Shortening a Long Joist Span
derekr
| Posted in General Questions on
I’m getting the house ready for drywall picking where I want lights and things
While I’m doing this I notice my 2nd floor joist span might be a little over the limit
The wood is southern yellow pine, my first floor is 16 OC but the 2nd floor is 24 OC, in order to have 30 psf on the 2nd floor it looks like the span is supposed to be 20 feet 3 inches or less but my span is 21 feet 8 inches
if I add 2 rows of blocking will that fix this or do I need to do something else
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Derek,
I think you left out a few crucial details.
Is the span of 21'8" the clear span, or is there a beam in the middle to cut the span in half? There's no grade of commonly available SYP in regular dimensional size lumber that will span 21' within any compliant deflection limits.
Can you tell us the size of the lumber, what subfloor is on top of it, if it's glued/screwed, nailed, or some combination? also if there's rafters or trusses on top?
There are walls supporting the floor underneath in every location except for the loft area
The loft area has no walls under it, it’s 7/8 inch osb floor that is glued and nailed
All the lumber for the floor is 2x12, actual is 11 and 1/8 inch - 1/4 inch
Interesting,
Just to make sure I'm completely sure about it:
You've got a span of 21'8" using dimensional lumber, SYP, with no intermediate supports, correct? What size are the joists?
2x12, actual is 11 1/8 - 1/4
Was that lumber ordered special? for this loft? @ 24" OC I'm having a hard time believing it honestly. Can you grab a pic and post it if you don't mind?
Yes is was special order 24 foot length then cut down
I could feel a slight bounce on the floor when i walk but nothing major, I added some blocking and pretty much feel nothing now
I was more concerned about the floor breaking than movement
This is a very old picture when the house was still being framed it’s the best one I can find of the 2nd floor loft though
There's nothing to say it wont work, or fail, it's just the deflection of the system that I'm curious about. With that kind of span you probably wont feel much bounce, as it's already draped, if that makes sense. I suppose they could have all been crowned in just the right way, but if you use the AWC span calculator, that's way over spanned for even a L/180 limit.
Replied with a picture from your other comment,
so is there anything else I should do besides the blocking?
The rest of the upstairs is fine since it has walls below it?
I did a few quick calculations based on the design values for SYP from the AWC specs, and at that span, and loading, you're well within the limits of the design parameters, but I'd expect a deflection in the floor, mid span of somewhere between 1" and 2" depending on how much stuff you have up there.
Is the underside going to have drywall or be exposed as the finish?
It will be drywall underneath, I’m within the limits even at 24 OC?
The blocking I did helped a lot with the movement
At that length 17" over isn't a little, it's a lot. Note however that isn't the overall length of the joist, it's the unsupported span. So if your joist is supported by a 2x6 wall at each end, the span will be 11" less than the length.
The right way to fix it is to add more joists, which probably isn't what you want to hear because joists of that length are expensive and adding them after you've put the subfloor in is a lot of work.
That said, your house isn't going to fall down. The limit for joist length isn't determined by failure strength, but by deflection. An under-supported floor is bouncy. It feels disconcerting to walk on, and the flexing can cause problems. Most commonly, this shows up in the ceiling drywall cracking. If you have ceramic tile or similar for your floor it can crack. A joist that length is going to move about 1/4" under the weight of a person, which can cause problems with wires, pipes and ducts that run through them, repeated flexing can cause them to fail.
Ah ok, yea it’s supported in the roof with the wall below it so the span is like 5 inches over then, 20 feet 8 inches
I was just contemplating a few fixes that may work, since it's not going to be an exposed floor, that doesn't require the deconstruction of the subfloor above.
Here's a few off the top of my head, in no particular order
(1) Preload the loft using some weights (A lot of bags of concrete) until the drywall is installed, and take it up before it's mudded and taped. That'll offset the initial weight of the drywall and help the joints close, assuming you can get your crew to cooperate on leaving a small gap between panels. It wont fix the deflection, but I certainly hope there's not going to be any holes drilled for plumbing through those joists.
(2) I'd have to do the calculations here, but sistering a 2x4 on either side of the bottom of the beam, glued and nailed, with the 2x4 staggered so the joints don't line up on the joist, would effectively create a T beam, with a wide flange on the tension side where it's needed. This would probably be the easiest route at this point. Now that it's already up there's really no way to slot additional joists in, practically because they're heavy AF, and because the joist on either side will be in the way. With a live load of 0 psf, I'm showing a dead load deflection of about 1/2" at the moment, which probably wouldn't require the effort of supporting the center of the joist to take out any already built-in sag.
Derek,
Not trying to scare you here, but DC alluded to what I was trying to ease us into. It's not well designed. It doesn't mean its going to fail, it just means you'll want to take additional steps to reinforce it.
There will be no holes drilled for plumbing, I will have a few half inch holes for wiring the lights though
generally the small holes drilled for electric can be ignored, so long as they're near the middle of the beam. They're no worse than a knot.
Drill 3/4" holes. You don't want the wire catching if the joists flex, you want it moving freely. You can get a ratchet effect where the wire can only move in one direction so every time it flexes it tightens and never releases. Tight wires cause problems.
Ok thanks, some holes for lights have already been drilled at 5/8 inch is that enough?
5/8” holes should be fine, just try to run only one cable through a hole. The goal is to avoid binding, so if the cables can slide freely through the holes you should be ok.
Note that with a bouncy floor like this, you want to leave some slack in those cable runs so that they don’t get pulled tight when the floor is loaded. I would play a little loose with the code here in terms of cable staples, always making sure there are slack loops near boxes and no tight staples. You want to be absolutely certain there is no possibility of floor movement pulling a cable tight and pulling it out of a clamp at a junction box. Make sure to leave a foot to 18” or so of slack loop between each box and the first staple, and form that “loop” into a sort of “Z” shape. That means the staple would be fairly close to the box, with a big bend of cable between the staple and the box to act as a spring to ensure no force transfers to the box through the cable. Be sure the cable between joists also has a fair bit of slack in it so that it doesn’t pull tight with floor movement.
Bill
1-2" deflection is far more than you'll find comfortable. Engineers I work with limit deflection to 5/8" maximum, regardless of span. Being a little over on the span tables will almost certainly not result in catastrophic failure, just more bounce than most people find acceptable.
Blocking helps each joist share its load with adjacent joists. At 24" o.c. spacing you might find it helpful, but at 16" o.c. and Adventech subflooring it doesn't do anything because the Adventech is stiff enough to share the load with adjacent joists.
Do you have a bathroom on the second floor? The code of 30 psf (live load) is for sleeping rooms only. For bathrooms, you should use 40 psf live, and in most cases a 20 psf dead load.
Is your drywall 5/8"? Standard 1/2" drywall isn't rated for 24" o.c. on ceilings.
I'm not sure what span table you're looking at; this is the one I use: https://codes.iccsafe.org/content/IRC2021P1/chapter-5-floors#IRC2021P1_Pt03_Ch05_SecR502.3.1.
There’s a bathroom up there but there’s alot of walls below it and there’s a few spots where it’s 16 OC in the bathroom because it’s right beside the stairs
Drywall hasn’t been done yet
Michael, see my reply just above and give me your thoughts on some of those.
Kyle, I don't think "pre-tensioning" the joists is a good long-term idea.
To reduce deflection, you need to increase the stiffness of the framing members, which can be done by making the existing joists into T-beams as you suggest, though fastening the new "flange" to the existing "web" requires a lot of glue and screws.
I think it would be easier to add additional joists; doubling them would probably be about the right amount. The new joists could be sistered to the existing joists and would not need to be supported at the ends, though additional fasteners should be used at the ends to attach the new joist to the existing joist.
That's a good point on the sistering. I'm showing that at 15 lb dead load / 20 lb live load the shear values have plenty of capacity on a single joist.
I threw a 3x12 into my beam calculator and I'm getting about L/360 with normal loads, while it's not flat as a pancake it would meet typical design goals. That might be the best, and easiest to implement solution (with two guys).
Oh that was another thing, there’s not much dead load on this floor it’s not supporting any walls or roof above it
Dead load is the sub floor and finish floor.
If add osb underneath I will have to do it every where because the ceiling will be lower in that one spot than the rest of the house
I may just hope the blocking is enough because it’s not 1.5 feet over the limit like I thought it’s only 5 inches over
I didn’t consider where it was supported on either side of the far walls
Yes I’ve already put in 2 rows of block on either side of the center
The center already had some bracing
That kind of rules out sistering an additional joist to every joist without removing those. Perhaps someone else will chime in with an option that doesn't require removal.
Here's another thought. glue/screw another layer of 3/4" OSB to the underside of the joist? If the fixtures aren't in you still have time to make up for the depth, while essentially making a giant box beam of the entire floor diaphram.
It adds cost, and effort, but probably no more than the additional joists.
Bonus is that now installing drywall is extremely easy.
Deleted
What are some drywall alternatives I could put under the loft? So I don’t have to worry about drywall cracking
Could I have the framer build me 2 posts underneath that go from from the back of the loft to the front in the center? It would be 13 feet long front to back
Basically an open wall with just 2 supporting posts then I could cover the posts with something
I think we were all reacting in the same line of thought - that you wanted to keep that space below open. If you're up for modifications underneath, two beams with post running perpendicular to the joists would do wonders, and solve the problem all together.
It would still be open I would just have 2 posts there
derekr,
Please take this in the spirit it is intended.
In all the threads you have started on GBA there is a common theme of getting well into a project with no real understanding of what's at play, and then trying to extract yourself from the problems you encounter. You need a different way of approaching them - of getting expert advice on things you don't have experience in, so you can make informed decisions well before the work starts. Relying on advice you get off the internet half way through - whether from here or elsewhere - isn't a viable way to run construction projects.
Well I didn’t draw the plans for the house or frame it, I figured both of those people knew what to do
The guy that did the plans I told him what I wanted, then I gave the plans to the framer
I didn’t do any part of this myself like I did with the deck
Who drew the plans and designed the framing? Are you subject to building inspections?
I didn't realize how many questions you were asking here. GBA is good for the occasional question, but the extent of yours makes it clear that nobody involved understands what should happen. You need a qualified general contractor, and ideally a skilled architect or designer.
Yes inspector came a month ago and said everything was fine
Apparently he doesn’t know either
The simplest way to beef up your undersized floor joist is by laminating a strip of steel to the bottom of the joist. Your local commercial drywall place has this in 100' or 150' coils for bridging strap for steel stud construction (ie CS2.5-100-43 is a start but exact size needs to be spec'd by a professional).
You can attach it to the bottom of the joists with construction adhesive and TEK screws. This is the same idea as building a T beam as Kyle suggested earlier but done with steel. Drywall can be attached directly to this using drill point drywall screws (again stock at any commercial drywall place). The benefit is that it doesn't take up much space and can be done without removing anything.
If you want to make the drywallers job a bit easier, strap out the ceiling with resilient channel and hang the drywall on that instead. The resilient channel does a decent job of sound isolation for the bedrooms above as a bonus.
And this wouldn’t need posts? It would just run down the center? Screwed and glued on?
Is this what your talking about in the picture?
That is the stuff. As for, it is enough? Maybe.
You need an engineer to run the numbers to tell you how wide the strapping needs to be, what thickness and screw pattern.
The strapping does bump up the stiffness a fair bit, less than the 2x4 laminated to either side, I'm not going to guess if it is within reasonable code limits, that is the job of a professional not a random dude on the internet.
I’m going to get the framer to come back and let him feel the bounce and get him to do what he thinks would fix it, I may just do a lot of cross bracing
Inspector and everyone said everything is fine, so I don’t always trust people even the ones that are supposed know what they are doing, just because they have license doesn’t always mean they know everything
Maybe if I went through 10 different inspectors and engineers maybe I would finally get one that knows what they are talking about, but I’m starting to have enough of that
I mean I’ve had 3 different inspectors in a year and I’ve caught a few things that none of them even mentioned
Cross bracing only helps with a single person bouncing. Two people and just as bad as no cross bracing. If you look at most code tables, cross bracing only bumps up the span by a couple of inches. For example, in our code for 2x12 joist, bridging bumps up the span by 5". Far cry of the many extra feet you need.
You need to fix the stiffness of the assembly. No shortcut to this. Without increasing the stiffness, never mind bounce, the floor joists will sag overtime. I'm in the land of old houses with undersized joists and having 1" or more of sag in the center is not uncommon. At your 22' span with 2x12s, you are looking at more than that in a couple of years.
If sistered is equal to 12 OC going by the calculator it could span 21 feet 4 inches for 30 psf
Even if it was no. 2 grade southern pine
I may try to get him to put a beam down the center with 2 posts underneath it then, this would probably be the most secure solution and best for cost
Unless he wants to sister 22 foot boards, the first and last board are already sistered so it would only be 5 more, there’s 7 joist total, not sure if that would be as good as the beam and posts though
Sistering the joist allows its span to be doubled right? So that would put it well over the limit, even 2x10 sistered to the 2x12 would probably be enough
The loft is the only room like this, the longest span I have upstairs besides the loft is 16 feet then all the other upstairs rooms span is 12 feet or less
"Sistering the joist allows its span to be doubled right"
Not even remotely close. Doubling the joist increases the max span by about 15% to 25% depending on the wood species and grade.
I would check your local code, your sistered joists at 24OC is about the same as 12OC which there should be tables for. Right grade plus right species it might work.
Thinking about it a bit more, if the sistering can work, I would use an LVL. Straighter, stiffer and less chance of sag over time. Plus easy to get long lengths. Unfortunately not cheap but you only need a couple.
In the corner of the plans is a box for a stamp of the person responsible to make sure the plans are compliant and it seems clear they are not even close.
The city is not suppose to accept the plans without the stamp.
Any first-year framer knows what was built is wrong.
My uneducated guess is an engineer would have put steel I beams where the span exceeded 12 feet.
The question is are you ready to pay the engineer to design a fix and build it or are you going the wait until someone decides to have a dance party up there and see it fall.
I know who ever stamped the plans and I would be having a discussion.
Walta
I think I might just have the framer come back and add many rows of cross bracing
The floor isn’t bouncing to the point where I think it could fall, there has be multiple people walking up there for me to even feel it
Once again, bracing will do little or nothing for you.
No idea on the end walls (options for support) but if you're looking to minimise ceiling intrusion, you may be able to install an LVL flush beam (bisecting your 20ft floor joists) to half that span. Just a bunch of cuts and joist hangars. Supporting the point loads at both ends will require some framing additions as well. If you have an open loft, that end point can be addressed by another LVL or steel beam across the span.
Otherwise, sistering with LVL would be my suggestion.
You need an engineer who knows what they are doing to take a good look at your options. I'd 100% get it resolved while you're framing as going back after will be very painful. It's unfortunate that you had to be the one picking up on this issue, vs the designer. There is nothing to lose by going beyond code in situations like these.
I would be careful cutting those joists in the center. The structure looks like it may be a sort of A frame, in which case those long joists may be acting as collar ties. If that's the case, then the joists are in tension too, keeping the rafters from spreading apart. If you cut the joists to add a flush beam perpindicularly down the middle of the span, you have to "strap around" that center beam to keep the joists capable of handling the tensile load too. Regular joist hangers aren't suitable for tensile loads like this.
I do agree that making the center beam out of a few sistered LVLs is probably the best option here. Cutting the span in half with a center beam and some columns would completely solve the sagging/bouncing floor problem.
For the OP: keep in mind that if you add the beam and columns, those columns need to transfer the load all the way down to the foundation, so they can't just sit on the floor below -- you need to continue the column under that lower level too, typically down to a concrete pier (which is usually just a thick "block" part of the slab) to handle the load at the very bottom.
Bill
Bill, Simpson carries tension ties which I believe can be spec'd in this application (if the beams are used as collar ties) for use with the truss hangars. Thanks for reinforcing the support load issue. If you're introducing beams, the loads need to taken via framing to your foundation which may limit options in this case.
LVL sistering with PL adhesive and structural fasteners is likely the easiest fix in this case.
Connectors rated for the application would be fine, regular joist hangers are not though. You can run steel straps over the top and bottom, or use a connector rated for tension as you mentioned.
I've seen people try to use "regular" joist hangers in the past like this, and they put the tension along the nails, which means the force tends to pull the nails out. Nails are much stronger in shear, resisting sideways forces, and not very strong at all for pull-out resistance.
Bill
I thought about doing this but I rather keep it simple and just have the joist rest on top of beam instead of cutting them to put the beam inside the floor
But the other thing is my posts the beam will be resting on are going to be on a floor that’s over a crawl space, so I guess the framer needs to put blocking in the crawl space directly under where the posts will be unless they happen to be on top of a joist
Edit: actually the posts might line up perfectly with my crawl spaces piers that run directly down the center of my crawl space, I may not have to do anything underneath
Yea it’s A frame
The second floor isn’t being used as collar ties, it’s resting on the first floor wall but is connected to the roof just to help keep it in place but all the weight of the second floor is resting on both walls at either end, then the second half of the roof was built on top of that and connected to the top of the first floor
Small notches were cut in the roof rafters on the first floor and they are resting on the sub floor and the wall of the first floor, (one notch 7 feet up on the first floor wall and one notch at the very bottom on the sub floor)
So I have 7 foot straight walls on the first floor of the A frame then I used the back side of those as closet space and to run wire and duct
Derek, you have a challenging situation. At this point a licensed structural engineer is the best person to help you out of it.
Yea the person that did my plans for the house was an engineer, not much good that did me
I figured he knew how the long the joists needed to be
I would put money on him not being a licensed structural engineer. Anyone can call themselves an engineer, unlike architects who strictly control use of the term.
Going from calculators people showed me to use in here everything else about the house is fine, the roof, the first floor, and all of the second floor except for the loft
Looks like I’m in luck, I went under the crawl space to do some measurements and the posts will be right on my center support beam and piers
One post would be right on top of one pier the other one would be a couple inches off but still on the double 2x10 support
Did the “engineer” put his stamp on your plans with his license number?
If so, he will have no problem redesigning this at no charge as he would lose his license if the board got a look at your plans.
If a real licensed engineer did draw these plans, it would be a huge mistake to vary from the plans in any way. As doing so would shift all the liability and responsibility from him to you.
Walta
I did make 1 change but I asked him first while he did the plans
I made the roof 24 OC instead of 16, I asked him if I could do that while he made the plans and he said either way would be fine
Everything is ok with the house though the loft the is only problem
The framer came and suggested I just sister the joists so I won’t have a post in the middle of the room, it will be no. 1 and no. 2 yellow pine glued and nailed, does that sound sufficient for 21 feet?
Sistering is equivalent to dropping the joist spacing in half, from 24" to 12". Check your span table but it sounds like it would work.
Yes, see post 21
Thanks I remember you mentioning that now but I forgot, I’ve been worried the last few days and couldn’t keep up lol
Will this also stop the floor from sagging over time though? Like in 30 to 50 years could it still sag even though they are sistered or should it lessen that too?
I wish there was a calculator for actual failure of a floor instead of just deflection, only because I’m curious what it is, I have this fear now of a floor falling on top of me from upstairs because of all this lol
I’m glad I noticed this myself before they drywalled it, because the inspector didnt, all of you on here have given much more information than the inspector has, when he said everything was fine I thought i was good to go
I've seen the guts of hundreds of old houses and barns and you'd be surprised how undersized much of the structure is. It takes a lot for wood to reach catastrophic failure--aka breaking and falling on you. It can be calculated by using what's called the "extreme fiber in bending;" each wood species and grade has a different point where the wood fibers actually start breaking, but it would take far more load than most houses experience. From a design perspective, "failure" is just an uncomfortable amount of deflection, where the floor feels bouncy, drywall can crack, etc.. So don't worry about that.
Unless a piece of lumber has knots or cracks it will twist or buckle long before it snaps in two.
About two years ago I was working on a house where an unfinished attic had been converted to a second floor. The floor joists had originally just been ceiling joists and were 2x6. On the second floor they had put in two back-to-back bathrooms with lots of tile and cast iron tub in one. Those 2x6's were twisted about 30 degrees in the middle, but hadn't snapped.
Not that it wasn't dangerous. When any part of the framing starts to move you get stresses in directions that weren't anticipated. What's probably more likely is a joist rolling all the way over than snapping. Once one rolls the whole floor can follow.
What does the engineer say about the bouncy non code compliant floor he designed?
It is hard to imagine a responsible engineer condoning such a major design change verbally but general truss design and spacing falls the engineers employed by the truss supplier so it is not his problem.
Really you are taking advice from the guy that knowingly built this code violation just because that’s how it was drawn?
"I just sister the joists" Will that make it code compliant I dont think so.
I got to ask did someone decide to eliminate the wall/walls under this thing after the plans were approved?
Walta
I asked him and he said the same thing, add more joists or do support beam
So I’m going with more joists, I only need 5 because 2 are already doubled
It’s only going to be a $500-600 fix not worth arguing with him over why he got plans wrong for that
I think I can see how he made the mistake though, the loft is 21 feet wide but only 15 feet of that is considered usable because of the roof slanting in, so the room in the plans say it’s 15x13 even though it’s actually 21x13, so he probably just thought of the room being 15 feet, that’s just my guess
Is the engineer going to give you a set of stamped plans with that change???????
My guess is no! because it is still a code violation and it will still do the limbo.
A real fix is made of steel and costs 10k.
Walta
No plans he just reviewed what he already did for me then told me what I could do, I don’t think rules are as strict here out in the country
Going by what DC said with the calculator and making it 12 OC it adds up to working
If i use no. 1 lumber it will actually be beyond what I need, no. 1 12OC can go up to 24 feet I only need 21
I would ask about MSR lumber (Machine Stress Rated), which is graded by a machine and not just visually. MSR lumber often costs about the same as #1 visually graded lumber, but MSR lumber guarantees you strength and not just appearance. I spec MSR lumber for anything that is critical but not big important enough to warrant stepping up to an LVL.
Bill
I think a lot of my no. 2 yellow pine could be no. 1 going by appearance, some barely has any knots, I guess some get through every now and then when being graded
If you are ok with an under slung beam and have good support to the foundation, that would be my pick in your situation for "sag" down the line. Likely cheaper too. If that loft portion is open, and you don't want a post on the interior end of the beam, then you can use a 2nd LVL (or steel) to take the load across to the walls. LVLs are pretty easy to work with as they can be ordered and cut/built on site.
If you get into the structure into some of the 100 year old balloon framed places around here, you would be pretty surprised they are standing. There's something to be said for old growth timber and ship lap siding when it comes to rack/sag resistance.
Yea there was a house atleast 100 years old that had rocks under it for a foundation not far up the road from me that had been rotting, it’s been torn down now but even if we had a storm that had like 70 mph winds it was still standing
I’m just going to have the framer sister more joists underneath to keep it simple, calculator says it works for 360 deflection and 30 psf live