I am thinking about extending my existing deck, 10 ft wide by 18 ft long and replacing the deck boards with composite decking. I would like to know how far can I extend the deck width without adding another beam at the end of the joists?

Currently, the 10 ft joists are connected to a ledger on one end, while the other ends are set on a beam supported by 4 posts. How far can the joists be extended (cantilevered) past the beam, 3ft, 4ft?

If the joists can be cantilevered, can I extend the existing 10 ft joists with another 2x8? Or, do I have to replace all the joists with new 2x8's of a length equal to the deck width?

Thank you.

  • "Desk" or "Deck"? The first is office furniture for doing paperwork or computer work types of things. The second is a wooden structure for standing on.
    – wallyk
    Commented Mar 31, 2016 at 5:32
  • The composites I have used do not hold up well for a cantilever they sag quite a bit. If a hand rail is needed this weight is added to the load.
    – Ed Beal
    Commented Mar 31, 2016 at 13:01

2 Answers 2


The rule, at least here in the US, is a cantilever can extend no more than 1/3 the supported length (so, you could go ~3 feet further). Personally, I try to keep cantilevers no more than 2 feet, regardless of total supported length: while it might be OK structurally, it will likely sag the further you go, and can feel bouncy.

To do the best job, get some 12 foot 2x8s and sister them with the existing joists (run them the entire length, nailing with galvanized nails at the top and bottom every 16 inches). Then put a double joist hanger at the ledger against the house.

One of the critical things with a deck cantilever is how you attach the railing posts at the end of the cantilever. You will be nailing a 2x8 rim joist at the end to finish the deck, but you need to make sure the posts for your railing are tied in securely to the joists, and not just the rim joist. The method can vary depending on whether your existing railing posts are notched into the decking or fastened to the outside of the deck.


Post tie in, I agree is important. Here is a pic how I did it. Blocked in the post between the joists, then used two 1/2 inch carriage bolts through the ledger and block. I suppose to be extra cautious I could have also used carriage bolts through the joist and post as wellenter image description hereenter image description here

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