I'm starting the process of building a small house (about 430sqft plus loft) and while designing the framing in Google SketchUp, I realized that in my original plans I had given the loft a 3ft cantilever. I know a 3ft cantilever is possible, but I'm trying to figure out how to safely do it without compromising headroom too much, it's already about 5'3". The cantilevered section of the loft will not have heavy loads bearing down on it. The most it will hold is maybe 350lbs if myself and my wife are both sitting on the ledge. Although I'd of course want to build it to carry more than that. You never know what may end up on that ledge.

The loft is positioned above a bathroom and an office. I plan to run the floor joists into an exterior wall, supporting that wall and the roof above, to give the cantilever some weight at the other end, preventing uplift. The joists will extend over about a 6ft clear span and then be supported by the walls of the bathroom and office. Then cantilever out 3ft with no support posts beneath.

I've read that you can cantilever 1/3 the total length of the joist, but I bet this is factoring in 16" oc. Currently, I have it drawn up with 2x6s on 24" centers (the whole house is 24" oc staggered stud). The clear span is 5' 8.5" with a total joist length of 9' 7.5". According to the rule of 1/3 total length, I should be safe, but if that's factoring in 16"oc, maybe not so safe.

What size floor joists will this require? Should I keep it 2x6 and do 12" oc? (12" oc would line up with all my wall studs)

Here are some pics:
http://oi63.tinypic.com/66hvo4.jpg
http://oi67.tinypic.com/2u927eq.jpg

We need a drawing.

Section 500 of the 2015 IRC covers floor systems. There are span tables for cantilevers used in decks and balconies. The one for decks has 2X6@24OC, but the deck section(507.5) only allows 1/4 of back-span length cantilevers. Your 5'8 clear span(with an allowed 17" cantilever) is also close to the max 6-3 span allowed for a 2X6 at ANY spacing. A 2X8@24OC is good up to 9-1 clear span(plus a 25% cantilever)...

The section on balconies has longer length cantilevers at the size loft floor you are talking about, up to 42" I think(back-span to cantilever ratio must still be greater than 2:1) but it only goes down to 2X8, @ 12 and 16OC joist spacing. You also need to account for weight load on balcony(local code design criteria should have live and dead load numbers you must build to), and an appropriate uplift restraint on the opposite end of the joists...

Looks like at the very least you will have to up to 2X8 joists, and depending on the table used, possibly move that load bearing wall a little bit...

Good Luck

Posted By TWhite on 13 May 2017 01:55 PM
We need a drawing.

At the bottom of my first post I put 2 links to some pictures of the design concept. You may have missed them, because I didn't know how to put line breaks in the forum yet, so everything was kind of smushed together. I made them hyperlinks now. Sorry, I haven't had to type in HTML in about 10 years

Posted By ronmar on 17 May 2017 09:29 PM

Section 500 of the 2015 IRC covers floor systems. There are span tables for cantilevers used in decks and balconies. The one for decks has 2X6@24OC, but the deck section(507.5) only allows 1/4 of back-span length cantilevers. Your 5'8 clear span(with an allowed 17" cantilever) is also close to the max 6-3 span allowed for a 2X6 at ANY spacing. A 2X8@24OC is good up to 9-1 clear span(plus a 25% cantilever)...

The section on balconies has longer length cantilevers at the size loft floor you are talking about, up to 42" I think(back-span to cantilever ratio must still be greater than 2:1) but it only goes down to 2X8, @ 12 and 16OC joist spacing. You also need to account for weight load on balcony(local code design criteria should have live and dead load numbers you must build to), and an appropriate uplift restraint on the opposite end of the joists...

Looks like at the very least you will have to up to 2X8 joists, and depending on the table used, possibly move that load bearing wall a little bit...

Good Luck

Thank you very much for you reply. I forgot to mention that I have no codes, inspections or permitting where I am building. I don't say that because I intend to build anything below safety standards. I only mean that I can build whatever I want, so if I am able to build it to what is deemed to be safe within margin, even if it is disagreeable to what the building codes want.

Maybe you're looking at a different span table than I am, but the one I see shows that the maximum span for a 2x6 (#2 Douglas Fir) is 9-4. I think I found the the IRC section you're talking about and for my scenario it would be under table R502.3.3(1) this is also where I've been getting my span tables from. It seems that the determining factors for the length of the cantilever are the width of roof, depth of joists and spacing of joists. This is all fine and dandy, but the tables don't seem to play very nicely with small houses like mine. I'm not sure which direction it's talking about when it says "roof width", my place is 16.5' x 27' interior, with 7.25" thick walls. If you look at the images, the joists for the loft are spaced along the 16.5' section.

The table only goes down to 24'. I'm pretty sure I can't use the section on balconies, because I think that is describing a situation where the 2nd story floor joists continue through the wall to create an exterior balcony. That would mean the backspan on those joists would be pretty long and also have plenty of weight bearing down on them to prevent uplift and also has to factor in other loads such as snow and wind. Regardless, that isn't what I an trying to achieve, so I think I can disregard that section.

I may still be able to use 2x6 @ 12oc or like you said I may have to move up to 2x8 joists. I'd really like to keep from getting an engineer involved, as I have no idea what that may cost me.

Not having to build to code is nice, but you also can't really go wrong using the prescriptive tables in the code and building to your local design criteria(snow load, roof and floor live and dead loads, wind speeds and exposures ect). It will also probably get what you need without needing an engineer

Yea the floor cantilever section is confusing. The part of that section you referenced 502.3.3(1) is refering to an external cantilever that is supporting a wall and roof load ONLY(second floor larger than main floor) which is why they are interested in roof spans... The table that more appropriately applies to your situation IMO is the next table 502.3.3(2) for exterior balconies. You are basically making an internal balcony right?

In that table a 2X8@16 will alow for a 36" cantilever. At a 5-8 span and 3-0 cantilever, you are just under the minimum allowed 2:1 ratio. you could move the load bearing wall out a few inches to get a 6-0 span, which would get you a little larget loft(9-0), or shorten the cantilever to 34". The table just so happens to give you the uplift you need to account for where the backspan attaches(end opposite cantilever) at a 2:1 ratio for 3 different floor loads. At 30# PSF(would be surprised if your local floor design criteria is higher than that) you need to provide for 151# of uplift force on each joist. A Simpson H1, H2.5 or H3 Hurricane tie will do that and then some($0.42-$0.71 ea at Home Depot)... You will also need to use full depth blocking where the joists pass over that load bearing wall... This would probably give you the least compromise/change(2" thicker floor, small wall relocation) to the plan you described above...

Good luck.

I ended up increasing the size of both the office and bathroom by moving out the wall supporting the cantilever. This now gives me a cantilever of only 1-7 and a new clear span of 7-1 and total span of 9-7.25. That's a backspan of 7-8.75. Due to the increased clear span I lowed the ceiling height in the bathroom and office areas by 1.75" and switched to 2x8 joists 24" oc.

Think this should fix me up?

That looks like you are under the 4:1 deck cantilever so you should be fine, especially with a 2X8. Just make sure the backspan end is attached secure.

Good luck with your project:

That looks like you are under the 4:1 deck cantilever so you should be fine, especially with a 2X8. Just make sure the backspan end is attached secure.

Good luck with your project:

An upside down joist hanger would be a better connection for uplift resistence than a twistie.

You have no codes, the epitome of our disappearing freedom to do what you want. Someone will chirp in chastising me for suggesting this, but its the internet so I dont care, make a mock up on the ground with correct bearing points, lock down the back end using a car or something and bounce on the cantilever. Grab a partner and you both bounce on it and see if you can snap it. Bet you cant. Once you add the other members and the sheathing it will only get stiffer, not magical rock solid but load spreads more than you think. If your bounce test deflection is unsuitable to you, upsize or tighten your spacing or both if your paranoid.

Or...4x6 joists, leave exposed. Would look cool.

Greenpee, you're kidding right? Getting high & posting again?

"Grab a partner and you both bounce on it.."

Lets dance on structure to test it heck with code, how nice!

Follow ROMAR on this thread.

Nope.
No codes, we dont need your math.
It will work, no need to over analyze or follow codes cause you dont have to, you (op) get to use common sense, cherish that.

Ive got stuff on Lake Vermillion I built as a kid 25 years ago that is way sketchier than a 3' cantilever that would make you question your existence that is still supporting load. 3 story tree house, flat roof with a trap door. Full deck on 4th level, great views. You (para) would probably stroke out upon inspection, saying "but...how could this be...my modulas elasticity analysis based on six sigma principles says it should be lying in ruins."

Ive bore witness to turn of the century homes hacked up so bad you scratch your head about why a particular section hasnt collapsed, and its 115 years old, hacked up joists done maybe 85 years ago when indoor plumbing came to town. Again, so defiant of our codes yet, gasp, its still completing its function of supporting and distributing load.

Get the point? Do I want you (op) to build something, you and your bunny go up for a round on the cantilever made of 2x6 and the cantilever snaps off? No, but it wont, so I stand by the redneck deflection test based on experience.

I am helping my son remodel an 1840 cape cod in Boston. Post and beam. NE white pine over 4x6 joists four feet on center (maybe; maybe not). The floor was like a trampoline, but it's hard to argue with a building that's been around 180 years.

Posted By toddm on 29 May 2017 12:28 PM
I am helping my son remodel an 1840 cape cod in Boston. Post and beam. NE white pine over 4x6 joists four feet on center (maybe; maybe not). The floor was like a trampoline, but it's hard to argue with a building that's been around 180 years.

I know of a similar vintage house a few miles west of there (in Wayland, MA) built that way where a 9 year old girl jumping on the bed in an upstairs bedroom ended up crashing through onto the dining room table. (Fortunately the dining room floor held, and she didn't end up in the basement.)

Yes, it's possible for much lighter framing to still survive, but that doesn't make it a good idea for new construction. The average quality of the old-growth wood used in a lot of those early 19th century & older antiques is remarkably better than current standards for framing lumber too.

As if the building code span tables and overhang limits are just a hair over failure point.

In the context of what this guy is doing it's a nonissue, it won't fail unless he tried to make it fail on purpose.