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Just
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# Posted: 26 Feb 2018 22:09
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Double rim joists are usually used if you are not using a girter on very small cabins. I would not use a double rim on the20 ft. Walls just on the12ft. Walls and make sure the giters are under both the joists and rims that’s all you need no hangers . Ice is a pro and he will not lead you astray me I just build them .
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ICC
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# Posted: 26 Feb 2018 23:04
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Quoting: caducus was using rim joist to describe all joists around the perimeter.
Understandable. But when a recognized name for a component is used for a component where the proper name is different it can lead to confusion.
Quoting: caducus I've seen a lot of frames with doubled up rim joists.
Not knowing where you are seeing a "lot of doubled up rim joists" makes it difficult to critique the individual examples. There are some places joists are doubled. However, I'll say this; if you are seeing this scattered about the internet..., well the internet is full of examples of how someone did something. Not all examples are good or necessary, some are plain wrong, may lead to a failure of some kind down the road a time or may even be dangerous.
Have you looked at the IRC? It is not a DIY set of instructions. It is a rule book. There are no reasons given for the illustrated methods. The IRC is broken down into sections for foundations, wall, floors, etc. Methods that have been found to work well over the decades are shown or talked about.
The section on floors calls for rim joists to be a full depth member (same height as the floor joist, not assembled from two pieces stacked) and with a 2" nominal thickness. (a 2x has a nominal thickness of 2 inches.... 1-1/2" in the S4S lumber that we all use.) No mention of doubling up standard rim joists. There are places where doubled joists are needed but rim joists are not one of them. Sometimes people assume if one is good more is better... sometimes but not always.
No point in using hangers to mount rim joists.
You might want to use H1 ties to connect the floor joists to the beams though. If you measure out the spacing and nail the h1's to the girders (beams) first it simplifies setting the spacing when installing the joists as well as helps to provide uplift resistance for the floorassembly.
Before you start building you might want to calculate the precise length to cut the joists to. The rims add to the width of the building floor. You do not want the subfloor sheathing to end up needing a narrow strip as a fill in under a wall plate. The subfloor sheets tie everything together but should not be less than 16" in any direction anywhere in the floor. 24" is better.
Note T&G subfloor sheets end up NOT covering 4 feet in width because the T&G joint is machined into a panel that starts at 48" width, then gets reduced. Lots of folks have been surprised by that.
Quoting: caducus extra width would mean that most of the wall was being carried by the rim joists.
It ends up not being quite that bad as the subfloor sheathing helps spread out the load. But still doubling the rims is usually a bad idea. Costs extra as well as being not necessary.
Shed roof means only the 20 foot walls carry the roof load down to the floor. Just FYI.
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caducus
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# Posted: 26 Feb 2018 23:30 - Edited by: caducus
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"No point in using hangers to mount rim joists."
I'm trying to keep the footprint of the foundation narrow. There's reasons, not really worth explaining. As such I'd like to overhang the 2x10 joists by the permissible 9 inches. If I do that I'd think I need the hangers since there would be no girder directly under the rim joists. Thoughts on that?
A lot of good info above. Thanks for helping me through this. A lot to learn on a first cabin build.
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ICC
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# Posted: 26 Feb 2018 23:56 - Edited by: ICC
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Quoting: caducus I'd like to overhang the 2x10 joists by the permissible 9 inches. If I do that I'd think I need the hangers since there would be no girder directly under the rim joists. Thoughts on that?
The wall loads are carried by the floor joists, not the rim joists. The rim joists are mainly to (a) prevent floor joist rotation (b) be something large enough to nail the subfloor to securely. (c) finish off the joist ends... but wall sheathing could be used for that. Back when balloon framing was commonplace that is what was often done. The old house Dad built in the late 40's was like that.
If we didn't need something sturdy to nail the edges of the subfloor sheets to you could do without the rim joists on a short cantilever like that. Instead we could use solid 2x blocking cut from the same size material used for the floor joists. Solid 2x blocking would then be installed between the joists directly over the girders. The purpose is to restrain rotation or falling over of the floor jousts. This solid blocking over a girder or beam is actually required by code but when the floor joist ends are only 9 inches away from the girder(9"cantilever) you can get away with using rim joists instead, especially because you need the rims for subfloor nailing.
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caducus
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# Posted: 27 Feb 2018 11:51
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I guess I'm still a little confused. I hear what you're saying... that it's the joists carrying the load. And I do get that the subfloor helps to distribute the load. But still, 2x4 a wall resting on the edge of that frame has 2 inches resting on the joists and 1.5" on the (non structural) rim joist. That's a nonissue?
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ICC
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# Posted: 27 Feb 2018 18:04 - Edited by: ICC
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Nonissue.
I'm not an engineer so I can't provide a technical explanation, other than it is commonplace practice. I have built many floors/walls with the depth of joist overhang. Most off plans that were approved. Some were larger overhangs and those had an engineers approval stamp. My own home has several cantilevers, all on second or third levels so very much like cantilevering floor joists over the support beam... I drew those up myself and had an engineer check and stamp the plans.
Maybe it will help understand how wood and loads get along by thinking about this. A floor joist such as a 2x10, #2 grade SPF, 16" OC and using a 40 PSF live load along with a dead load of 10 PSF can span 15'5" and only be supported by 13/16" at each end. I know that is only one joist, but if loaded to the maximum ratings that works out to about 500 pounds on each small bearing surface at the 2 ends.
The wall assembly is rigid once the sheathing is applied. The upper ceiling joists/rafters keep the wall tops aligned. The load on the wall travels straight down to what it sits on which is the subfloor. The bottom plate is not going to roll sideways and somehow overload the rim joists. The load will be borne by the floor joist, through the subfloor. If the floor joist were to fail then there might be an issue with the rim joist too. But that is a far fetched scenario IMO.
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95XL883
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# Posted: 28 Feb 2018 14:25
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"Note T&G subfloor sheets end up NOT covering 4 feet in width because the T&G joint is machined into a panel that starts at 48" width, then gets reduced. Lots of folks have been surprised by that. "
Sounds like something I need to know as I get closer to starting my "cabin". Please help me understand the exact dimensions and how quickly the T&G dimensions become a surprise. Nominally I would expect a 4'x8' sheet to cover 48" by 96" and thus would plan on installing joists 16" on center. But if the sheet covers only 47.5" v 95.5" because of the T&G then I would have to make sure that the center of the 7th joist is 95.5 inches from the outside of the first joist, right? The result would be the tip of the tongue (assuming 1/2" is the correct amount) would be past the center of the 7th joist but the tips of the groove (of the next piece of plywood would be on the center of that 7th joist, correct?
Hmmm? Another thought just occurred to me. Is the outside edge of the T&G to be trimmed flush so there is no T&G on the perimeter? That would mean another 1/2" loss (or whatever the exact dimension is) that would have to be accounted for when placing the joists, correct.
Do I understand the "surprise" correctly? Thank you.
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caducus
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# Posted: 28 Feb 2018 14:39 - Edited by: caducus
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Got it. Thanks. That explanation makes sense.
Where can I find span tables that would cover the girders for this build? All the ones I've looked at, now that I think about it, are basically beam tables for decks. Figuring out how large the beams need to be, and the amount of footings on each 20 foot run, is important.
The tables I looked at said that a 4x8 over 5 footings (spanning about 55 inches) would be fine, but as I said, I think that probably more for decks than small buildings.
And I second the question above about T&G subfloor and joist spacing to account for it.
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ICC
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# Posted: 28 Feb 2018 21:33
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Re T&G width loss. It can be confusing at times. Most of the T&G sheets I recall effectively cover 47-1/2 inches width. I seem to recall some years ago that was more like covering only 47-3/8 maybe 47-7/16. The Huber AdvanTech I bought several months ago covers 47-1/2. But, surprise, a few hours ago I was in the Lowe's in town. Pushing a cart down the sheet goods I stopped to measure the OSB T&G. Surprise, surprise. The stock they had measured out to cover a full 48" width! About time somebody got smart.
The T&G are not cut in AdvanTech 4 foot panel ends; only the long edges are T&G. Same thing for the T&G OSB in Lowe's today. However, in the past I've used subfloor T&G OSB that were T&G on all sides.
So, I revert to what was going to be my first answer to what to do - - check the stock the local supplier carries and plan accordingly. But measure again once you go to start nailing; it seems you never know what you will get. I have no idea how many mills turn out true 48" wide coverage panels. I haven't bought any std T&G OSB in years.
The length, when no T&G is present is usually 95-7/8" That's to allow an eighth inch panel end gap to allow for possible panel change due to moisture level changes. Note that the T&G are machined in a manner that leaves an apparent edge gap. That is by design. Do not mash the edges up tight with a big hammer as I have seen some people do. When the T&G edges are pushed together there should be a surface gap of about 1/8 inch.
ALWAYS install the joists and wall studs on 16" (or 24) centers. Exceptions can be at floor ends where you may want a panel end to lay at the outside edge of the last joist rather than in the center of the joist. Same thing at wall ends or at corners if you want the sheets to go to the edge of the stud rather than the center.
For a floor the easiest way to avoid shrinkage due to the T&G loss would be to design and build the floor assembly an inch or two narrower. Lay, nail and then cut off the narrow strip on the last outside edge.
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ICC
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# Posted: 28 Feb 2018 21:53
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Don't confuse beams for decks with beams for covered porches or houses & cabins with ceilings, and roofs as well as floors. Decks do not have roofs. Ss soon as there is a roof the deck becomes a porch which is handled like a room. Seen lots of porches that became rooms once the walls were fitted with windows and such.
It is hard to find a table to be used for sizing a beam for a pier foundation because the codes don't have prescriptive solutions for pier and beam foundations. That's because P&B requires an engineer to be code approved.
You can get close by using Table R602.7(1) in the IRC. The portion of the table that is for roof, ceiling and one clear span floor would cover a cabin such as the one caducus has described. Two 2x8's nailed into a beam with the lowest roof load that is calculated for shows a 5 foot maximum spacing of the beam supports.That is good for a building up to 20 feet wide so should be fine for a 14 foot width. It is possible to extrapolate the building width but why bother. Err on the side of using members larger than necessary or spans smaller than those listed will often keep trouble at bay.
southernpine.com also has tables for girder sizing. There is also a calculator for sizing beams online at http://timbertoolbox.com/ but that does require looking up several variables for the species in question, entering that and knowing what the load on PSF will be. Very doable but takes more user input. Note that when usding tables always look for and read the footnotes.The footnotes may exclude or include variables that can either make or break tings.
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ICC
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# Posted: 28 Feb 2018 21:55 - Edited by: ICC
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Re subfloor panels; they always go with the long dimension across the floor joists. Plus the panels should be staggered; never have four corners of 4 separate panels meet at the same place. Edges must always be supported by a joist or blocking. That IRC Chapter 6 also has tables for how many nails, what size and how far apart on edges or in the center of the panels. Note the nail sizes given are spelled out in inches. I mention that because nail gun nails are almost always thinner and thus not as strong. There are a few guns that are made to use a full size nail.
Common nail = full size.
Box nail = more like most nail gun nails.
Deformed shank = ringed or twist shank
Ringed are great for sheathing.
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95XL883
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# Posted: 28 Feb 2018 23:41
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ICC, Thanks for the explanations. i should have guessed that there really isn't a specific standard but it is good to know that the standard length is 95 7/8. That makes it easy to space joists 16" on center. As long as the difference is in the width, then it is a lot easier to make sure there is a short overhang that can be trimmed off. Definitely don't want to be short a little short there. Thanks again..
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caducus
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# Posted: 1 Mar 2018 15:29
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ICC, good to know about the plywood subfloor. So it looks like I'll just have to see what my sheets work out to.
You mention the gap and not forcing them together. So those T&G sheets are built to include the 1/8" gap I'd normally leave for expansion? You just snug them up and they're good to go?
Looking at that header table 2x2x8 at those spans does seem doable, but I would probably do well to go a little bigger to be on the safe side.
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ICC
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# Posted: 1 Mar 2018 18:52
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Quoting: caducus You mention the gap and not forcing them together. So those T&G sheets are built to include the 1/8" gap I'd normally leave for expansion? You just snug them up and they're good to go?
Yes, I once had a drawing that illustrated the interference fir that was designed into the sheet goods T&G joint. They go together relatively easy just so far. There is still a gap at the end of the T; that is the T does not bottom in the G. That is to leave the space needed if expansion occurs. So it is possible to whack the sheets tight against each other but that defeats the purpose of the gap. Try a search for an image or when you get the sheets on hand you will be able to see.
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cspot
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# Posted: 1 Mar 2018 20:50
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I would seriously consider going with Advantech for the subfloor. I never knew it existed until I read about it on this forum. I think it is rated for 500 rains or something like that. Anyway since we were mainly building on the weekends mine got wet several times and it never swelled or delaminated. The price is pretty good on the stuff too. Another nice thing is we ran out of time to finish the inside last year, so the subfloor is currently our floor. Not a big deal if it gets some wet a little bit when tracking in snow.
When I got my floor it did not cover 48" wide. If your width is in mulitples of 4' you may need to put a 2' wide piece or something to start so that your final piece isn't just a 1" sliver. Downside is that you end up wasting a few 1/2 sheets to do this.
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ICC
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# Posted: 2 Mar 2018 00:08
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Huber AdvanTech is wonder stuff! The leftover cutoffs almost always seem to be good for something.
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