Framing nails to excess (C) Daniel Friedman Deck Construction: Joist, Beam, & Floor Framing Spans & Tables
     

  • FRAMING TABLES, SPANS - Framing span tables for decks, porches, other common building floor structures
    • Deck beam span table
    • Deck joist span table
    • Deck joist rule of thumb
    • Deck framing tables, codes, specifications
  • POST a QUESTION or READ FAQs about allowable spans for deck, floor, or porch framing.
  • REFERENCES

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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 LUMBER. See Rot-Resistant Deck Lumber & Flooring for a discussion of lumber and flooring alternatives to preservative-treated wood.

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

Deck Joist & Beam Sizing Tables & A Quick Rule of Thumb for Sizing Joists

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 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.

Example:

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" (Doug fir-larch).

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:

Size Spacing Visually Graded Lumber Machine Stress Rated Lumber (MSR) Machine Evaluated Lumber (MEL)
DSS No.1 No.2 No.3 2400f - 2.0E 1650f - 1.5E 1500f - 1.6E M-14 M-29 M-12
   
Recommended Maximum Joist Span
2x8 16" O.C. 13-7 12-10 11-10 8-11 13-10 12-7 12-10 13-1 13-1 12-10

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 4-7: Maximum Beam Spans in Feet for Exterior Decks (C) J Wiley, S Bliss

Table of Maximum Joist Spans for Exterior Decks

[Click any image or table to see an enlarged, more detailed version.]

Table 4-8: Maximum Beam Spans in Feet for Deck Joists (C) J Wiley, S Bliss

-- 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.

Changing Wood Framing Span Tables & Modern Framing Lumber Strength

As we comment at DIMENSIONAL LUMBER,

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/

More Reading

Green link shows where you are in this article series.

...




Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about allowable spans for deck, floor, or porch framing. .

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References