Okay before y’all get mad at me for not using a structural engineer just relax this is a simple problem but I have a question on something that seems to me to be overlooked.
I want to beef up my attic floor joists to be able to safely store a bunch of stuff up there. Right now I have ‘real’ 2by4 old growth red wood joists that span ~11′. Using Span Calc 2002 I get these numbers. With a 30live 10dead loading and using Doug Fir #1 grade (12″oc) it says I can span 12′ with 2by6’s. That seems a bit much to me, but what do I know. I figure if I glue/screw the 2by6’s to the existing joists and block between them and glue/screw 3/4″ T&G plywood to them I should have a strong enough floor?? Right now the attic is only storage but maybe someday it will be some sort of livable space so I want to make sure that I do not shoot myself in the foot in that respect.
Heres’ the part that seems to be overlooked to me. I have old growth red wood joists that I am gluing and screwing the new joists too. These joists are impressively hard. This HAS to be stronger than just a 2by6 floor. Its almost like a T-beam sorta. Ha, I guess this is where a real structural engineer would come in handy. If the engineer could figure out that the new 2by6 ‘T’ beam was as strong as a modern 2by8 you save money in the lumber and save a lot of additional weight on the lower floors of the house. Maybe the money saved in lumber would be a wash with paying an engineer but you would definitely add a lot less weight to the house.
Daniel Neuman
Oakland CA
Crazy Home Owner
Replies
I never go for a 30# live load on a floor system.
With that span, I'd suggest 2X8s. I thnik 2X6s spanning that far would be pretty thin.
BTW - How's married life?
I believe that for bedrooms and attics the 30-10 is some sort of standard? I don't have any sort of feeling for what will make a non-bouncy feeling floor but 12' sure seemed like a long span to me.
Since this is an attic every inch of headroom counts and if I can get away with 2by6's then I really want to go that way. Hence my question about the 'ignored' contribution of the existing joists. With a 40-10 load, spancalc gives me a max length of 10' 11" for a 2by6.
Married life is fine. One thing we screwed up is changing our last names. We were told that we could do it easy at the SS office but no go. Now we both have to go thru the whole rigamarole of a legal name change. If we had known the facts beforehand I could of changed my name before the wedding then she could of taken my last name and that would of saved us a bunch of money.
Married life just got a bit stanger as my new job is working for my wifes boss so we both work outta the same building and see each other often. She better not catch me eyeing the cute interns.....
Daniel Neuman
Oakland CA
Crazy Home Owner
Have you ever read the thread on Floor Vibration ? It will maybe answer some of the questions of subjective "feel" of a floor.
You asked about the 30# design load for a floor sytem. My understanding is that 30# is allowable for 2nd floor sleeping rooms only. All other floor areas have to be 40# minimum.
Even if the 30# is allowed, I've never once used it. It allows floors to be grossly underdesigned with the lighter loading. Kinda like in your situation, with 2X6 joists spanning 12'. I think that would be a mistake.
TV is chewing gum for the eyes [Frank Lloyd Wright]
You're absolutely right. I even took a peek at the 2000 and 2006 IRC and 40 PSF for nonsleeping rooms, 30PSF for sleeping rooms is the code. But I stay with the 40PSF for all. The baths are down the hall from the bedrooms, bedrooms become libraries, etc. The floor should not bounce no matter what label you give the space.
Just curious if you looked at the Floor Vibration thread too.
Didn't know if you were aware of that research regarding floor design. I sure learned a lot from it.
By the time a man can read a woman like a book, he's too old to collect a library
I just took a look at the Floor Vibration and it sounds interesting. Regarding the 11 7/8" TJIs, of course you know that whether its a Pro 150, Pro250,Pro350, or Pro550 allow you more span with your L/360 (min. per code) and L/480. I'm not sure if the study takes that into account as well as the different wood species. But it was interesting.
The stifness formulas have the modulous of elasticity built into them for floor trusses and conventional lumber. For I-joists, the manufacturers have a "Stiffness constant" that's used in the formulas.But - Those numbers don't make NEAR as much difference as adding depth.Depth = stiffness.
I'm only God when I get paid [George Burns]
I just now read the thread. Makes some sense. So for someone like me who has not built hudreds of floors and then walked on them. Should I design my floor for a L/480 spec??Daniel Neuman
Oakland CA
Crazy Home Owner
I wouldn't design a floor for less than 40 PSF, or less than L/480. Don't know how exactly that would work out joist-wise, as I don't have span charts for joists. And I never wrote a spreadsheet for floor vibration with joists. For a "rule of thumb", I go with an 18-to-one ratio for all types of floor members. By that I mean 18" of span for every inch of depth.So with your 12' span, that would mean a joist that's 8" deep. But a 2X8 would probably be close enough.
In politics, an organized minority is a political majority. [Jesse Jackson]
I found this online joist span calculator from the american wood products council.
http://tinyurl.com/ympeyh
It says that Doug-Fir #1&better grade 2by8 16"oc at L/480 with the 40/10 loading would have a max span of only 12' 2". So yea it looks like y'all were right on the 2by8 call.
A 2by6 in similar conditions has a max span of 9'2'. Looks like I'll be going with the 2by8's which really aren't even overdesigned for the job.
Daniel Neuman
Oakland CA
Crazy Home Owner
".... it looks like y'all were right on the 2by8 call."
I wasn't hired for my looks, ya know.
(-:
If blind people wear dark glasses, why don't deaf people wear earmuffs?
For a non experienced person like myself it sure would of been nice to know beforehand that the L/360 'standard' is considered substandard by the pros. I think that what I am hearing here is that L/480 should be the min for a reasonable floor. Its a bummer though that I am going to have to lose ~4" from my room height....Daniel Neuman
Oakland CA
Crazy Home Owner
"...it sure would of been nice to know beforehand that the L/360 'standard' is considered substandard by the pros."
I wouldn't say the standard is worthless. But maybe it's getting to be outdated.
One of these days I figure there will be a widely accepted performance standard based vibration. But that may be many years away...
Muppet Newsman: Here is a Muppet News Flash. An international spy ring is trying to sneak ridiculous stories into the news... In other news, a black and yellow striped mackerel was elected king this morning. (from The Muppet Show)
If a little bit more headroom is worth a whole lot of work to you, you could save yourself the thickness of the subfloor by nailing 1x3 cleats to the sides of your joists, 3/4" down from the top, and putting rips of 3/4" plywood between the joists, flush with the tops. This trick is usually used to get tiled bathroom floors closer to the adjacent hallway height.
-- J.S.
Wow, I had to read that twice to understand what you were talking about. Dang that would be a lot of work to save 3/4"....since this is going to be a storage area and then someday maybe some sort of limited use room I think that the 3/4" loss is the least of my problems.
What I need is something like 4" deep parallam 'beams' that I could but 24" OC and have that be as strong as 2by8's 16"oc and have them be cheap too if its not too much trouble.Daniel Neuman
Oakland CA
Crazy Home Owner
Okay my goofy comment to Jon Sprung just got me to thinking. What way-out there ways can y'all think up to give me a strong floor 12' span on the joists and keep the head height loss to an absolute minimum. I wonder if there even is a way to (cost being no object) do it within the 4" of the existing joists.
Steel joists? Some sort of engineered lumber?
Okay I just thought of a crazy one. What about somehow jacking the entire roof up (like you jack a house up) say 4' and building a short wall all around the perimeter and then setting the roof back down on it. Bang, instant tons of headroom.....Daniel Neuman
Oakland CA
Crazy Home Owner
You might ask these guys:
http://betterheader.com/default.asp
-- J.S.
I wonder if there even is a way to (cost being no object) do it within the 4" of the existing joists.
Stressed Skin Diaphrams With tensional members.
Uh. . . That's 4" + thickness of the Diaphrams.SamT
I've read about them in the archives. This is where you remove the lower ceiling and install a plywood membrane top and bottom? This does sound great but I don't have access from below. Actually the ceiling is the original in good shape plaster with some fancy detailing that I would really hate to lose.
This definately sounds like something that an engineer would have to design.
Daniel Neuman
Oakland CA
Crazy Home Owner
If this ceiling is wood lath and plaster, you can make it a whole bunch safer to do this work. Before you do any structural stuff, go up there with the shop vac and gently get rid of the dust and broken keys on the top of the lath. Then dribble yellow woodworking glue between the lath and the plaster. This'll really secure your plaster to the old shrunken lath, making it a whole bunch better able to withstand the sistering job. Let it harden over night before you start breaking out keys where the sisters will go.
When I did this on my place, IIRC, I got about 250 - 300 sq. ft. from a gallon of TiteBond. Of course this won't save you if you slip and put a foot thru the ceiling. But I did slip once and put part of my weight on a knee onto the top of the lath. It held without damage, which I'm sure it wouldn't have without the glue.
-- J.S.
If this ceiling is wood lath and plaster, you can make it a whole bunch safer to do this work. Before you do any structural stuff, go up there with the shop vac and gently get rid of the dust and broken keys on the top of the lath. Then dribble yellow woodworking glue between the lath and the plaster. This'll really secure your plaster to the old shrunken lath, making it a whole bunch better able to withstand the sistering job. Let it harden over night before you start breaking out keys where the sisters will go.
When I did this on my place, IIRC, I got about 250 - 300 sq. ft. from a gallon of TiteBond. Of course this won't save you if you slip and put a foot thru the ceiling. But I did slip once and put part of my weight on a knee onto the top of the lath. It held without damage, which I'm sure it wouldn't have without the glue.
I want to do this but am having trouble visualizing the process. Do you mean you just squirted a bunch of titebond all over the ceiling? Will that soak in enough to the wood lathe and plaster to really work? I don't know but it seems to me that gluing the plaster might take a more specialized adhesive?
I'm thinking of some kind of thinner adhesive that could be sprayed on in a nice even coat that would sink down and really bind the lathe to the plaster. Oh, what about a glue that wouldn't dry super hard to give the ceiling some flexibility?
Help,
Daniel Neuman
Oakland CA
Crazy Home Owner
What I did was put TiteBond in an old plastic ketchup bottle. The hole in the lid lets it out a lot faster than the OEM squeeze bottle. I worked from the top, shop vaced out all the crud and broken keys, then squirted the glue between the lath and remaining plaster. In areas where the plaster was sagging down from the lath, I pushed it up from below with the ceiling jack until it dried. Titebond and wood lath have a small amount of "give" to them, enough for this to work fine.
I suppose you could thin the glue with water if you want it to go deeper, but if the lath is sufficiently shrunken, there's plenty of room for full strength titebond to flow in. You want it between the lath and plaster. Spraying the whole top would put most of it on the upper face of the lath, where it doesn't do anything.
When you jack the plaster up, put a carpet scrap on top of a big scrap of plywood, like a couple feet square or more, on top of the ceiling jack. If you have cracks where glue might leak thru to the carpet, add a sheet of thin plastic to the rig. I never had any glue come through the cracks, but it's better not to take a chance on gluing the carpet to the ceiling.
-- J.S.
Daniel, I just ran some numbers and it looks like you get very close to L/480 if you sister each existing 2x4 joist with a 1 3/4" x 5 1/2" LVL.
Your "t" beam theory is good, and it's how things like parking garages are made, but there are too many variables to make the formulas work with wood. Will it help? Yeah, but I can't say how much.
There's no way extreme fiber in bending (per GHR) is going to have any effect at these levels of stiffness.
I can post the formulas, or email them to you if you're interested, but the end result is above.
Disclaimer: I have a degree in structural engineering, but no stamp. I work as a building designer and carpenter. Anything I post here could be pure horsepucky.
Mike Maines
Hi Mike and thanks for running the numbers for me. I just got an email from an engineer at trusjoist that basically said the same thing. So it looks like to save the 2" from using a standard 2by8 I would have to spend a ton of money on fancy engineered lumber. Guess I have to get right with the 4" loss of headroom.
Though I have not found out yet if there could be a steel joist solution (just for fun that is). Dang, if only it was possible to pop the roof up 4' and add a kneewall.....Daniel Neuman
Oakland CA
Crazy Home Owner
I don't have any experience installing them, but it looks like 550S162-68 steel joists will work for you. Nominal size is same as 2x6, 14ga steel, 16" o.c. at L/480 and 40# live load.
Or you could pop that roof up 4'. Why not? I've seen some of the other stuff you've made, you're up for it!
The Clark Steel guys have a 2by4ish steel joist that would do it also. So far they are the winner for strongest joist with the least depth.
Man I just wish there was a way to pop the entire roof off in one piece and add a short knee wall. Though I suppose everyone and their uncle has wished something like that before.
I've talked causually with a couple of engineers and archetects before about this and I really have not been impressed with their seeming inablitly to come up with an outside the box solution.Daniel Neuman
Oakland CA
Crazy Home Owner
I recall seeing a product made in northern Europe that is 2x4's glued edge to edge like a giant cutting board, used for floors. If your criteria was 40#LL and went back to L/360 deflection, you could do it with solid 2x4's that have a modulus of elasticity of 1.4x10^6, which would be any of several select-grade softwoods.
The 2x4's will work at 12" o.c., right? I was using 16" o.c. because you would need to brace each joist from a strong desire to roll. If you don't want to disturb the ceiling below you wouldn't be able to brace the bottoms. So I thought you would want to use the existing joists as bracing.
You want outside the box? How about you use the joist bays to form lightweight concrete post-tensioned slabs? You would have to cut into your sheathing to access the tensionsing bolts once the slabs have cured, but it could be done. If you're up for the challenge, that is!
Mike Maines
PS Popping the roof up isn't outside the realm of possibility, either. You have a Victorian-era home, right? You would need to add a structural ridge beam to keep the new kneewalls from bowing out. Rent a crane for a day and go for it.
No the 2by4's can not span the 12' required in any configuration. According to the span calc I need to use a single 2by8 at 12"oc to get the L/480. For bracing the joists I couldn't just use blocking between the joists and the 3/4" subfloor to prevent the joists from rolling?
I still don't know what spec I should be using L/360 or L/480, of course I would like to use L/360 but all the pros here shot that down as under spec'd.
It is a victorian house with 12/12 pitch roof-the roof is t shaped with gable ends at the three top ends of the t. Okay just for fun how in the world would you go about craneing the roof off?
Daniel Neuman
Oakland CA
Crazy Home Owner
Oops, I meant the STEEL 2x4's could span 12' at 12" o.c.?
You SHOULD use L/480, and I was one of the ones arguing that you should go for L/480, but compromising to L/360 wouldn't be the end of the world if you could live with the extra deflection. The floor would just have a little "give" to it, is all.
Craning off the roof would depend a huge amount on site specifics, but probably something like this:
Make the short cross of your "t" the primary beam. That gets supported at either end, with the load chased all the way down to the foundation.
The long cross of your "t" become two secondary beams, hung off the first beam and supported at the ends.
Get those in place, under your existing ridge, tied to the rafters with some sort of Simpson tie.
Bolt 2x6's about 1' or 2' up from the bottom of the rafters as temporary rafter ties, to keep the rafters from spreading or collapsing when you lift the roof.
You might be able to get by without a crane, if you set up properly supported jacks inside the roof structure.
Sawzall the rafters off at the base.
Jack the roof up to the desired height.
Build new cripple walls (is that a PC term?), 4' high or whatever.
Set roof down and attach everything.
At least that might be how I went about it. Depends on the site.
Mike Maines
Okay so here's what I have decided to do... I am going to sister each 2by4 with two 2by6's (one on each side). This should give me just about the same strength as a single 2by8' Then I am going to use 3/4" T&G Plytanium glued and screwed to the joists as the sub-floor. I plan on blocking the joists at both ends and the middle also.
I'm also going with the Memphis Folding Stair for the attic stair.
I would like advice on how best to sister the 2by6's to the 2by4's. The best way to make them act as a single beam. One book I have suggests nailing every 6" in an alternating pattern.
One more bouncy floor question..do the deflections (L/480) for the joists take into account the subfloor? Wouldn't a nicely done (not sure what that means exactly) subfloor contribute to making the floor deflect less under load?
We went up into the attic over the weekend and mocked up the headroom with the attic being finished. We basically get a 'room' that is 8.5' (that's the width above 5' head height) wide and ~50' long...more elbow room at the T gabled section. Its like ~600sq Ft. We have pretty much decided that it will only ever be storage...
All in it will probably cost us ~$2000 in materials to do this. Maybe that's a bit high just for storage but we pay $100/month to rent a 100sqft storage garage so in less than 2 years we will break even on the cost. Daniel Neuman
Oakland CA
Crazy Home Owner
Subflor that's glued on definitely does help improve the performance of a floor. Hard to say exactly how much, but it does help.You might check around to see if you can get a better grade of 2X6. around here the lumberyards stock 2X6 #2 or stud grade SPF. But you can go to a truss plant and buy 2X6 SYP in much higher grades. That may or may not be the case where you live, but it would be worth checking in to.
Ever wonder who tastes dog food when it has a "new & improved" flavor?
Glue and nails helps a floor a great deal. The APA website has enough information to determine how much.I agree with the poster who indicates that you will never load your floor to the design limits (fiber stress or deflection).
> I would like advice on how best to sister the 2by6's to the 2by4's. The best way to make them act as a single beam. One book I have suggests nailing every 6" in an alternating pattern.
Good idea, and offset the nails for the second sister 3" so they fall between the ones from the first side. You don't want to split the original joists with too many nails too close.
Glue the whole shebang together, use lotsa glue. A good construction adhesive like PL Premium is fairly easy to use. You could also use an aliphatic resin woodworking glue, but you'd have to apply it to the sides of the sisters on the flat, and slap them into place before it runs down too much. I've done some like that. Not as easy as construction adhesive, but at least as strong. My method was to slap some clamps on real quick to get the glue in contact over the whole area before nailing.
You can shoot the nails in, but finish off with a hammer. Hit the sister on both sides next to the nail, then seat the nail with a good hard hammer hit. The gun doesn't suck things together like hand hammering does.
First, though, stabilize your plaster using the glue trick described before. Then rake out the keys for a couple inches on both sides of the existing joists. Vacuum well so there's no crud in your way.
Crown your new sisters upward in matching pairs. With the plaster stablilized, you can gently jack the middles of the old joists upward to line up with the sisters as you dry fit them.
-- J.S.
Within the last year or so either JLC or FHB had an article where they raised the 2nd floor off the first and added segemnt to raise the first floor ceiling.Don't remember if on the 2nd floor they do someithing similar with the roof or if they torn off and rebuild the roof.
"Man I just wish there was a way to pop the entire roof off in one piece and add a short knee wall."
Is there any way to add a dormer to open up some space?
That would add some light, too.
If at first you don't succeed, aim lower.
Hey Boss thats my secret plan actually. SWMBO was pretty against 'finishing' the attic off the way I wanted to. But I got her behind the idea of just putting in flooring under the t section in the roof where there is the most headroom. My devious plan is to get this floor in and then have her go, 'gee this would make a great master bedroom suite if it was just a little bit bigger...' The best place for the dormers would be in the lower trunk of the t just past the cross part of the t.Daniel Neuman
Oakland CA
Crazy Home Owner
Remember as you get older you start to get shorter. The 4" will not mean anything in 20-25 years.
Ha, yea that's true. I just decided that I need to do a mockup of the head height in the area that I am building this storage loft to really see what height I can live with....Daniel Neuman
Oakland CA
Crazy Home Owner
Yes there is a 4 inch deep steel product that could be added to each existing 2x4 (full size) that would provide sufficient strength and stiffness. There are so many different manufactures and shape idiosyncrasies but something that is a full 4" and about 18 or 16 gage metal would work.
Be careful, this is similar to the light gage metal studs you might see in a store or office finish-out but those are typically very light gage. You will need the very heavy gage products so I would suggest that an engineer (or a good supplier) would be able to guide you best.
Basically what you are looking for is the combination of E (modulus of elasticity) and I (moment of inertia) for steel that, when multiplied together, will equal or exceed the E x I value for the 2x8 joists that are recommended.
Hi and thanks for the reply. I've poked around on the net looking for this type of thing but am not having a ton of luck. Can you suggest a couple of places to look?Daniel Neuman
Oakland CA
Crazy Home Owner
try this link:
http://www.clarksteel.com
Well thanks they do in fact have a 4" deep joist that would work 12"oc. This discussion has passed from the doable into the 'just for the heck of it I wonder if its possible' zone. I poked around some more on their site but have not seen how they would reccomend that this be installed. What I did see looked like using these steel members would be quite a PITA Though I am curious how they would be installed in a retrofit situation.Daniel Neuman
Oakland CA
Crazy Home Owner
Daniel,
One thing to consider when sistering is what all the pounding will do to that nice lath & plaster ceiling. The glue stabilization idea is great but nailing into any old-growth full dimension really dry wood is bound to cause a lot of vibration, even if you use a gun and just hand set the nails. Granted, alternating toenailed nails will be very strong, but you may well be doing a lot of plaster patching
I don't believe that screws have adequate sheer in this application (hope I didn't restart that little war with this statement!) so I'm wondering if through bolts would be a better solution than nails to build your T-joists.
Disclaimer: I have zero technical knowledge to back up this idea, and have found more times than I care to count that something that seems right just isn't...
Sorry to give you yet another option :-)
Wayne
With glue on top, my plaster stood up just fine to all the jacking and whacking. I never lost any at all -- good thing because the pattern would have been hell to match.
Drilling and bolting would certainly be gentler, but IMO, the necessary jacking to get the old and new wood to line up stresses the plaster more than the nailing.
-- J.S.
Hi John,
I don't think I understand why its necessary to jack to the old beams back into level? It sounds like a good idea but I don't understand why 'exactly'. SWMBO really is resistant to the idea of jacking the ceiling as she really does not want to damage it.Daniel Neuman
Oakland CA
Crazy Home Owner
In my case, the old joists had a visible sag to them. Pushing the middles up to match the sisters corrected that, and made sure that the ceiling load was carried by the new sisters. I didn't lose a single particle of plaster in the two big rooms where I did that.
-- J.S.
Interesting thread- I'm gathering that just slapping in a 'bunch' more full 2x4s (maybe so they end up being on 8" centers) wouldn't help? Taking this to the extreme for example, I would think a floor of SOLID 2x4s would flex a lot less that one w/ 2x4s on 16" centers- or is it ONLY depth that contributes to stiffness?Then how 'bout 2x6s 8" o.c.?If not, then I'd look into steel- maybe you wouldn't need it on every joist.
The formula for stiffness is width x height x height x height. Divide that by 12 and you have the moment of inertia for a rectangular object. Multiply that by the modulus of elasticity, which for the woods we're discussing here ranges from 1,400,000 to 1,900,000. Multiply those number together and you have a stiffness factor you can use to compare different shapes and materials.
The steel joist I suggested to madScientist has a modulus of elasticity of 36,000,000.
You probably meant to address this to 'all' as I am not the expert here. I don't know if the solid 2by4 floor would work. One thing I know is that you have to be concerned about the added weight. The solid 2by4 thing might work but its going to add a ton more weight than 2by8's.
Once I decide how much height I can lose exactly I will decide between the realistic options of 2by8 joists or the 2by6-2by4-2by6 sandwich which I believe will be just about exactly as strong.Daniel Neuman
Oakland CA
Crazy Home Owner
What way-out there ways can y'all think up to give me a strong floor 12' span on the joists and keep the head height loss to an absolute minimum.
Ooh, an easy one <g>--use 6" 18ga galvanzied metal structural studs (you need a 6" angle to hold them up, but they'll span 12' with 100# loading at 16" O.C.--or will for the storage loft the engineers just spun up for us)Occupational hazard of my occupation not being around (sorry Bubba)
Clark steel has a 4' structural steel joist that will do it at 12"oc. If I was going to go steel I would probably go that way. But the 1.5" of height I save just couldnt be worth the added hassle/expense.Daniel Neuman
Oakland CA
Crazy Home Owner
If I was going to go steel I would probably go that way
Cool.
Numbers were in my head since we just got through with a plan package for a metal building where the client added a 'storage deck' in at the end. Engineers ciphered roof deck out of product that our local Altex has in stock.Occupational hazard of my occupation not being around (sorry Bubba)
that the L/360 'standard' is considered substandard by the pros.
That can depend on which pros are asked.
Structurally, one uses l/240 as the maximum deflection for "flexible" wall materials (like wood paneling); and l/360 as the maximum deflection a plaster wall will "allow" without cracking. Neither standard of deflection really addresses whether a floor is "bouncy" or not (a stressed-skin floor can hit 1/720 but be as bouncy as a trampoline--'cause the whole floor is the diaphram).
I was taught both "ways" of structures. The "book" way, the way an engineer approaches such things, and the practical way, the way a frame and/or builder does (or ought). For all practical puposes, residential wood structures are never really going to be close to things like maximum fibre strengths, or "true" "l-over" deflections. This does not mean we can ignore those values in the look-up tables, though--just that the various safety factors each "whallop" the dinkier values pretty quickly.
Then, there's the whole debate that could be entered into about whether the average floor joist can be fastened with the sort of tolerance that measuring to "l" over anything . . . (for most lumber, it can deflect close to 1/3 of its own depth before the fiber strength begins to fail; but the 16d toenails will pull out long before that much bend occurs--mostly--).
Occupational hazard of my occupation not being around (sorry Bubba)
I was taught both "ways" of structures. The "book" way, the way an engineer approaches such things, and the practical way, the way a frame and/or builder does (or ought). For all practical puposes, residential wood structures are never really going to be close to things like maximum fibre strengths, or "true" "l-over" deflections. This does not mean we can ignore those values in the look-up tables, though--just that the various safety factors each "whallop" the dinkier values pretty quickly.
Are you saying that because of safety factors I can use the #2 and better stuff instead of the #1 grade?
Daniel Neuman
Oakland CA
Crazy Home Owner
Are you saying that because of safety factors I can use the #2 and better stuff instead of the #1 grade?
You can. A person can fo all sorts of things <g>. If the chart says #2 does the trick, then, that's good enough for me usually.
Personally, I don't like getting off the tables into specialty materials, like #1, but that's likely because I've never been comfortable at the edges of "the envelope" given the lifespan of buildings.
Some of that is from remodeling, though. Say a person wanted a very cool, skinny, balcony cantilever, and built a half-in/half-out stressed skin deck out of, oh, #1 2x4 Hemlock and used PL Premium and bolts & screws to make the thing aircraft-strong.
But, the repair dude, 15-20 years later, just sees "2x4" and does a repair with the wigglewood down to the big box and the 16d's in his bag . . . and the owners after that, are asking "Is this supposed to do [this]?" . . . Occupational hazard of my occupation not being around (sorry Bubba)
I agree that pushing the envelope is a bad idea. But I'd also put aside the best joist stock for the most critical locations, like long spans and under tile floors. This whole thing is long spans, so I'd pick thru the pile.
-- J.S.
Inche of depth? Nominal? 16" OC?
I don't know what you're talking about...
Statler: You know, the older I get, the more I appreciate good music.
Waldorf: What's that got to do with what we just heard?
Statler: Nothing, just thought I'd mention it. (from The Muppet Show)
You offered the rule of thumb 18" of span per inch. In and of itself that is worthless.
Real inche of depth or nominal?
12 OC? 16 OC? 24 OC?
The rule of thumb I suggested is not at all worthless. Did you read the thread on floor vibration? In that thread I pointed out that vibration is determined MUCH more by the length-depth ration than spacing or anything else. That's why I use the one inch of depth per foot of span as a rule of thumb. The spacing is means very little. BTW - I mean inches of REAL depth - Not nominal.
Waldorf: Greek music, very appropriate for the pigs.
Statler: The pigs are from Greece?
Waldorf: The pigs are greasy. (from The Muppet Show)
BossHog is right, Daniel. I have an old house with 2x6 attic joists @ 16" OC. spanning about 12'6". Attic is finished and has furniture. There is some sag in the floor. We are likely going to sister in 2 x 8's to beef up the floor.
Hi Mike,
Now is your sag in the floor from your house foundation settling? Is the floor 'bouncy' up there?? I don't know what to do because the head height is so important....it does seem like I will get a stronger-stiffer floor with the new 2by6's attached to the existing 2by4's...Daniel Neuman
Oakland CA
Crazy Home Owner
Basically, I think you MIGHT be able to get closely spaced 2x6's to carry the code required loading within the max stress but you WILL have a bouncy floor with 2x6's. How bouncy is too bouncy is up to you. I would not expect it to be acceptable. Go with the 2x8 joists and be done with it.
About the "T" beam ideas: in order for the decking and the joist to behave like a "T" it is necessary to fasten them together sufficiently to get the composite behavior. It requires a lot of nails or screws and glue has some limitations. You might get some composite behavior for awhile but over time the effectiveness of the glue diminishes.
Also, composite action will depend somewhat on the decking material. Not sure what you are using but I just would not count on much over the long haul.
The problem with the existing "old growth" joists is that even though they are nice and strong and hard, they are not very deep and so their stiffness (moment of inertia "I") is not very great compared to the deeper joists. As an example, the "I" of a full 2x4 is about 10 in*3 while a 2x6 (nominal)is about 20 and a 2x8 is almost 50. So, combined with a 2x6, it would attract about 1/3 of the load, that might overstress it and in any event, you would still have a relatively flexible floor. As you can see, a single 2x8 would still be almost twice as stiff as a combined 2x4 full and a nominal 2x6.
I would recommend 2x8's.
Thats true about quantifying 'bouncy'. One persons bouncy is another persons 'just fine'. Though I belive that the bouncyness is quantified in the L/ spec? The specs I quoted were for a floor that supposedlly had a L/360 spec. I think some folks spec L/480 for stiffer feeling floors.
Its interesting what you say about over stressing the orig joist. So even if I went ape sh*t with a high quality glue and screws and really stuck the two toghether-that could actually lead to overstressing the orig joist and what breaking it? The way you divided up the stress with 1/3 going to the orig joist is not obvious to me so I'll have to think about it.
Thanks for the thoughtful reply. I do realize that 2by8 is the obvious easy right choice I was just hoping that for the sake of headroom I can squeeze by with less.
Daniel Neuman
Oakland CA
Crazy Home Owner
Daniel,I was watching an old rerun of This Old House. They had undersized joists. They slid in 1/8" steel plates and bolted them to the joists. Of course this type of solution would require an engineer to size the steel and bolt placement.
I was watching an old rerun of This Old House. They had undersized joists. They slid in 1/8" steel plates and bolted them to the joists. Of course this type of solution would require an engineer to size the steel and bolt placement.
And I like steel....though definately an engineer would be needed.
Daniel Neuman
Oakland CA
Crazy Home Owner
I guess another thing this brings up is what is a bouncy floor and what is not? If the floor in question meets the L/360 spec is that going to feel bouncy to the average Joe? If so why does that seem to be used as a standard for floor deflection?
What floor deflection ratio is needed for a floor that most folks would consider 'solid'.
Are the joist span tables-programs useless if they say a certain config can meet the L/360 spec and most builders go 'Bah, that'll be a bouncy floor' and upsize the joists anyways.Daniel Neuman
Oakland CA
Crazy Home Owner
What constitutes a bouncy floor is somewhat subjective, but not totally. Those of us who have built L/360 floors can tell you that they are not all the same, which is why that isn't always a good minimum.
Over a short section of floor L/360 can feel stiff, while over a long span it feels quite bouncy.
Heck, L/360 isn't even up to the specs for applying extra large tile.
When old floor joists are sistered to new, it's not a simple matter to determine the load since the new joist has to pull the old joist back up straight. The old and new don't combine to equal something stronger, the old and new have to fight each other--unless you have existing joists that are flat (not likely).
Going up an additional step in joist depth is a good minimum, as would doubling up the minimum depth (2x6 in your case?).
How much weight are you going to have up there?
Beer was created so carpenters wouldn't rule the world.
Hi Don,
When you say a large span what do you mean? I have 12' from support to support-is that a long span.
Not sure how much weight I'm going to have up there. To start with just a ton of holiday decorations and stuff like that but I would like to keep the option open of turning it into a music room or something like that.Daniel Neuman
Oakland CA
Crazy Home Owner
Music room? You mean a piano?
You have been getting a lot of good advice about bouncy floors and joist depth.
I totally agree with this advice but another factor is where the floor is.
If the floor in question is your main living space or a frequently used area, vibration is very noticeable and it is important to take measures to prevent it. If the area is a bedroom, especially a child's bedroom or an infrequently used guest bedroom, vibration is less of a concern. Most people would not notice the difference.
Just something else to consider.
I've seen a dachshund bouncing things off the furniture in a house framed with floors at the L/360 spec. with tji's. Go with 2X8's and. put 1X4 pads (on the top plates) between the ex. joists to isolate the new floor from the ceiling below. Don't nail the 2X4s to the new joists, space them out, (will stop sound transmission). And hey Mad, the reason L/360 is used is CHEAP. Jim
I think that I am going to try to figure out what the 2by8's at 12"oc with a 12' span would be in L/? Everyone is pushing in that direction (I'm sure because its the right one) so I think it would be interesting to see what it designs out to. Daniel Neuman
Oakland CA
Crazy Home Owner
If headroom is very important, why not sandwich the existing 2x4's with 2x6's? That should get the same stiffness as single 2x8's by your calcs.BruceT
"If headroom is very important, why not sandwich the existing 2x4's with 2x6's? That should get the same stiffness as single 2x8's by your calcs."
Not at all.
Stiffness comes from adding DEPTH, not width. Adding 2X4s wouldn't likely do much of anything.
Apathy is becoming a major problem. But who cares?
I think Bruce is correct in what he is saying Boss. Stiffness is more easily (or efficiently) achieved by adding depth but look at the I values we are talking about:
Full sized 2x4 : I = about 10 in*4
2x6 : I = about 20 in*4
2x8 : I = just under 50 in*4
So for the assembly he is suggesting (a full sized 2x4 with one 2x6 each side) we have "I" total equal to about 50. Approximately the same as a single 2x8. Sure, it's a lot of wood but depending on what is important, (time, headroom etc.) and assuming the E values for the wood are sufficiently equal, I think it would be viable.
I don't follow your numbers, or what you mean by them.My understanding of what he meant was that he was just suggesting adding 2X4s alongside the existing joists. That wouldn't really accomplish anything significant.
Japanese scientists invented a camera with such a high shutter-speed, they can now photograph a woman with her mouth closed.
"why not sandwich the existing 2x4's with 2x6's?"
I think he is suggesting adding a 2x6 to each side of the existing full size 2x4's.
The resulting stiffness would be equal to the sum of the individual stiffnesses. The individual stiffnesses are a function of the moment of inertia "I" and the modulus of elasticity "E". I have indicated the I values for each of the components and assuming the E values are close to each other, the resulting stiffnesses should be about equal.
Not sure where this came from I am suggesting adding a single 2by6 to the existing 2by4's. Someone else suggested sandwiching the 2by4's with 2by6's and that would be as strong as a single 2by8.
Daniel Neuman
Oakland CA
Crazy Home Owner
Okay so by the same logic the single 2by6 sistered to the existing 2by4 should give me an I of...30 which is not as big as a single 2by8 but will give me a deflection of some measure better than L/360 which is supposedly what a 2by6 at 12 can do.
We need like a building exhibit. Have a stretch of L/360 floor then a stretch of L/480 and up...then I'd know if it would be acceptable to me.
Daniel Neuman
Oakland CA
Crazy Home Owner
Who said anything about adding 2x4's?BruceT
Daniel, much as I appreciate Boss Hog's expertise, in this case Houseboy is right--the stiffness of the floor really is as simple as adding up moment of inertia's. Well, to be more specific, multiply the moment of inertia of each member by its modulus of elasticity, then add those numbers up. Without running the numbers myself, I would guess that (2) 2x6's plus (1) old-growth 2x4 would be stiff enough for a 12' span.
Boss is right about never designing to 30# live loads. It is the absolute minimum acceptable by code, but storage areas can have a lot of weight up there and should be designed for at least 40#. I also usually design for L/480 deflection unless the span is very short, and also limit total deflection to 3/8" or 1/2" at the most, which often means L/720 for longer spans. Sorry to confuse you with all the numbers, but there is a science to this and many of us have spent a lot of time learning what all the formulas mean.
Mike Maines
Hi Mike,
Oh science doesn't scare me at all. I am the Madscientist after all.
You think the 2by6 sandwich would be somewhere in the ball park of L/480 for the 12' span? For a 12' span what ratio would you be shooting for??Daniel Neuman
Oakland CA
Crazy Home Owner
Before you get all excited about the help you are getting.
The floor is more than likely limited by the extreme fiber stress not the deflection.
Hire an engineer if you want an answer known to be correct.
(Vibration is not an issue in attic storage.)
Well yea as I said in my first post the answer to all these kinds of questions is 'hire an engineer' If I was honestly trying to use my T beam argument to get away with using a 2by6 then yes I think I would need an engineer.
But in this case it seems pretty straight forward doesn't it? Not caring about headroom 2by8's should do the job. Of course if you are saying that the free online spand calc are worth what I paid for them well....hmmm...Daniel Neuman
Oakland CA
Crazy Home Owner
Do you know what a 10 pound dead load means and a 30 pound live load means? If so and you say "I wnat to store ALOT of stuff up there" you might have partial answer to your own question.
I do know what the 30/10 loading means. It means the floor is designed to hold a 30 pound per square foot live load plus a 10 pound per square foot dead load.
If you are asking me to somehow calculate the loading of my holiday decorations man well-its all in bins and neatly stacked and it can be evenly distributed....
No, no piano, maybe a drumset and an electronic keyboard.Daniel Neuman
Oakland CA
Crazy Home Owner
Got it. Music room brings pictures of grand pianos to mind!
Why go for the 2x6, use 2x8's and plan ahead. After installing the 2x8's, away from the redwood, then remove the sheetrock,insulation and take ou the redwood for future projects, you will have a new ceiling and an attic that insulated right..