View Full Version : How far can a deck cantilever?

Erby Crofutt
04-19-2009, 07:00 PM
Kinda bouncy out on the end!

6' cantilever

Ted Menelly
04-19-2009, 07:16 PM
Kinda bouncy out on the end!

6' cantilever

Obviously all depends on the size of the lumber and that particular deck looks about maxed out. I can imagine a half dozen people out there bouncing around. As long as the limber is in good shape I doubt it will go anywhere but i would not want to be the one to test it. I can also imagine the movement that must be felt on the floor inside when the end of the deck is being bounced on.

Billy Stephens
04-19-2009, 08:00 PM
Cantilever Beam Loading Options (http://www.efunda.com/formulae/solid_mechanics/beams/casestudy_bc_cantilever.cfm)

Nick Ostrowski
04-19-2009, 08:46 PM
Looks like that railing post is already pulling away from the house.

Matt Fellman
04-19-2009, 10:25 PM
I think it's techincally a balcony if the outer edge doesn't bear on the ground.

As for how far it can stick out from the house? That's a bit of a complex question and one you can't answer without seeing or knowing how far the joists extend into the house. Most cantilevers have a 3:1 ratio. The most common example of this you will find is a deck that attaches to a house that is 12 feet perpendicularly away from the house. The outer support beam must be no closer to the house than 9 feet (or, no more than 3 feet can hang over the outer beam unsupported).

Applying that to the deck/balcony in question means the joists would have to exend 18 feet into the house for a total of 24 feet.... which seems unlikely. This is where the complexity and engineering comes in. It's possible and common to design cantilevers with less inside support but it goes way beyond what we do.

If it feels 'bouncy' I'd mention it document that you can't verify the design adequacy from what you can see.

Also, be very carful to look for spliced joists on cantilevered balconies/decks. It's very common for a knucklehead repair guy to come in and replace a rotted joist with a small scab of lumber.

IMO... once a cantilevered joist(s) rot out or is otherwise cut a balcony must be turned into a deck (posts installed at the outer edge). It's possible to repair the cantilever but it's much more complex and expensive than installing a couple posts and a beam at the outler edge.

Ken Bates
04-19-2009, 11:07 PM
Matt Fellman posited a 3:1 ratio that sounds reasonable unless the cantilevers are 2x3's.

I don't spend my time checking if the dimensions fall within various engineering and code formulae.

I check for deterioration if I can.

Russel Ray
04-19-2009, 11:45 PM
Does look like a balcony. Terminology aside, how far can a house cantilever?

Ted Menelly
04-20-2009, 02:34 AM
Does look like a balcony. Terminology aside, how far can a house cantilever?

Thats a serious pic you have there Russel. Must be some costly Real Estate and they are get every square foot possible.

Jerry Peck
04-20-2009, 08:21 AM
Does look like a balcony. Terminology aside, how far can a house cantilever?

However far the engineer and architect designs it to cantilever.

One 'could' sink a specially designed foundation into the earth/rock and with steel/concrete beam cantilever it out quit a bit, basically the entire house could be cantilevered out.

Not sure I would want to live cantilevered out over the edge of the mountain, but I am sure it is possible and probably has been done.

Jerry Peck
04-20-2009, 08:37 AM
Getting back to the question ...

As I recall (without looking it up) the amount of cantilever is 1/3 of the span rating.

Not 1/3 of the length as implied in some posts above.

Let's say a certain size joist is allowed to span 15 feet on two bearing points (no center bearing) placed on 16" centers. With that presumption (and without specifying the joist size, just using this as an example) if you were to get those joists 20 feet long, one end bearing 15 feet inside, another bearing at the exterior wall, then the cantilever could be 5 feet out.

But lets take the above to the 6 foot cantilever Erby found, that would mean the span rating would need to be 18 feet, and the length 24 feet, which means that a #1 Southern Pine 2x12 could be used, or, if you could get it you could use SS Southern Pine 2x10. Both on 16" centers.

Go to 24" centers and your options drop considerably: SS Souther Pine 2x12, or SS Hem-Fir 2x12, or SS Douglas fir-larch 2x12, but all would have to be SS grade and all would have to be 2x12.

Above from Table R502.3.1(1) using Dead Load = 20 psf with a Live Load of 30 psf.

Jim Luttrall
04-20-2009, 08:38 AM
Russel, that looks like the "before" picture in a disaster film. You know when the earthquake, storm or bomb makes the house go tumbling down the hill.
I'm with Jerry, not sure I would want to live in a house like that.

Steve Frederickson
04-20-2009, 01:27 PM
Like they say..it depends. :)

Kirk Hersee
04-20-2009, 03:09 PM
I think it's techincally a balcony if the outer edge doesn't bear on the ground.

Not that this is the answer, but in the insurance inspection business, this is a balcony being that there are no posts supporting it from the ground.

Jerry Peck
04-20-2009, 03:16 PM
From the 2006 IRC.

BALCONY, EXTERIOR. An exterior floor projecting from and supported by a structure without additional independent supports.

DECK. An exterior floor system supported on at least two opposing sides by an adjoining structure and/or posts, piers, or other independent supports.

Mike Truss Guy
04-20-2009, 03:26 PM
The cantilevers at Fallingwater

Fallingwater's structural system includes a series of bold reinforced concrete cantilevered balconies; however, the house had problems from the beginning. Pronounced sagging of the concrete cantilevers was noticed as soon as formwork was removed at the construction stage.
The strong horizontal and vertical lines are a distinctive feature of Fallingwater.

The Western Pennsylvania Conservancy conducted an intensive program to preserve and restore Fallingwater. The structural work under supervision of Louis D. Astorino was completed in 2002. This involved a detailed study of the original design documents, observing and modeling the structure's behavior, then developing and implementing a repair plan.

The study indicated that the original structural design and plan preparation had been rushed and the cantilevers had significantly inadequate reinforcement. As originally designed by Wright, the cantilevers would not have held their own weight'.

The 2002 repair scheme involved temporarily supporting the structure; careful, selective, removal of the floor; post-tensioning the cantilevers underneath the floor; then restoring the finished floor.

Fallingwater - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Fallingwater#Structural_problems)http://farm1.static.flickr.com/149/331253588_d807508ff4.jpg?v=0

In other words...even if you think you are as smart as Frank Lloyd Wright...if you're going to cantilever a balcony...have somebody check your calculations prior to construction. :D

Cobra Cook
04-21-2009, 09:01 AM
I believe that most respondents here are going beyond the scope of a home inspection. The inspector’s job is not to design or redesign a home or building, it is to report what we see. If you can not see that what you are not looking at is unsafe i.e.: rotten, loose, ect. And, put that into your report then simply write that a professional be consulted who can make a better judgment of it. When home inspectors start telling customers how to redesign something out of the ordinary that’s when your E&O insurance goes to the moonwhen your design fails. Cobra

Mike Schulz
04-21-2009, 02:20 PM

Your comment is quite common for new people on the board. Even though it is above what we need to report it's for our own unquenchable thirst for knowledge.;)