How to Cantilever Deck Beams and Joists

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Blog entry by EricWrights posted 08-01-2009 09:52 AM 27896 reads 4 times favorited 1 comment Add to Favorites Watch

Deck building is a polular do-it-yourself project with homeowners who can take advantage of how-to-do-it clinics and tools rentals at home improvement retail outlets to construct the decks of their dreams.

Cantilevering is a deck construction technique that can be used by do-it-yourselfers on almost all decks. It allows the builder to add several feet to the width of the deck without increasing the maximum span rating for the beams or the joists. This is especially true when homeowners are building bigger and bigger decks with more levels and more complex shapes.

In some instances cantilevering makes it possible for a smaller beam size to be used to support the deck. As far as the joist size is concerned a smaller size could also be used by the using this innovative construction method.

Another advantage of incorporationg cantilevering into the construction of a deck is that the location of foundation columns, support posts and beams can be less precise than in the conventional method of construction. This is an important consideration for do-it-yourselfers who are not as proficient as professional contractors especially in this area of deck building.

However, it is important to remember that there are still guidelines that limit the distance that a beam can extend beyond a support post or that a joist can extend past a beam. These guidelines also assume that there will be no unusual loads placed at the other end other than the deck occupants. These guidelines are there to provide necessary standards to ensure the proper and safe construction of outdoor deck spaces by amateurs and pros alike.

Under the right conditions, technically it is alright for beams or joists to cantilever up to 40% of the distance that they are spanning between supports as long as the cantilever does not exceed four times the beam or joist depth. An example of this would be a joist that spans 10 feet between supports. Theoretically it could cantilever .40% x 10 = 4 feet beyond the supporting beam. Thus a 10 + 4 = 14 foot joist would be possible. But if the joist is a 2×8 (with an actual width of 7.25 inches) then the cantilever would be limited to 4×7.25 = 29 inches.

Unless you like bouncy construction it would be a wise decision to limit the cantilever to about 25% of supported beam or joist length. An even wiser decision would be to never exceed 3 feet of cantilever in the construction of the deck. This will result in the deck feeling solid even when one is standing over the cantilevered ends.

Cantilevering more than 3 feet would be better achieved by moving up to the next larger dimensional size of beam or joist.

In the initial deck design process it is a good idea to consult with the local building department regarding building codes and construction methods. Once you submit your application for a building permit along with your deck design drawings, building department staff will make alterations in your design where necessary. Cantilevering can be incorporated into this design and their expertise will ensure that the deck design will adhere to construction requirements and result in the building of an outstanding deck that will meet the needs and tastes of its occupants.

Cantilevering can be an important and very practical part of that process.

-- Sawing, sanding, scraping, cutting? Let Rockwell Sonicrafter do the job. & A more general blog at

1 comment so far

View a1Jim's profile


118144 posts in 4494 days

#1 posted 08-02-2009 12:06 AM

25% makes a lot more sense to me


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