My son is going to extend an upper balcony, doubling its size. His design gives me pause and I’d like to check here to see if anyone can tell me (1) if it’s a bad idea, and (2) if it violates code in any way.
The current balcony is about 45 inches deep and is supported by floor joists thats extend away from the house. No supports underneath other than the joists. The joists are 2X10s, but rough (they’re basically a full 2 inches by almost a full 10 inches).
He’d like to make the deck deeper and I tried to talk him into sistering 2X10s onto the existing joists and putting a beam about 18 inches from the end of the joists, which would be somewhere between 8 and 9 feet long.
His plan is to put a 4X12 beam directly underneath the end of the current joists (they would end in the middle of the beam) and sister 2X10s to the existing joists so that the overhang (from the new beam) would be about 48 inches or so). He’d put 2 6X6 posts underneath to support the beam (which will be 18 feet 4 inches long.)
My thinking is that the overhang … 48 inches or so, with the joists exerting upward pressure when weight is on the outside … is too much and that the deck will either be too floppy or too dangerous. If he does this I’ve suggested we bolt the sistered joists to the current joists, in addition to nailing.
Note that the deck will be ipe (very heavy) and the railing will have 4X4 ipe posts with a 2X6 ipe railing. There will be metal fencing in between the ipe posts. So there will already be a lot of weight on the outboard end of the joists even before people go onto the deck.
With either design we’re proposing to put a new ledger on the house to support the house end of the sistered joists, as a precaution. It’s probably not needed, but I’m conservative.
Not being a framer by trade I’d like the considered input from the experts. Thanks in advance for sharing.
John
Replies
What he has now is a poor design to begin with because the joists penetrate the skin of the house and lead water in and air out no matter how tight he might believe it to be.
The general rule of thumb is that for every foot of cantilever out, the joist shoulld be counter balanced by two feet back of the beam. So if he places a beam at 48" from the house, the deckcould be six feet wide.
BTW,depending on placement and whether there is other points of attachment to suitable spot on the house, the span of the beam is too great unless he wants a trampoline.
Of course, whether he lives in a snow area affects what load a deck can be epected to handle too.
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Piffin,
The house was built in the 1930s (my best guess) and the deck looks original. Near as I can tell, there has never been a leak ... but your observation is correct, IMO.
I was mistaken when I said the beam in his design would be a 4X12. It will be a 6X12 (the posts are 6X6s and that would look weird with a 4X beam). The beam will be 18 feet, and if the posts are 5 feet in from the ends, that would leave 8 feet between the posts. 4 feet in from the ends leaves about 10 feet between the posts. Is that too much span? What would be correct for two posts? Are three required?
I'll start by saying I'm not a framer, but my recollection is that for a 4 by beam and 4 by supports, if the supports are x inches apart, the beam needs to be x inches wide (high). So, if this were 4X posts and a 4X beam, I think a 4X12 would be sufficient with two posts. Am I mistaken? For my design I've chosen to recommend 3 posts, since it's 4X lumber, both for looks and for more strength. I think it's overly conservative, but I'd rather over-engineer than under-engineer.
John
That post placememt sounds fine. My earlier comments were based on not knowing if the posts were to be under the ends of the beam which would then be spaning close to the full 18'BTW, it was totally clear to me that it was the BEAM and not the posts that was 18'On the cantilever thing counting for the extend back into the building...maybe. You are assuming that your attachment connection sistering these would be as strong as the wood fibre in both compression and tension
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> His plan is to put a 4X12 beam directly underneath the end of the
> current joists (they would end in the middle of the beam) and
> sister 2X10s to the existing joists so that the overhang (from
> the new beam) would be about 48 inches or so). He'd put 2 6X6
> posts underneath to support the beam (which will be 18 feet 4
> inches long.)
Sounds solid to me. I think it will be pretty ugly. He will have to run some sort of cross-bracing between those 18' posts to keep them from bowing in the middle. I would lean towards old-fashioned buttresses to the wall, but those are hard to do with dimensional lumber.
> My thinking is that the overhang ... 48 inches or so, with the
> joists exerting upward pressure when weight is on the outside ...
> is too much and that the deck will either be too floppy or too
> dangerous. If he does this I've suggested we bolt the sistered
> joists to the current joists, in addition to nailing.
I think you're wrong about it being too floppy or dangerous, but I support your idea to bolt things together. Run the bolts in the center area of the timbers, where there are no compression stresses.
> Note that the deck will be ipe (very heavy) and the railing will
> have 4X4 ipe posts with a 2X6 ipe railing. There will be metal
> fencing in between the ipe posts. So there will already be a lot
> of weight on the outboard end of the joists even before people go
> onto the deck.
Maybe 8x8 supports would be a better idea for an 18' high deck. I don't have my tables handy.
> With either design we're proposing to put a new ledger on the
> house to support the house end of the sistered joists, as a
> precaution. It's probably not needed, but I'm conservative.
I agree that it's probably not needed, and I can't see how it's going to work with the existing joist ends protruding from the exterior wall. Is your new ledger going to be several chunks 14.5" longs between the joists? I'm not sure that would do much good.
I guess I wasn't clear on several points.
1. The deck is now 18 feet wide by almost 4 feet deep. It is roughly 8 plus feet from the concrete decking below.
2. The current joists which support the deck run through the house and are at least 20 feet long.
My son's proposal to attach (sister) new joists to the current joists might be seen as logical extensions of the current joists, and since they are probably 16 feet long inside the house, having them be extended 4 feet past the beam (which will be 18 feet 2 inches long) they "meet" the one third rule quoted earlier.
I do agree that it will be somewhat unsightly, but his view (I haven't been on site to look at things and do a mock up) is that the design which uses posts clost to the house will look better.
I'll have him check the local codes, since if an overhang of more than 2 feet requires engineering in at least one locale it very well could at his location. And building it and then having to redo it isn't something either of us wants.
Any more input?
John
A 6x6 will be fine for an 8' tall support. IIRC, there's no need for cross-bracing on that, either.George Patterson
George,
Thanks for the input on cross bracing for 6X6 posts. If we go with the other design, with 4X4 posts, will those require bracing? I actually think the design will be nicer with bracing, and we'd probably put it in anyway ... but I'm curious nevertheless.
John
Well, I misread the height (as someone pointed out). With an 8' height and an open deck, decent 4x4 posts are ok without bracing. Add a roof, and bracing becomes a necessity.Personally, I like the effect of curved braces, as you see sometimes in older porches. They're pretty labor-intensive, though.George Patterson
My son's proposal to attach (sister) new joists to the current joists might be seen as logical extensions of the current joists...
This is free advice from a non-PE... but here goes:
Assuming 40 psf live plus 10 psf dead load, the total weight on the "new" cantilevered deck is 4' x 18' x 50 psf = 3600 lbs.
Assuming 2x10 joists at 16" oc, the load on each joist is 267 pounds. This puts a shear of 19 psi on each joist, which is much less than the 85 psi allowable shear I see in my tables for hem-fir.
Assuming E=1.3E6 psi for #2 hem-fir (and using a simplified load analysis), I calculate the deflection at the end of each 2x10 joist to be only 0.03". Due to the cantilever effect, I think the real world deflection will be greater than that.
The maximum bending stress I calculate is 299 psi, which is much less than the typical allowable Fb of 1555 psi for #2 hem-fir.
Note that I didn't analyze the supporting beam, but otherwise, the above numbers look fine. A big question is -- as Piffin remarked --- whether the sistering will be an effective load path. We did a similar "scabbing" of some interior joists one time, although it was not a cantilever (just a simple "extension" to the next bearing wall, but there are many similarities). If I recall correctly, the engineer called for two rows of 16d nails (one near the top edge of the joist, the other row near the bottom edge) at about 12" centers. We also had much more contact length; you've got 4 feet of contact for 4 feet of cantilever. We had more like 10 feet of contact and an "extension" of 2 feet to the next bearing wall.
I'm not sure on this, but my guess is that we should treat the placement of the new beam as critical in interpreting the cantilever arrangement. As your son proposes it, I see a 1:1 ratio. I'd feel more comfortable putting the new beam about 32" from the end of the 8-foot-deep deck. This is in compliance with the 1:2 rule of thumb for cantilevers (32" + 64" = 96").
Again, this is only a free consultation, but hopefully it will help the discussion.
Edited 8/15/2007 2:59 pm ET by Ragnar17
Thanks to everybody for their input.
I've convinced him to put the beam at the 8 foot mark, so the cantilever will be minimal ... probably 12 to 18 inches at most. To make things look better, we'll be sistering two joists per each current 4 foot extended joist ... one on each side. We'll also put a ledger on the house below the joists and this should also help with things.
The big question I have now is how in the world we're going to get a 6X12 by 20 foot green beam from the street into the back yard (he's on a hill in Berkeley and the side of the house drops two stories over about 30 feet ... quite steep. And then there's the small matter of getting that mother of a beam up and onto the two 6X6 posts.
Life is never easy ... :)
John
(My guess is we'll simply drag the beam down the side steps, staying away from its path in case it decides to go on its own. But that only gets it down. Up onto the posts has me truly stumped. I guess we could throw a BBQ for 20 Breaktimers and have a go at it ...)
John,
I'm a little late to the fray here, but I''l try to add something worthwhile,
As to the "double sistered joist" , I would just do the regular sistering then put blocking in between the new sister joists, nailed into the ends of the existing cantilevered joists. This will help stabilize the ends of the cantilevers and tie the sister joists together as well, making for a more stable connection structure. The blocking should all be through-nailed into the ends of the cantilevered joists and through-nailed through each new joist, no toe nails.
second, I would not do a ledger, your only making for a new entry point for moisture with no real structural gain to the assembly, since the bearing point(s) is/are the ext. wall and the new added beam (with only an 18" cantilever) the ledger will not offer any support to your extension.
Geoff
Calcs aside, the 1:2 & 1:3 thumbs for cantilevers are only half of the equation. One has to factor in the size of the joist also- just assuming extension could lead someone to think they could, for example, extend their 2x8 floor joists out 12' so long as they had 24 or 36' running into the house.This poor logic led to a lot of poorly built decks throughout the 50s, 60s, 70s, even into the 80s. The wood will support it for a time, but you have to calculate the bending moment, stress points & paths into the equation.The "rule" I generally go by is 3 x the height of the joist- ie; a 8" joist can cantilever out 24", a 12" can go 36". It's a bit conservative, but is much more reliable.
Calcs aside, the 1:2 & 1:3 thumbs for cantilevers are only half of the equation.
You're right, of course, in pointing out that one can't simply ignore joist depth in determining a cantilever limit. Nor can one reasonably extrapolate the 1:2 rule of thumb to design a 20' cantilevered balcony. ;)
But, according to the rough calcs I showed in my previous post, it seems his proposed 4-foot cantilever worked fine.
Do you have some calcs to show otherwise? On the surface, a four-foot cantilever on 2x10s doesn't seem to be unreasonable.
I'm not picking a fight -- just trying to learn to do things right and see if I was missing something in my previous post.
Don't know about your code, but our's specifies anything over 2'-0" cantilever requires an engineer.
I'm not a framing expert, but I did recently extend my balcony quite a bit and I used Trex which is also very heavy. The existing cantilevered joists extended about 4' from the house (and about 10' wide). I extended the balcony to about 10'6" from the 4'. I did this by sistering the joists (2 x 10) to the existing joists using P/T joists and galvanized lag bolts to attach them together. From there I notched the end of the balcony to allow for a 2 x 10 "bottom plate", and then another 2 x 10 on the very outside to act as a end plate. I supported this increase using polycast fiberglass columns. These columns hold like 6,000 or 7,000 each, they are easy to work with, and can be painted any color.
What I did above worked extremely well, and this thing doesn't move at all. I also adding bridging between the joists to controls movement, etc. and this also worked very well. I staggered the bridging in various spots throughout the framing. You can jump up and down on the balcony and it won't budge at all. It's very solid and this is with very heavy material (trex).
The one thing I didn't do and I wish I did was add some type of transparent screening in between the joists and deck boards to control things like hornets, bugs, in case you drop something, etc. I would have cost me about $30 + 1 hr of time and this would have saved a lot of problems later on. I didn't do this, but I might consider some type of flashing on top of the joists to further protect them from rot. I think I saw a similar product in one of the FHB magazine ads for this product. It's relatively cheap, and it's likely overkill if you use P/T lumber, but you might want to consider it.
If you want pictures, or anything else, please let me know. Good luck!
Either your description or your deck appears to be missing a beam.
F.A.T.'s , no offense meant, but I couldn't resist the acronym :),
I think you missed the part about the two columns supporting the deck AND the use of a "bottom plate" notched into the bottom ends of the sister joists, w/ a rim joist also, this being his "beam"
Geoff
Well the rim joists alway there, so the addition of the notched-in plate is the beam?
I'm still puzzled.
No offense taked at the acronym. Not fat, just fat-headed.
In the mind of the poster,yes, that "plate" is a beam, not in my mind though. Brian points out the flaws with the posters design, I was just clerifying what I thought the poster was saying, although I don't condone the method.
Geoff
Geoff..... you're wrong buddy. That poster's deck does not have a beam and his butchering of the nomenclature of various framing components does nothing to support the argument. His rim joist is just that..... a rim joist. And his "bottom plate" has actually weakened the framing system by notching the bottom of the joists.
Additionally, the size of the columns doesn't really matter if the beam is undersized. A rim joist could technically be considered a beam in some situations. However even a flush header in a floor system (for a stair opening or something similar) almost always needs to be at least a doubler even if it's only carrying two or three joists. I suspect that the posters deck has more than two or three joists in it..... I believe that's a safe assumption. Therefore, he has a single 2x10 rim/beam carrying however many joists are between columns. An undersized beam sags..... and will always sag regardless of what size columns you put under it.
DIY'ers tend to get very defensive about their concoctions so I don't expect this post to be received very graciously. However I'd be doing a disservice to the forum by cosigning that kind of seat-of-the pants engineering and letting another reader assume that what's been described is an acceptable framing practice. Just because it passes someone's "I can jump up and down on it" test doesn't mean a thing in the real world. If I tried selling that design or that "test" to an inspector I'd never be taken seriously again.
View Image
Edited 8/18/2007 7:38 am ET by dieselpig
Totally agree on all points, I wasn't really condoning the method or technique described, just clarifying to "fingersandtoes" that the OP thought he had a beam with the concoction he had built. My bad for encouraging the OP.
Geoff
Sorry, not the OP, but "Grandchat" was the poster.
Edited 8/18/2007 9:21 am ET by Geoffrey
"DIY'ers tend to get very defensive about their concoctions so I don't expect this post to be received very graciously. However I'd be doing a disservice to the forum by cosigning that kind of seat-of-the pants engineering and letting another reader assume that what's been described is an acceptable framing practice."
I know where you are coming from, but the tone of his post was nothing but helpful. I think it's a bit early to label him a defensive DIYer.
LOL....... like I said.......View Image
You have been around here a lot longer than me. I defer to your experience in these matters.
Here are some pictures of the balcony:
Hi
I'm redoing a similar project and it's my first time although i have experience with wood framing.
The balcony is 4 feet x 14 feet. It's on the third floor of a triplex.
It was covered with metal sheet and underneath the wood board were completely rotten. I remove everything up to the joists (coming out of the brick wall) They don't look bad. the end plate though has to be changed.
I'm going to change the handrail + the three posts (4x4) attached to the little roof above the balcony.
I want to have sistering 2x12 joists lagbolted to the existing joists, new end plate. And then i want to lay down plywood. Then i'm not sure.
I want to cover it with an elastomerre membrane and then wood board for the finish. But i don't know howto proceed yet.
I would be glad if you could send me some pictures of the construction of your balcony, it would for sure help me a lot!
thanks
Eric
[email protected]
I don't think I saw anything about footings...what is holding up your
posts? I am sure that someone in here can size them for you. 12x12x12
seem to work for most decks, depending on the soil. There should be
some positive attachment to the posts too, like post bases. Good luck
with the beam! If you can get the beam to the bottom of the posts,
sometimes I will stabilize the posts and then make a 'ladder' with
blocks nailed to the side of the posts about every 2' vertically.
Then you lift one end, then the other, up the 'ladder' until you are
at the top. This way it can only fall 2', not all the way off of a
ladder. Hope that was clear enough, if not give me a holler and I
will try to clarify it. Rob Z