I want to put up a couple of ceiling joists in my garage and lay some plywood over them to use for storage. The span is 20 feet. I have some 12 ft 2x8s left over from my remodel that I could use.
What is the best way to join two of these 12 footers to cover the span and strong enough to hold about 2 or 300 lbs? Would another 12 ft 2×8 nailed and glued across the span be enough? Or should I add an 8 foot piece of plywood in between?
And what is the best glue to use?
Thanks for any feedback,
Steve
Replies
2X8s spanning 20' will hold virtually nothing.
Sex takes up the least amount of time and causes the most amount of trouble.
hmmmm the 20 footers that have been up there for 30 years seem to be holding up just fine. I dont know how much wait I have put on them but at least 150 lbs or so.
What if I doubled up, used 4 12' 2x8s staggering the joints of course, glued and nailed. Would that be strong enough or should I sandwich with plywood?
Steve
" the 20 footers that have been up there for 30 years seem to be holding up just fine."
The fact that you've gotten away with it so far doesn't mean it's a good idea.Eagles may soar, but weasels don't get sucked into jet engines.
I really appreciatte all of the responses. I apologize if I appear to be beating a dead horse here but I'm trying to understand wood and its strengths and what can be done to make it stronger. So I appreciatte the whys and why nots. (I also am not trying to prove anyone right or wrong)
A lot of the responses appear to be directed to a single board spanning a length. That I understand. But how do you make it stronger? Make a beam? Can you make a beam from different lengths and materials laminated together?
From some of the responses it sounds like it's possible to make a beam this way. Bolting the pieces together is something else I considered. I'm not an engineer but laminated wood seems to be everywhere these days. I would think pieces of 2x along with 3/4 plywood glued/nailed/bolted together would be a lot stronger than a single 2x.
So again, it's not a matter of cost or alternatives I want to know how this works. Also, I have a head cold so if I still don't make sense just tell me to _ off.
Thanks,Steve
All the plywood does is overcome the non-existant strength in the joint where you butt the two together. The best glue is PL Poluurethene construction adhesive, IMO
It turns out that what you want to do is build a high shelf which is different than the original Q.
It may be the cold medicine that is making you thick headed but you asked for the right way to do something and then responed as though you haven't heard what you want to hear, so you are going to keep trying until someone tells you yes, Steve, it is perfectly acceptable to do this.
So it's clear that you have your heart set on doing it that way. Yes, Steve, it can be done that way, though ti is a waste of time and energy when you can do it better for similar cost and less effort.
The torsion box is the right way to do it. With plywood glued and screwed to the bottom as well as the top, the whole unit braces itself back to the wall.
Now take two aspirin, drink lots of fluids, get some rest, and look over the thread again in the AM with a new perspective on life.
Good luck.
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
My question has always been the same, it's sitting on top of the walls between the rafter ends which according to the Guide is called a ceiling joist (I never mentioned anything about a floor). I would have to imagine that since I am asking about putting up joists that it would be assumed this is stick framed not trusses. I suppose for added clarity I could call these Collar Ties but they are sitting on top of the walls. Yeah I know, my limited knowledge is getting me into trouble...
I did not do the original work but it passed inspection in the 70s. Stick Framed roof but there are only four 20 ft 2x8s running from wall to wall. Not only have I thought there should be a bit more support but I also have been looking for storage - both things mentioned in my first message.
Now we know that single 20 foot 2x8s are not good enough. So why even suggest I go buy more material? I suppose I could get 2x10s but they won't fit with the rest of the roof very well and would raise my 'shelf'.
But, wouldn't a custom built joist/beam/collar serve better than a single 20 foot 2x, even a 2x10? I am learning here so my understanding of wood is limited. The reason I am still pursuing this is that you turned it into a shelf but I still want to support the rafters.
I'm not sure my 'beam' idea is clear so hopefully the drawing comes through. Top view of 5 10ft 2x8s - one in the middle which overlaps two on each side by 5 ft. Span is 20 ft. Could be nailed/glued/bolted together. Each end will fit over the ends of the rafter and attached with bolts. Doesn't seem like a lot of unneccessary work to me unless I need to start adding more support.
WSA and UW are about to start so I'll see how bad you beat me up in a little while - LOL
Thanks,Steve
Not trying to beat you up.
I know you didn't use the word floor but by function that is what it is when you put plywood across it and then load it with stored items. A floor calls for 40#per sqFt live loaad rating.
The position is like that of ceiling joists but all they hold is SR and insulation and thirty pounds per square foot is more typical for them and you can expect a higher allowable deflection with them.
But since this will not be a continous system but only four feet oput from the gable end of the building with an open end, it is a shelf. Not much diff structurally between a shelf and a floor.
Now this is the first I remember hearing that you want to support the rafters too. Are they failing? Can't imagine why if the other ceiling joists are still doing OK.
OK - so here's my last shot - Only because I know you are going to use those 2x8 you already have. Here's a drawing of how you can build that thing as a torsion box. Maybe double up the outside joist and be sure the one on the wall is stuck really good before adding the horizontal plywood. Instead of jointing them all at ten feet in the center, you could stagger those joints by doing one at twelve and eight left and one at twelve and eight right.
.
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
Piffin, I know you aren't intentionally but not being a pro it's not too hard to get beat up in here...
Thanks for the drawing. That's makes a lot of sense. So you think plywood by itself would be enough?
Thanks,Steve
No I don't. But two slices of 3/4" glued to both sides is better than one 2x8 stitched to one side. The drawing you had up placed one splice piece between two split joists so the only strength was in the single sister at center. The other two half joists were adding nothing but weight. Now if you did like you drew for the outside one and lapped onto the rafter but then added the plywood sisters outside laps and staggerd the center joint, I might feel even more comfortable with it, provided you didn't give up on the torsion box too.
I still view this as a tremendous waste of time for you to use up a couple of leftover 2x8s tho' and it's not engineered, just better than you'd have done alone. .
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
Nice diagram! What program do you use to draw with?
Softplaan tho for these simple line drawings, I should be using my Turbo cad or Intellicad to get the practice..
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
Steve,
For your amusement, here's beam theory 101 (Boss can correct me on anything I screw up):
Bending strength is a function of the size, shape, and material of a beam. The maximum stresses occur at the extreme top and extreme bottom edges. Bend a pencil and look closely at it while it is bent into a curve. The fibers on the inner radius are "scrunched up", i.e. in compression, whereas the fibers on the outer radius are stretched, i.e. in tension. In the middle of the pencil, along it's centerline, is an imaginary axis where the forces shift from tension to compression. Along this imaginary "neutral" axis, there is no stress at all.
Also, the bending strength of a beam is directly proportionate to something called the moment of inertia, "I", where I = 1/12*b*h^3 (for rectangular cross-sections). For a 2x8: b=1.5", h=7.25"; therefore I = 1/12*1.5*7.25^3 or 47.6 in^4.
From the above paragraphs, it may be observed that an efficiently designed beam should have lots of depth (h). For example, doubling (i.e. sistering) the 2x8s would only yield I=95.3, whereas a single 2x10 would yield I=98.9. Therefore, the 2x10, with only 28% more material, is more than twice as strong (in bending) as the 2x8.
When designing efficient cross-sectional shapes, note that material near the beam's neutral axis is essentially "wasted", since stresses at the neutral axis are zero, and increase linearly as the extreme upper and lower surfaces are approached. Thus, steel sections are typically produced as I-beams. This shape puts the greatest proportion of material at the extreme edges, where it is needed most to resist bending. (As a side note, this is also the reason you should never notch the bottom of floor joists.)
The strength of solid lumber in bending is derived from the continuous grain fibers along the length of the beam. To illustrate, consider an 11.25" long piece of 1x1, and a 3/4" offcut from a 1x12. Both pieces are exactly the same size. However, if spanned across a gap, the 1x1 would support a fair amount of weight, whereas the offcut would almost immediately snap when bent under load.
Now, when you look at the prospect of scabbing together "short" pieces of lumber to make a beam, you can see some of the problems. There are no continuous fibers. You could, in fact, sandwich the butt splice between two splints and through-bolt, etc., but it's not cost-effective.
I don't know exactly how long you would have to make such a splint: I would guess it would have to be at least 4' long overall, if not 6' or 8'. There's probably a formula somewhere that will tell you the length of the splint as a function of the size of the beam, along with the number and size of fasteners required. Again, this would not be cost-effective for a short span where you could just buy a continuous joist, but if you had a long span and required some sort of truss.... maybe Boss Hog could comment on this (?).
Ragnar
Thanks Ragnar, looks like you beat me to my last post. After reading your post, I do recall reading something about that a long while ago. I guess that's why engineered trusts look like I beams?
So if I nail another 2x4 across the top I would have the same depth as... LOL just kidding.
Now, any suggestions on how to convert my car to a long truck to pick up these 20 footers?
Thanks again everyone,Steve
Oh Boy!
You're using twenty footers!
Just stick them in the rear driver's side window and let the excess stick out the rear passenger side window. Drive real fast so you can get it home before anyone notices. Take along your pilots license and watch out for bicycle riders..
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
LMAO!
Steve,
Your "joke" about nailing 2x4s across the top actually works. If you can get the flange (top part) completely secure to the web (vertical part), you get the beneficial performance of an I-beam section.
In my old engineering job, we did this all the time to stiffen temporary structures. I've never read study data, etc., to determine the ideal way of fastening the flanges to the web, but construction adhesive and framing nails worked fine for what we did.
***
I'm beginning to see at last what you're trying to do now. I had the impression you were going to lay plywood across all the ceiling joists and make a big storage area: that is a lot different than making a shelf at one end.
If you're not going to put on a whole lot of weight, you will probably be able to get by with the existing 2x8s and the contraptions you're talking about bolting together. Also, some blocking between the joists will help distribute the load a bit, and the plywood will help a lot with that, too.
If you want to get serious about the storage space (i.e a place you can actually walk on and load up to your heart's content), here's a suggestion: get enough 2x12s in 20' lengths to create the amount of storage area you want. According to span tables, 2x12s at 16" o.c. will span 20 feet. "Dog ear" the top outside corners of each joist so that it fits within your roofline (this won't affect the bending strength of the joists at midspan, where you need it). Face nail the joists to the rafters where possible. Sister 2x12s onto the existing 2x8s as need be. At the wall parallel to the joists, just put in a 2x4 nailed to the studs to act as a ledger. Then run your plywood and you'll have a nice strong area that you can load up and never worry about.
The final question: how do you get the 20-footers home? I've seen a situation where one guy was in the front passenger seat, and one guy in the rear passenger seat. They each had their right arm out of the open window and were holding the board! Hopefully, they didn't have far to go. <g>
Most lumber yards will deliver for $25 or so.
Hope this has been helpful,
Ragnar
"According to span tables, 2x12s at 16" OC will span 20 feet."
According to the book I use, The U.S. Span Book for Major Lumber Species (Canadian and U.S. Species) published by the Canadian Wood Council, 2001 edition
in Table 2.1..... Floor Joists...all rooms except sleeping rooms and attics...40 PSF Live and 10 PSF Dead....Maximum allowable spans for a 2X12 is as follows:
Using No1 grade and or No. 2 graded wood:( most commonly used for regular joist stock)
12 inch OC:
SPF......20ft-7inches Doug Fir-Larch...20ft-4 Hem-Fir...21ft-11
16 inch OC:
SPF.....17ft-10 inches Doug Fir-Larch...17ft-7 Hem-Fir...19ft-1
Moral of the story: according to this particular book, it would be safer to space your joists at 12 inch OC rather than 16".
Davo
Davo,
My table says 2x12s @ 16" o.c. will span 19.5' with #1 hemlock, 20.3' with #1 southern yellow pine, 20.8' with #1 Douglas fir. I know your source says something a little different -- I'm just citing this for a point of reference. Different tables say different things, and code seems to be constantly degrading allowable stresses in framing lumber.
On a slightly different topic, have you ever read what Boss Hog has written about floor vibration? He seems to say that low frequency vibration is the stuff that makes people "feel uncomfortable". Also, he seems to say that using fewer but deeper joists generally avoids the low frequency ranges. Anyway, I thought you might find it interesting.
Ragnar
Is new lumber weaker? My house is about 50 years old and built with 2x4s. Some rooms are about 18 feet or so wide. The roof is still holding up with a bit of sag the average person wouldn't notice.
My garage roof, added on in the 70s, was framed with 2x6 rafters and four 2x8 joists holding it together. Now we are saying 2x8s won't work and I need a 2x10 or maybe a 2x12. Which I may have trouble getting something like that in place. It seems like every so often the requirements go up.
Which brings up another question: what do you do in places where you can't bring in the larger lumber? Say I wanted to beef-up the roof inside the house, how would I get a 20 ft 2x12 up in the attic and position it in place? Seems to me you would have to piece something together? Or else tear the roof off.
Ragnar, what do you do in Seatte?
Steve
Steven,
I posted something at 7 of 27 not that it means anything but I can't tell you how many two car garages I've framed with 2x8 ceiling beams 22' long.
We put double rows of strongback 2x4 flat 2x8 on edge on with verticle 2x6's nailed into the side of the strongback and up into the rafters never had a problem, that's how they were all drawn and from all different architects.
Your not building a house with rooms and bathrooms on it you just want a simple shelf for some storage space, same thing as an attic.
In my eyes do like I said in my first post using your 10' 2x8's or go with 20' 2x8's they will work.
As far as your other question, if there was a serious problem and you had to get a long beam into the attic we've taken out a gable louver and brought big beams through there.
One time we took the gutter off and cut the fascia out in between two rafters and slid the beams through there.
So you have a little work to fix on the outside but at least you get your full beam in without doing in pieces.
Yes, some wood is weaker now, some is stronger. That's not the point. What was put in before was only intended as rafter ties and not for load bearing. What you are building now, you intend to load with weight.
You've got a few solid suggestions. pick one and start cutting!.
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
Piffin,
Do you really want me to cut those 20 footers? LOL! Hey, it's not often I get to pick your brains in here. But I really do appreciatte everyone's responses.
As far as the extra lumber thats been bugging me, I plan to take it back and get some 20 foot...2x2s. I'll post pics of the sag.
Thanks again everyone and have a great Holiday!
Steve
Hey Steve,
Piffin was right to point out that your "joists" were not intended to be used as such. But I was thinking your question was more of a general nature.
I'm going to go out on a limb here and say that new lumber IS weaker. For one thing, more and more framing lumber is spruce or hemlock, as opposed to Doug fir. It's my understanding that hemlock grows faster than fir, so that's why the tree farms are going with it. Also, the tree farmers want to get their product to market, so they cut the trees down after they're done with their growth spurt, and don't wait around for them to become "old growth". I'm not a tree farmer myself, so I can't tell you the exact particulars, but when I'm out hiking around and whatnot, I take an interest in trees that have been blown down in storms and cut clear from the trail. Based on my casual observation, it seems that fir takes about 50 years or so before the grain starts getting nice and tight. With tighter grain (less sapwood), comes greater strength, along with the fact that it's lighter and much less likely to split or blow out when you're machining it. Also, the older trees have huge trunks of clear wood.
Another culprit is declining standards in grading. More allowable knots, etc.
Finally, I think the Nanny State has a lot to do with the fact that code keeps requiring bigger and bigger framing. You know: some bureaucrat in DC always knows more about building than the folks who actually do it for a living. Funny thing, though: a number of years ago we had some heavy snow (well, heavy by Seattle standards), and it was the stuff built in the 70s and 80s that experienced major structural damage. The old houses from the 1910s and 20s, despite decades of neglect and despite the fact that they don't come close to meeting code, seemed to do just fine.
About what I do in Seattle: I'm a residential remodel contractor and specialize in houses from the 1910s and 1920s. Lots of my work is cosmetic stuff, like unbungling remodels from the 50s and 60s through today. I put in lots of period-appropriate trim and architectural details, traditional windows, leaded glass, etc. I've been doing this full time for about six or seven years now. Before that, I worked as a mechanical engineer.
depending on species and quality, 2x8@16"oc will only span about twelve feet for floor loading - 40#/sf live load plus 10# dead.
Joining two shorter ones is even weaker.
The ones you have? You saying 150# total? Over how much area? And you can say with a straight face that it has how much deflection???
To do it right, you would need a beam in the center so you are only spanning ten feet. What you are proposing might do something for you but It'll be hard to get someone to recomend it. You get strenghth by using deeper lumber or spacing it closer together, not by scabing scraps together. For instance, 2x12 #2 @ 12"oc will span about twenty feet for you.
Excellence is its own reward!
"The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are." --Marcus Aurelius
The Graphic Guide to Frame Construction has a chart that shows 2x8 joists spanning from 17.5 to 18.5 depending on species, 12 in on center. It doesn't say but I am assuming they are considering drywall nailed to this which is quite a bit of weight itself.
My span is actually 19.5 ft since my 20 ft is outside of the walls and the joists rest on top of the walls. I'm pushing it but maybe 30 years ago the wood was rated better.
I guess I really want to know if you can create a beam by overlapping 2x8s and sandwich plywood in between? And if so what is the best glue for this situation. I don't plan to do the entire garage, this is 4 feet at the back end with the back side nailed to the back wall. It will serve as a storage area and place to hang lights above my bench. I will probably want to cover the underside with drywall.
Anyway, I have about 8 12 ft 2x8s sitting here and thought I might have a use for them. But maybe what you are saying is that I could do this but only to store Christmas wrapping.
Thanks,Steve
I feel your pain. But I don't understand why kind of roof you have in your garage - truss? What holds it together? I can't visualize this.
In any event, the best glues would be resorcinol or epoxy.
From what I understand, you could overlap your two 12' pieces 4' to get one 20' piece. Use thru bolts in addition to the glue. As others have noted, a 2 X 8 that long is not up to serious floor loading standards for that length but you could tie it perhaps to the ridge piece. Or you could support it with some sort or bracket or brace since you mention installing it against the back wall.
A torsion box is another idea. You apply a solid layer of plywood to both the top and bottom of the joists. The connections between the individual pieces of ply must be strong enough to handle the compression/tension forces [top & bottom] respectively. This is sort of like a hollow-core door.
-Peter
"NotUsed is unused and must be zero." - Herbert Schildt
Steve,
I guess buying some 20' 2x8 is out of the question?
If you have to splice these together, never seen it done befor but you can glue and screw 3/4 plywood on each side of the splice and run 4 verticle 2x6's on each side of the joint bolted to the 2x8 up to the rafters.
Seems like alot of work than just getting 4 new 20' 2x8 with some strong back and some verticle pieces up to the rafters.
Hope this helps.
Joe Carola
Steve,
Perhaps you misread the chart cited. If you're looking at page 34 of Graphic Guide to Frame Construction, the typical allowable span for 2x8's @ 16" o.c. ranges from 12.5 to 13.3 feet (for a 40psf live load, 10psf dead load, and L/360 deflection).
The thing to remember about your garage is that those joists were obviously not intended to act as floor joists. Rather, they were designed to support a ceiling and hold the rafter tails together to keep the roof from splaying.
The 2x4 ceiling joists in the top of my house will support my weight, but it's not something I do all the time. I think you'll be alright if you use the area for storage of light items, but don't overdo it.
Your original question was how to splice scrap 2x8s together to make a new joist. I think the answer is: you don't. A joist should be continuous across its span. Can you return the 12-footers you have and exchange them? If not, save them for some other project. For about $0.75 per foot, go and get yourself some new ones if you need them.
Ragnar
Edited 11/23/2002 2:03:47 AM ET by ragnar
Ragnar, I don't have the same book as you but in two books I do have I think you have correct span for 2x8 under load as you described in the books I have looks like you can span from about 11 to 13.5 ft with what they so descriptively call "Strong Wood"
Think about it this way do you want to save a couple of bucks only to be working on something on your work bench one day and hear the cracking of lumber only to look up and see your storage coming back down at you, chances are it might not happen, but is your saftey not worth a little extra money to get the right materials
Just a thought here people , why not install some 9" Steel I Beams & use the joists you have left to span between them. Just keep the joists at the appropriate centres depending on how many steels you intro.
Not pretty but pretty effective