Options for breaking the 3:1 cantilever rule when cost is less of an object


Looking at options for adding 3 rear balconies to a brownstone co-op on the upper west side of Manhattan.    Joists unfortunately run perpendicular to a cantilever so I am curious about engineered solutions that would reduce the back span requirement.   The building was rebuilt in 1980, current joists are 20' spans (the building is 20'W x 50'D) 2 x 12s.   The 5th joist in from the rear wall is doubled...located about 7' inside th rear would seem a shame to have to cut back and re frame beyond this.    If there is a way to engineer something that would allow a 7' backspan terminating at this double joist to support say a 4' (3.5'? balcony) the double joist seems to be the sensible place to stop cutting.

Outdoor space directly attached to apartments in this neighborhood add significantly to what are already outrageous prices in this zip code.    Money is always a factor but in this case getting the job done, and done right is more of a concern than just the $$.   The design/engineering and permit process would be amortized over 3 balconies and as mentioned before would add significantly to the value of the apts they are attached to as private outdoor space trades at a premium here  in NYC.

A cantilever seems most elegant and is the first option I would like to explore.  I'm looking for pointers and advice that will help me understand what is and isnt possible before I start talking to engineers and architects.     Ive seen hints that "exotic materials or engineering" can reduce the 3:1 cantilever requirement... but I haven't found any detail on these options.

Another option might be a metal brace underneath the balconies.. but clearly a clean cantilever would be the most elegant... if the building is willing to spend some $$ what options might be available and where might I learn more?

Thanks for any help the community can provide...


Jul 10, 14 3:01 pm

You should really contract with an experience consultant to figure this out. 


Without seeing a drawing.... it is hard to say, but it sounds feasible. Send me $125 and I will tell you what strategies I would present to my consulting engineer for consideration.

Jul 10, 14 4:12 pm
A cantilever might be more elegant if the building is already Modern in style. If the building is historic - upper west side brownstone? yeah - a cantilever likely will look odd.

IMO an angled brace is the more Modern solution, anyway, as it's a smarter use of materials than difficult retrofitting to create a cantilever under less-than-most-efficient conditions.
Jul 10, 14 4:41 pm

Consulting starts @ $250 / hr. plus expenses.

Jul 10, 14 7:06 pm

Too expensive for the modern client - I'll move your period to correct - "Consulting starts @ $2.50 / hr plus expenses.


For $2.50 I'll do $2.50 of work.


I think that comment was $2.50 of work, so time to go home : P


agree with donna...

why would you go through the headache of digging into a bunch of existing floor structure, utilities, and who knows what the hell else, when you could just design building face-mounted solution, epoxy that brace/balcony right into the wall of the building and be done with it.  get a structural engineer to run calcs.

Jul 10, 14 7:21 pm

Another suggestion if cost isn't a factor...

Tear down the rear facade (assuming it's not a landmarked building); add a moment frame that ties into the bearing walls (to prevent overturning); weld steel sections to the moment frame and run joists perpendicular. I would even go as far as suggesting new foundation(s) for the moment frame. This is a a bit extreme, but I think it works...

Jul 10, 14 8:13 pm
Non Sequitur

Has anyone suggested sky-hooks?

Jul 10, 14 10:50 pm

helium balloons work too!

Unlimited budget? Hold it up with stacks of money. 

Jul 10, 14 11:10 pm

demo the entire floor, backspan 2 huge steel beams to the opposite end of the building, run new columns to a new underpinned foundation on both ends, rebuild the floor.

Jul 10, 14 11:23 pm

I cheat.. Don't bother with the cantilever.  For smaller decks: Create a beam out of the rail system basically you end up with a 36-42" deep beam to work with, so it really comes down the attachment to the building being able to resist the lateral forces.  The floor of the deck is hung at the bottom of your beams.  You can even prefab it and hang it by crane.

You can also use tension steel rod or cable and hang it from above. (think upside down gusset). If you want it to look lighter.  This looks more like a tension cable from the parapet to the rim joist like a swing.

With both, you'll need to reinforce the internal building framing to resist lateral loads and pullout of fasteners.  But usually that is adding bracing to the existing, not replacement.  Like blocking those parallel joist back that 2/3 the deck to create a shear diaphragm.

Jul 11, 14 2:51 pm
Erik Evens (EKE)

Some tube steel beams should easily be able to cantilever 3-1/2 feet with. 7' back span.

Jul 11, 14 3:19 pm

just use gorilla glue.  put your money into the lawsuits instead of the decks.

Jul 11, 14 3:41 pm


Jul 14, 14 10:43 pm

^^that's some ugly ass balcony

Jul 14, 14 11:05 pm

that's all he's getting from me as far as free advice

Jul 15, 14 1:40 am

I always rely on Sky Hooks.

Jul 15, 14 7:13 am

LOL I love that balcony. So honest.

Miles your comment about stacks of money is still cracking me up days later.

Jul 15, 14 10:42 am

And stop replying to 4 year old threads like you're part of the conversation ;)


Prettier?  It just uses a gusset to direct the forces back to the wall...  Antoni Gaudi

Jul 15, 14 12:04 pm
go do it

Just pop out some 2x12's and deck that bad boy!!

Here is the trick... you turn the joist hangers upside down.

Jul 15, 14 1:57 pm

Pool balconies are what you want. 

Water Balcony

Jul 15, 14 2:31 pm

You can preserve the integrity of the building structure with a bolt on balcony system. Here's one where the architect opted for a color scheme to match the retrofit of the building, with a streamlined joist free balcony look.

Nov 10, 17 2:39 pm

Look at some of the balconies in the classic houses of New Orleans and Charleston, SC. Some are cantilevered, most are not. Seems you would be opening up a very large can of worms to go the cantilever route.

Nov 10, 17 4:34 pm

When in doubt, air it out.

Nov 10, 17 6:51 pm

-- Louis C.K.


Using thin iron poles at ground level tend give the appearance of a cantilever especially when contrasted with the wider filigreed columns on the second floor.

May 25, 18 3:18 pm

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