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Recommended or allowable joist & beam spans: this article explains typical spans for deck joists, deck beams, and deck flooring, giving both standard span tables and a quick "rule of thumb" that works pretty well. Our page top photo shows a hybrid deck structure that encompasses a boat. No fasteners were found securing the boat to the deck however. This article explains deck framing. Basic building framing information is found at FRAMING SIZE & Spacing, Age, Types.
Spans, sizes and spacings for deck joists and deck beams are shown
in Table 4-7 and Table 4-8 (shown below in this article) as well as in most carpentry texts and framing guides. Here we give a rough "rule of thumb" that will provide a reasonable guess at allowable deck joist spans for most lumber species and grades:
A Quick & Easy Rule of Thumb for Estimating Allowable Joist Spans
While it is by no means a substitute for a proper table of spans and sizes of framing lumber, home inspectors often use a quick and dirty "rule of thumb" to guess whether or not deck floor framing has been obviously over-spanned when joists are spaced 16" on center.
[(Depth of the Framing Member) - 2 ] x 2 = rough deck joist span limit for Southern Yellow Pine spaced 16" o.c.
If you see a deck framed with 2x8 pressure treated lumber (typically Southern Yellow Pine or SYP), spaced 16" o.c.,
(8" -2) x 2 = 12' = maximum allowable span for a generic "deck floor joist"
A look at an actual current joist span table will show that depending on the wood species, the allowable spans for a 2x8 floor joists with 40 pound live load with deflection limit of span/360, ranges from 10'5" (Western red cedar) to 12'9"
A check with the current (2014) Southern Forest Products Association's (SFPA) span tables for southern pine, and assuming loads of 40 pound load (psf or per square foot), 10 pound dead load, and maximum deflection of 360, and choosing again a 2x8 joist spaced 16" on center for the same example, we found the following data:
Visually Graded Lumber
Machine Stress Rated Lumber (MSR)
Machine Evaluated Lumber (MEL)
2400f - 2.0E
1650f - 1.5E
1500f - 1.6E
Recommended Maximum Joist Span
You can read that for visually graded southern yellow pine joists, the rule of thumb will get you within the range of the table-given specifications for joist span for 16" on center spacing. Immediately below are current wood span tables from Best Construction Practices.
The Southern Forest Products Association's (SFPA) current suggested lumber spans are found at http://www.southernpine.com/span-tables/
Table of Maximum Beam Spans for Exterior Decks
[Click any image or table to see an enlarged, more detailed version.]
Table of Maximum Joist Spans for Exterior Decks
[Click any image or table to see an enlarged, more detailed version.]
Our opinion is that modern dimensional lumber is not the same product as it was in 1833 or even 1940. Modern 2x lumber is produced from trees that have been developed to grow rapidly to a size at which they can be harvested. Rapid tree growth means wide-spaced growth rings which may mean softer, weaker wood than dense-grained first-cut timbers or lumber. That combined with the increasing number of knots (as 2x's are cut from ever smaller trees) means that the building frame must rely on additional materials (such as plywood or OSB sheathing) for a critical part of its strength.
Details about the evolution of lumber standards are at FRAMING SIZE & Spacing, Age, Types. The Southern Forest Products Association's (SFPA) current suggested lumber spans are found at http://www.southernpine.com/span-tables/
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Questions & answers or comments about allowable spans for deck, floor, or porch framing. .
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 Paul DeBaggis is a building inspector and certified building code official with special interest in the history of and standards for wood products. (The American Wood Standards Committee). Mr. DeBaggis has served in the Easton MA. Building Department since 2002 , has worked as a building trades instructor, and also writes about land use regulations, building regulations, and standards. He is a past president of Southeastern Mass. Building Officials and is currently writing a book on the Massachusetts building code. Email: email@example.com
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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