Modular roof hinged truss or rafter (C) Daniel Friedman Building Framing Size, Spacing
Details give Building Age & History

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This article gives details about the standard or traditional size and spacing of wood framing members in residential construction. By taking a look at the actual dimensions of framing lumber as well as its spacing you can often determine the age of a building or of its various parts.

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Wood Framing Spacing & Framing Member Actual Dimensions can indicate building age

What are the actual sizes of dimensioned lumber studs, rafters, and joists?

In North America, up to about 1930 it was common for dimensional lumber to be full-sized - a 2x4 was really 2" x 4" in cross section. Modern wood framing wall studs 2x4's (a modern dimensional lumber "two by four" is actually 1.5" thick by 3.5" wide) and larger members (x" deep by 1.5" thick).

Common Spacing Intervals for Dimensioned Lumber Framing Studs, Joists, Rafters

Here are some common intervals or spacings used in frame construction in North America.

  • non-standard wood framing spacings, typically in post and beam or hewn beam frame construction
  • 14" spaced wooden stud framing - early balloon framing
  • 24" spaced wood stud and wood rafter framing, based on a 24" module - all framing, wall studs, floor joists, roof rafters are spaced 24 inches on center. See Arkansas framing above. 24" spacing is also often found between modern roof trusses.
  • 16" spaced wooden wall stud and wooden roof rafter or wood roof joist framing
PHOTO of post and beam framing with joint number markings.

Here is a photograph of post and beam framing with joint number markings.

The observation of framing materials, framing markings, and framing styles provides considerable information about the probable age of a house.

We discuss framing materials and styles here as an aid to house age determination.

Also see our article on " Saw Cuts and Tool Marks" (links at the "More Reading" links at the bottom of this article ).

Log framing and both modern and antique log construction are discussed at Log Home Guide.

Antique and modern trusses are distinguished and modern laminated beams and I-truss beams and wood joists are discussed.

Keep in mind that even when we can identify specific types of building materials and building methods, precise dating of the time of construction of a building remains difficult: old building materials were often re-used, so beams, siding, and other components may appear in a building built later than when the materials were first made.

Also, in the U.S. various states had machines for making cut nails, screws, and sawmills at different times. For example, New York State was industrialized earlier than some western or southern states, so machine-made nails appear earlier in New York than elsewhere.

History of U.S. Lumber Standards Since 1900

Below, quoting a US FPL article & courtesy John McDaniel and Paul DeBaggis is a historical document published by the Forest Products Laboratory report entitled "History of Yard Lumber Sizes Standards"[1]. Mr. DeBaggis points out that The first national U.S. lumber standard was composed in 1924.  Apparently from nearly from the beginning, 4 inches was not quite 4 inches when citing lumber dimensions.

Prior to 1970 the size of a 2x4 was 1-5/8 x 3-5/8 and a 2x8 was 1-5/8 x 7-1/2 regardless of the moisture content.

The sizes of standard dimensional lumber changed in 1970 when separate size standards for dry and green lumber, under nominal 5-inch thickness were established.

In 1970 the minimum dressed size was related to moisture content and this lead to a dry (19% MC or less) 2x4 being 1-1/2 x 3-1/2 and a 2x8 being 1-1/2 x7-1/4; a green (over 19% MC) 2x4 being 1-9/16 x 3-9/16 and a 2x8 being 1-9/16 x 7-1/2.

Prior to 1970 the lumber standard was called Simplified Practice Recommendation R 16 and after 1970 the lumber standard was called Voluntary Product Standard 20 (PS 20). This history of the lumber standard can be found in Appendix D of that document. The history of U.S. lumber standards is summarized below.

Early in 1922 Secretary of Commerce Herbert Hoover, responding to a request from the lumber industry, offered the cooperation of the Department in activities directed toward simplification, standardization, and development of adequate quality guarantees for the lumber consuming public. This cooperation led to the development and publication in 1924 of Simplified Practice Recommendation R 16 under the guidance of the Department's Division of Simplified Practices, which was to become a part of the National Bureau of Standards (now the National Institute of Standards and Technology).

The history of R 16's development and its subsequent revisions is summarized in editions issued, respectively, in 1924, 1925, 1926, 1929, 1939, and 1953.

R 16-53 was revised in 1969 and superseded by Voluntary Product Standard PS 20-70 American Softwood Lumber Standard. The significant provisions added to PS 20-70 were:

1) separate size standards for dry and green lumber, under nominal 5-inch thickness, were established in order to achieve greater uniformity in the dimensions of seasoned and unseasoned lumber at the point of use;

2) an independent National Grading Rule Committee was created to establish and maintain a national grading rule for dimension lumber conforming to PS 20;

3) an independent Board of Review was formed to assure uniform approval of grading rules and of agencies to grade under these rules, and to enhance enforcement of the accreditation program ;

4) the composition of the American Lumber Standard Committee was expanded to reflect a broader representation of interests; and

5) uniform methods for assignment of design values were accepted. Non-substantive changes were made to the Standard in 1985, 1991, 1992 , 1999 and 2005. The Standard was reviewed in 2009 to assure it reflected the needs of manufacturers, distributors and consumers."

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.

Paul DeBaggis commented further at the start of 2014:

The southern pine industry has recognized (as you state in your work in "dimensional lumber") today's trees grow faster, and hence probably have lost some strength.  Comprehensive testing has been done in southern yellow pine, and the southern pine bureau has announced a new set of spans.  Unfortunately, it seems to me some politics is at work, because the ICC has reviewed the data and stated, in effect, since the matter is not an emergency it will have to wait for the 2015 code for adoption.

I am a huge fan of southern yellow, but if I were going to use it tomorrow, I would use the pine bureau's suggested spans rather than the current code.

Depending on the span and the size of the joist, the new numbers are between 4 to 24 inches less.

Don't know whether this is urgent to know, but it seems good to know.

- Mr. DeBaggis is a building inspector and certified building code official with special interest in the history of and standards for wood products and a contributor to [2][2a]

The Southern Forest Products Association (SFPA) current (2014) suggested lumber spans are found at The SPB provides this disclaimer about these recommended joist and rafter tables, wood framing member size tables, and allowable load tables:

These easy-to-use tables were compiled by the Southern Forest Products Association (SFPA) as a service to design/build professionals and other Southern Pine users.

SFPA does not grade or test lumber, and accordingly, does not assign design values to Southern Pine lumber. The design values contained herein are based on the Southern Pine Inspection Bureau’s Standard Grading Rules for Southern Pine Lumber and modified as required by the American Wood Council’s National Design Specification® (NDS®) for Wood Construction. Accordingly, neither SFPA, nor its members, warrant that the design values on which the span tables for Southern Pine lumber contained herein are based are correct, and specifically disclaim any liability for injury or damage resulting from the use of such span tables.

The conditions under which lumber is used in construction may vary widely, as does the quality of workmanship. Neither SFPA, nor its members, have knowledge of the quality of materials, workmanship or construction methods used on any construction project and, accordingly, do not warrant the technical data, design or performance of the lumber in completed structures.

- Southern Forest Products Association, [4]


More Reading

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Frequently Asked Questions (FAQs) about framing damage & the proper size & spacing of wood framing beams, joists, rafters

Reader Question: how to deal with a floor joist that is arched upwards

I have a floor joist in our cottage I’m about to install a bathroom in. During the inspection and layout, we have one joist directly under where the tub will go which rises approximately ¾ to 1”. I look through several articles, but only find this brought up on roof trusses.

I am proposing to fix this by bolting a metal angle iron on the joist, then a 4x4 post on the bottom side, then connect the two using nuts and all-thread. After connection, tighten the system till the joist comes back in line with the other joist.

My alternative is to cut the joist from top, till near the bottom, then applying a sister joist on the side of the cut a minimal of 3’ on each side.

This joist is under the house, therefore have room to apply the 4x4 post on the bottom side. Any input would be appreciated. - J.D.B. 11/18/2013


The arched roof trusses you mentioned (TRUSS UPLIFT, ROOF) are a special case of arching that probably does not pertain to a bathroom floor - rather that arching is traced to temperature and moisture variations caused by burying the bottom chord of a roof truss in insulation. The result can be cracks at the ceiling/wall juncture.

You are describing an arched floor joist that occurred for a different reason, possibly simply that when the framer (quite correctly) placed the joists "arch up", one of them was particularly arched or curved. The effort you describe to pull the arched floor joist down into position seems a bit more than necessary to me, and possibly risks some future arch-back that messes up the bathroom.

With the warning that I don't have a complete picture of the building, structure, age, condition of framing, cause of the "Rise" in one floor joist - all of which could change the recommendation, in general, if you simply have a single floor joist that happened to be more arched than its neighbors, and presuming we're talking about standard floor framing, 2x10s for example, I would not hesitate to simply chop off the arch with a power saw, getting back to a sufficiently level floor that when the subfloor is installed things will be flat and level.

IF the framing is sub-standard in dimension, for example some older homes used 2x8's or even 2x6's for floor framing, you may indeed want to add joists or sister joists.

IF you find that the joist arched because of some structural damage or problem, I'd look into that further before just making things nice and flat.

IF you are planning on a ceramic tile floor and the joists are smaller in depth you may want to add joists or sisters anyway - to be sure the floor doesn't flex - it depends of course also on the spans. In that event take a look at FLOOR FRAMING & SUBFLOOR for TILE and also FRAMING TABLES, SPANS (where the deck joist spans are typically required to have the same load-bearing properties as the interior building floors)

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