Cantilever Deck Addition

by Cliff Dawson
(Alpine, Alabama)

I have an existing 28 x 8 elevated deck. I want to extend it by 4 feet to allow enough room to screen in the deck. The joists are 2 x 8s on 15" centers.

How far out can I extend a cantilever without having to put support posts on the end?

I planned to use 8ft 2x8s bolted to the existing joists and putting 4 diagonal supports back to existing posts at 8ft intervals and securing them to the fascia which would tie to all the joist ends.

Am I building an inherently weak structure?

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May 20, 2010
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Do it right, not over!
by: Anonymous

A 4' cantilever... DANGER! DANGER!

From everything I've read online and in books, codes nationwide will allow no more than a 2' cantilever over a beam and for good reason!

Try this, take a brick and put it on your shoulder. No big deal. Now remove the brick, raise your arm horizontally with the ground and bend halfway at your elbow. Then rest the brick in the bend of your elbow and forearm. It's a little more difficult but there's no real risk of your arm bending downward and dropping the brick. Now fully extend your arm and rest the brick on the back of your hand. It gets exponentially more difficult.

Your shoulder is the beam and the brick represents all the guests you have at your deck finishing party. I suggest you add more posts and another beam. If you build it with a cantilever out of code and you try to sell your house the inspector will FAIL your deck. Do it right, not over!

I am anxiously awaiting my Titan Post Anchors so I can install the posts and railings and finally put the finishing touch on my giant 10'x36'x8'tall deck. (It's giant to me because I built it without help from anyone other than my brother for the ledger and beam b/c they were heavy, long, high off the ground and needed to be perfectly level).

I was forced to cantilever the deck out 2' because of a tree and it worked out great. I used 2x8 joists. Be safe and Good luck!

Jun 07, 2009
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Cantilever For Existing Deck
by: Editor - Rich Bergman

How far one can cantilever a deck from an existing supporting beam is a common question. The question arises when people decide they want to extend the length of their deck.

Certainly it is easier to just add longer joists rather than move an existing supporting beam and post structure.

The short answer is yes a four foot cantilever is not out of the question. But it may not be possible and you can find the right answer by getting a copy of an applicable building code in your area.

The building code has sections that refer specifically to the guidelines and limits of building floors in homes. This includes cantilevers for balconies that emanate from an upper floor.

They explain how many feet out the cantilever can extend in proportion to how many feet inside the house the other end of the joists must go. Follow these recommendations.

While they don't usually refer to decks, the same physics applies. You will find out if a four foot cantilever requires using 2x10s or even 2x12s and whether they have to be spaced closer at say 12" centers.

If you can extend the cantilever by adding joists of the existing size then you will save a lot of time and effort. But you may find that the effort required to add another support beam closer to the new edge of the deck and then just removing the old post and beams isn't such a headache.

You mentioned adding triangular trusses against the support posts to help support the cantilever. This may work.

But if it were me, and I do this frequently when building houses, I would talk to a local structural engineer whose specialty is approving building plans and run the idea past him.

He probably knows the charts and numbers from memory and it would not cost much.

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