What is a Cantilever and Why would Your Deck want one?

Cantilevers come to us from bridge architecture, where engineers have to span wide rivers with few supports.  We can apply their cantilever¹ engineering to deck construction to good advantage.

Say you are designing a large deck and want to use materials efficiently.  Standard deck structure is simple:  extend the joists from the house out to a girder at the front edge.

You can enlarge this deck by moving the girder further out, but only to the limits the joists can span  — to 12′ 10″ for 2×8 joists 16″ on center or to 14′ 2″ if you place the joists 12″ on center.²   More?  You want more?  Then spend the money to upgrade all the joists to 2×10.  Or you can use some clever engineering:  cantilever the joists.

Instead of a girder at the end of the joists, you’ll need to put a beam under the joists, so they can extend past that support.  This framing style is called post and beam, and the overhang is a cantilever.  Now your deck can extend 17′ out:  a 14′ span from house to beam plus a 3′ cantilever.³  Nice.  For a thorough explanation of post and beam deck construction and comparisons with alternatives, see my article “What is Post and Beam and Why would your Deck Care?”.

The overhang by itself seems unsupported.  Is it solid?  Would you walk on it?  It is safe — if you build it properly.  There are different forces at work here, and you must follow some special rules.

1. The cantilever length.  How far out a cantilever can safely overhang is determined by the strength of the wood – the species, grade, and size – and by the spacing of the joists.  Here we show #2, 2×8 Pressure Treated Southern Yellow Pine (SYP PT) joists, spaced 12″ on center to support our 3′ cantilever.  The Building Code (IRC 2009 or IRC 2012) allows a 40 ½” cantilever for a live load of 40 pounds per square foot (psf);  your engineer may allow more.  Upgrade to 2×10 joists, and the Code says you can cantilever almost 5 feet.³
2. Backspan. In addition to the limit on a cantilever’s overhang, a deck must also be proportioned to balance weight on its cantilever. The backspan of each joist – the distance from the beam back to the house ledger – must be sized properly.  The Building Code requires a deck’s backspan be at least twice the cantilever extension. (footnote 4)   This 2:1 ratio is liberal;  I prefer a more conservative (safer) 3:1.  (For more on backspan standards, see Note B, below.)
3. Ledger attachment. A downward weight on the deck cantilever causes an upward thrust on the joist (the beam acts as the fulcrum of this lever).  This upload must be resisted at the other end of the joist, where it attaches to the house with a joist hanger.  In our example, a full design load on the cantilever causes an upload of 148 lbs on each joist.³  A Simpson LUS 2×8 hanger, properly fastened to a PT ledger, can resist uploads up to 1,335 lbs. (footnote 5)

If this engineering seems complicated, consider the alternative.  To extend a deck 17’ from a house with traditional girder construction, you’d need to upgrade all the joists and the ledger from 2x8s to 2x10s.²  This brute force approach would add approximately $4 more per joist;  for a deck 16ft wide (including the 17 joists, ledger, rim joist, blocking, and joist hangers at the girder) that amounts to an additional $100.

Cantilevers do not save money on every deck.  You can build 14’ out using 2×8 joists on a simple girder frame.  The cantilevered version of this deck still wants 2×8 joists (2x6s would not suffice)², but it creates a stronger structure.  The maximum span for SYP PT 2x8s 12” o/c is 14’ 2”², so a girder at 14’ 0” is close to the maximum.  Conversely, a two foot cantilever puts the beam only 12’ out, well short of the limit.  Thus the cantilevered deck creates a stiffer platform. (footnote 6)

So cantilevering the joists allows you to build larger decks at lower cost or build stronger decks.  That’s good — and it gets better.  Cantilevers offer two other benefits.  They pull the beam and supporting columns back, which softens the look of a deck.  Consider:

Best of all, by divorcing the deck edge from the supporting structure, cantilevers give you the freedom to creatively shape your decks.  You can efficiently build decks like the following without support columns at every corner.

In all, a cantilevered deck can be larger, stronger, cost less — and be more interesting and even elegant.  A winner.

Want to see more?  This gallery displays a wide range of deck designs that all benefit from cantilevers.

Now, here’s an impressive cantilever:  the Skywalk over the Grand Canyon.  Would you walk out there?  Really?  To the far edge?

NOTES

A.  For these design examples, I selected a live load (and snow load) of 40 pounds per square foot (psf) and dead load of 10 psf.  Your local design loads, especially snow loads, may vary. (For a detailed discussion of snow loads on decks, see my blog article “Will Snow Damage my Deck?”)  The specifications from the International Building Code 2009  that I reference here may seem aggressive. Check with your local building officials or your engineer;  they may want more conservative parameters.

B.   IRC 2009 and 2012, Table R502.3.3(2) address balconies, but that is the closest they come to deck cantilevers.  Other backspan standards vary.  The American Wood Council publishes a deck construction guide, Design Code for Acceptance, DCA 6, which specifies deck standards intended to augment the IRC.  Because DCA 6 prescribes generic standards safe for any deck, without specific engineering, it is very conservative.  DCA 6 prescribes a 4:1 backspan ratio.

C.   For a discussion of other deck terms, see my article about understanding your deck contractor.

FOOTNOTES

1. The term cantilever first appeared in English the 1660s, as a combination of the word cant from the Old French, meaning “corner” or “edge”, plus lever, from Old French levour, “to raise“.  From Webster’s New World Dictionary, second edition, 1986.
2. From the International Residential Code 2009, Table R502.3.1(2), for #2 Southern Yellow Pine 2×8 joists, 16″ and 12″ o/c, supporting 40 psf live load + 10 psf dead load.
3. Ibid, Table R502.3.3(2), interpolated for 40 psf live load.
4. Ibid, Table R502.3.3(2), Note b.
5. From Wood Construction Connector Catalog from Simpson Strong Tie, online, the table for LUS hangers fastened with SD screws
6. Actually for this cantilevered configuration, you could save a few joists and a little money by setting the joists 16” o/c rather than 12” o/c.

OTHER SOURCES

1.  Encyclopedia Britannica, online, http://www.britannica.com/EBchecked/topic/93144/cantilever.

2.  2013-2014 Wood Construction Connector Catalog from Simpson Strong Tie, online, http://www.strongtie.com/ftp/catalogs/c-2013/C-2013-p071-073.pdf.

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