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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. Also see Deck Nails, Screws, Hidden Fasteners and Deck & Porch Connections: Ledger Boards. And see Preservative-Treated Framing Lumber. See Rot-Resistant Deck Lumber & Flooring for a discussion of lumber and flooring alternatives to preservative-treated wood.
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As explained in Best Practices Guide to Residential Construction, Chapter Four, Best Construction Guide for Building Decks and Porches
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 Deck 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 over-spanned when joists are spaced 16" on center.
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 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"
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
-- Adapted with permission from Best Practices Guide to Residential Construction.
Details about selecting the proper fasteners for constructing decks and porches and how they are installed can be read at Deck Nails, Screws, Hidden Fasteners.
Also see DECK COLLAPSE Case Study (collapse of a new code-approved deck) and DECK FLASHING LEAKS, ROT Case Study for an example of an older deck with rot and collapse due to improper construction and missing building flashing.
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