QUESTION & ANSWERQ Is there a maximum allowable height for a 4x4 deck
Glenn Mathewson, a building inspector from
Westminster, Colo., responds: Unfortunately, there are few
pre-engineered and universally accepted standards that address
The International Building Code and the International
Residential Code (IRC), the foundations for most of the
building codes in the United States, specify only that a post
must be at least a 4x4. This isn’t meant to imply that
4x4 posts are good for all conditions, merely that
there’s no condition where less than a 4x4 is
acceptable. For most decks, post size is overlooked unless an
inspector notices the unusual height of a particular deck. A
short 4x4 can carry an awfully big load, and many deck builders
use 6x6 posts as a standard, regardless of the height or load
of the deck.
Though the height of a deck is often questioned only in regard
to the requirement for guards, it is a factor in determining
the height or “span” of the posts. When an
undersized post is compressed along its axis by the weight of
the deck above, the post can bend or bow, although complete
failure is rare. Taller posts of a given cross section are more
likely to bend than shorter ones. Additionally, the tributary
load that each post will carry has a direct effect on its
minimum required cross section.
The tributary load is the area supported by a single post
multiplied by the expected live and dead loads, typically 50
pounds per square foot. To find the tributary area, halve the
joist and beam spans on all sides of the post (except that you
must include the entire area of any cantilever adjacent to a
post) and calculate the resulting area. Multiplying that area
by the expected load tells you the weight a post is expected to
carry. Deck builders may be more familiar with this exercise in
determining the loading on footings. That said, without
engineering knowledge or a table of prescribed post heights,
knowing the tributary load isn’t very useful.
Although the IRC says little about post sizing, the two sources
described below go into greater detail. Either of those, or an
engineered design, can be submitted to the regulatory
jurisdiction as alternatives if requested.
Published by the American Forest and Paper Association
(AF&PA), the DCA6 (Design for Code Acceptance #6)
Prescriptive Residential Wood Deck Construction Guide
is an alternative to the IRC that local authorities may allow,
but they may require it to be used in its entirety (the
document is available free
The DCA6 recommends minimum 6x6 posts with a maximum 12-foot
height. Although the DCA6 contains no mention of the tributary
load, its joist and beam spans provide a maximum tributary load
that should be within the capabilities of its post
cross-sectional and height limitations (for more information
about using the DCA6 to build a deck, see
May/June 2008; free at deckmagazine.com).
Another document that provides a flexible post-sizing table is
Wood Decks: Materials, Construction, and Finishing
(Kent A. McDonald et al., The Forest Products Society, 1996).
Tables 6 and 7 in this book show a range of tributary areas and
post sizes; they were developed by the AF&PA and should
still be valid for most designs. The one caveat is that the
AF&PA has reduced its recommendations for the maximum
post height. While the tables allow posts to reach a height of
17 feet in some instances, the AF&PA now recommends
going no higher than 14 feet.
Otherwise, based on these tables (for Douglas fir or southern
pine posts and a combined live and dead load of 50 psf), a
4-foot-tall 4x4 post could carry a maximum tributary area of
256 square feet, but a 4x4 that’s 10 feet tall (the
maximum allowed height) could carry only 36 square feet.
Increasing the size of the post to 6x6 would mean that at 13
feet in height, the post could support 256 square feet.
Of course, it is up to the local inspector whether to accept
this documentation. Extraordinary loads — say a 6x14
glue-laminated beam supporting long-spanning joists —
are beyond any code or code alternative, and an engineered
analysis would be necessary.