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Deck Cantilever Rules and Limits - How far can it span?

The distance your joists can safely cantilever or overhang a drop beam is determined by the size of the joists, the wood type and grade of the lumber and the spacing between joists. Many building departments limit joist cantilevers to 24" maximums.  You must use at least 5 framing nails to attach the rim joist to the ends of your joists.

The Wood Frame Construction Manual, Published by the American Wood Council  states that cantilevers are limited to 1/4 the span of the joists.  2x10 joists spanning 14 feet, for example, could cantilever up to 3.5 feet.


Comments, Questions and Reviews


I am building a raised deck for a playhouse. The deck is 6'' wide and 10'' long, with a 2'' cantilever in the 10'' direction. If I run the joists in the 10'' direction (8'' span), will this work? The playhouse will be 4'' x 8''.

posted by Dogmelissa at 8/14/2015 11:31:56 PM

I am building a deck with 12'' x 12''. I have sauna tubes and cement posts spaced 16" apart. The deck joists that sit on top of the cement posts are 2 @ 2" x 8" (nailed together with carriage bolts and nails). The cantilever at the end turned out to be 33" long. Is the current set-up adequate?

posted by gwoodley at 7/30/2015 9:03:55 AM

Addendum: There are two 2" x 8" beams attached to 4" x 4" pressure treated posts (which are attached to the sauna tubes and cement, under the deck - running parallel to the outside 2" x 8" joist in front (as a part of the frame). The support underneath is the 4" x 8" doubled up beams, not the deck joists (I believe they are the runners?).

posted by gwoodley at 7/30/2015 9:10:24 AM

Add an appropriately-sized beam under the joists, and you can cantilever up to 1/4 the backspan (distance between beam and house.) If you have a 12'' deck, you can cantilever it 3''.

posted by alanram at 6/9/2015 12:00:18 PM

The cantilevered decks should include water protection such as metal flashing to prevent wood rot such as occurred in Berkley California. All of the media coverage appears to be people trying to avoid blame. Do building codes address this obvious failure?

posted by Bob Berschauer at 6/24/2015 6:41:42 PM

I am Helping a friend extend an existing deck THREE FEET that has 2x8 joists. I plan to canterlever the extension using a combination of deck screws and two carriage bolts for each joist. How many feet should the new 2x8s overlap the the existing? This deck is just above the ground, should I plan a support at the end of each joist? Thanks.

posted by Wes Pickreign at 4/6/2015 8:08:30 AM ya can''t do et. 2x8''s can only be cantilevered 1/2 of the width of the wood. A 2x8 is actually 1 1/2 inches, so according to code, you can only extend the deck 3/4 of an inch. I''d recommend using 6x12''s and adding a post to every joist, about every 3.1415 feet (I like pi(e)).

posted by yadumfuk at 5/24/2015 8:55:24 AM

You should use 2 by 12 joists on 24 inch centers.Be sure and provide metal flashing to protect the beams from wood rot.

posted by Bob Berschauer at 6/24/2015 6:45:42 PM

Leave someting to be remembered by.

posted by martinsnines at 7/3/2015 8:08:45 AM