ties the floor assembly together.
Have a look at the AWC joist calculator... You'll see the average floor joist on a 20 wide only needs about an inch or less of bearing surface on the ends. More is better but not necessary. Look at the size of the bearing surface provided by joist hangers.
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xinull
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# Posted: 21 Sep 2020 09:01am
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well it's good to know that the studs dont need to line up with the joist. I was under the impression that they absolutly needed to be. That means i dont necessarily need to do my floor joist at 12" OC to matchup with my walls. I can do 16" OC, which i believe will be adequate...and my walls can be 24" OC, so will my roof rafters
I also didn't know you could get 4x9 or 4x10 panels. You mention OSB. From another thread i recently created, i was recommended not to use OSB for wall panels, since they dont hold nails up very well (and that sometimes the nails go right through if using a nailgun). But this sorta made me wonder why so many new houses are built with them. Maybe they use a better quality OSB?
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ICC
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# Posted: 21 Sep 2020 10:24am
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Quoting: xinull I also didn't know you could get 4x9 or 4x10 panels.
You prbably will not find the longer sheets at a big box home improvement store. But there are other sources.
Quoting: xinull i was recommended not to use OSB for wall panels, since they dont hold nails up very well (and that sometimes the nails go right through if using a nailgun).
I do recall someone making some comments about nailing to OSB, and I think I did make a comment, but did not feel like arguing technique, so left it alone.
First, OSB is great for what it was designed for; making a wall strong, having excellent anti-racking strength. It must be kept dry. I believe OSB makes a better wall sheathing than most of the plywood that has been available for the past decade or more. We used to use plywood for some wall and floor sheathing, but began to have more ply separations many years ago. Many of the plywood sheathing panels now have reduced numbers of plies and suffer from voids here and there and that can be a problem. Anyhow my opinion is use OSB for wall sheathing. Set aside enough tie to get the framing all done and dried in, in as short a time as possible. Plan ahead, make a bill of materials list and get it delivered on site, then get to work.
OSB and plywood can both be improperly nailed. Overdriving with an air nailer is operator error in virtually all cases. Maybe an old old nailer or a very cheap one? Or an ill informed operator?
Driving the nail head below the panel surface reduces the strength of the panel and will be red tagged on a inspection.
IRC chapter 6 covers everything the code has to say about walls AND has a series of tables that list sizes and numbers of fasteners for nailing everything, walls, floors, roofs. Pay attention to actual sizes of nails. Often nails made for air nailers are a smaller shank diameter. That can be taken care of by using 20 to 25% more nails.
If you are not familiar with the IRC it is the code that is used virtually everwhere in the US. Canada's code is siilar. The 2018 online version is available https://codes.iccsafe.org/content/IRC2018 The IRC is NOT an instruction guide, not a playbook. It is the rulebook. There is a lot of info in it. Well worth becoming familiar with IMO. Even if you are building in an area where there will not be any inspections, your building is subject to the same winds, rain and other forces of nature. The purpose of the IRC is to build something that is safe and will probably last. It is a minimum, so extra can be better.
Quoting: xinull Maybe they use a better quality OSB?
The OSB sheathing comercial home builders use is the same stuff we individuals buy. However there are also some products such as Zip System Wall sheathing that have advantages over standard OSB. You may have seen it; green panels with black taped joints. Made by huberwoods.com the same people who make the best subfloor material, Advantech.
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ICC
Member
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# Posted: 21 Sep 2020 10:26am - Edited by: ICC
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Two other online calculator resources that can be helpful and educational are
https://www.awc.org/codes-standards/calculators-software/spancalc
https://www.awc.org/codes-standards/calculators-software/connectioncalc
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Brettny
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# Posted: 22 Sep 2020 07:14am
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Xinull I wouldnt lap the floor joists in the middle. This will throw throw your plywood off or you will have a seam over the middle. Both of which you dont want. You want the plywood stagger its joints...if that makes sence.
But the ends of the joists and lap them with 2x material. It could even be one size smaller. This is really only there to keep the joists butted up end to end. There may even be a metal bracket you can buy that made to do this.
We just plan on scabing a piece on in the area your talking about.
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xinull
Member
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# Posted: 23 Sep 2020 12:12am - Edited by: xinull
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I think both solutions have their advantages and disadvantages.
Brettny, i understand what you mean if i overlap my joists, that my plywood will be off. From what i was reading, all you need to do is offset your plywood for the rows on the side that's offset'ed so they line up with the joist. But the middle row of plywood (which has both OC and offset joist), i'd have to place some support under the edge of the plywood. I calculated real quickly and it might only cost me an extra sheet of plywood, and 3 support for the middle row
I'm not sure if i rather overlap my joist and cut an extra sheet of plywood and a few extra support
or
if i rather cut 20 joist (correct that...40 joist, 20 on each side) to fit exactly end-to-end and have to cut another 20 piece of 2x to scab/lap them all together
I feel that it's faster and less expensive to overlap them. Structurally, i'm guessing they are the same since all plywood edges will always sit on top of a joist.
ICC, what I would like to confirm with you, is that from your comments, that if i do my joist 16" OC and my walls & rafters 24" OC. It's all good structurally. Something like the screenshot of the joists & studs below.
As long as i know my joist/studs/rafters are structurally sound, then at least i can decide later if i'll overlap or not.
thanks
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NorthRick
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# Posted: 23 Sep 2020 01:54pm
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Quoting: xinull if i rather cut 20 joist (correct that...40 joist, 20 on each side) to fit exactly end-to-end and have to cut another 20 piece of 2x to scab/lap them all together
Rig up a chop saw with block the correct distance away from the blade. Once you do that, you could cut all 40 joists in less than an hour. The scab pieces don't need to be exact so you could cut all those in under 20 minutes. You are going to make things hard on yourself when putting the plywood floor down if you lap the joists instead of butting them.
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ICC
Member
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# Posted: 23 Sep 2020 03:12pm - Edited by: ICC
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Quoting: xinull ICC, what I would like to confirm with you, is that from your comments, that if i do my joist 16" OC and my walls & rafters 24" OC. It's all good structurally. Something like the screenshot of the joists & studs below.
As long as you build the floor with the sheets of subfloor right out to the edges with at leat 16" width on the plywppd/osb, and build the walls with a bottom plate and two top plates as is the norm, you're good.
The problem with the illustration is that it seems that the bottom plate sits on the floor joists. The subfloor goes on first right out to the edge, nailed to rim joists and the floor joists. Then the wall is built flat on the floor and stood up and the bottom plate nailed to the rim joist.
The illustration is also missing the blocking or bridging that is required at least every 8 feet. Lapping the joists like that is fine (lapped 3" minimum with minimum of three 10D face nails) and makes it relatively easy to use solid blocking between the laps. See R502.7 and R502-7-1.... blocking or bridging is required at all intermediate joist supports and every 8 feet. That will keep those laps in place and prevent rotation of the joists. So, with a 20 foot width I'd use the solid blocking at the lap joints and 1x4 wood or metal bridging at two other places along the joists; like about midway between the outside ends and the center. Metal bridging is often cheaper.
Blocking/bridging adds strength to the floor as well as helping to keep the joists nicely aligned and preventing joists roatation (tipping).
In the illustration the unsupported butt joints in the doubled up jists acroos the nearest end is a no-no. All splices must be supported from underneath, from directly underneath, not just nearby.
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Brettny
Member
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# Posted: 23 Sep 2020 07:42pm
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Blocking is a good point and takes alot of twist and even can take bounce out of a floor. Also very cheap addition to a solid floor.
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xinull
Member
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# Posted: 24 Sep 2020 12:43am
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Quoting: ICC The problem with the illustration is that it seems that the bottom plate sits on the floor joists. The subfloor goes on first right out to the edge, nailed to rim joists and the floor joists. Then the wall is built flat on the floor and stood up and the bottom plate nailed to the rim joist.
Nah, that's just the angle. I have my plywood under my wall, all the way to the edge 
Quoting: ICC The illustration is also missing the blocking or bridging that is required at least every 8 feet
yeah, i hadn't put it yet. I'm still designing the whole thing. Trying to get it to the point where i know what i need to order from the mill. But you reminded me that i need it every 8 feet. I forgot about that!
Quoting: ICC Metal bridging is often cheaper.
Awesome, I'll take a look at it. It'll be that or solid blocking
Thanks
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