Designing with Floor Joist Span Tables

Part 2 of Residential Structural Design

On this page we will explain how to design with floor joist span tables. At the bottom of this page you'll find a joist span calculator. The content on this page will explain how to interpret the results from the calculator.

If you are just starting out, you might want to start at our previous page, residential structural design page which explains basic house structure.

See our Design Your Own House tutorial site map to go through the tutorial in order, or to jump around it as it suits you.

Using Floor Joist Span Tables

Continuing on from Part 1: Residential Structural Design Vertical Structural Forces, we were about to start learning about floor joist span tables.

Don't worry, you won't need to do a lot of calculations in determining the size and placement of the structural framing within your house design. You will however, need to become familiar with reading your local floor joist span tables for floor beams, floor joists, window and door lintels, ceiling joists, roof joists, roof rafters, and roof ridge beams, as well as the sizing and spacing of wood studs tables.

Before jumping into the span tables, let's first consider wood species and wood grades. In general, all lumber is stamped near the end of the lumber piece by the manufacturer.

The stamp displays several types of information. The information displayed varies between wood associations. But most will display:

• The wood species - often abbreviated
• The wood grade - could be a number or description
• The wood association of which the lumber manufacturer is a member
• How the wood was seasoned - this will give an indication of the moisture content
• The mill identification—indicated by name or mill number

Wood Species

The stamp above indicates in the triangle that the wood is Douglas Fir (D FIR). Other common designations are:

• S-P-F (or Spruce – Pine – Fir)
• Hem-Fir(N) (or Hemlock – Fir)
• D.Fir-L(N) (or Douglas Fir – Larch)
• N. Species (red cedar, certain pines, balsam and poplars)

Lumber Grade

The stamp shown above indicates the lumber grade is standard or better (STAND & BTR) this is the same as number 3 or stud. Common lumber grade designations include:

• SEL STR (Select Structural)
• No. 1, No. 2
• No. 3 / Stud / Stand & Btr. (or Standard & Better)

These designations and numbers will become important when you are reading your structural tables since the strength of a wood depends on its designations.

Now we'll look at how to read these tables. Let's start with a simple, very small house 12 feet wide by 13 feet long. The house will have a gable roof and will be framed with wood studs.

The table below shows the maximum span for a floor joist with the following designations:

• Douglas Fir-Larch (lumber species)
• No. 1 and No. 2 (lumber grade)
• All joists are bridged

Sample Floor Joist Span Table

The table excerpt above is simply a sample and may not be valid for your region.

Looking at this table you will see there is a choice in the size of floor joist (2 X 6, 2 X 8, 2 X 10 or 2 X 12) and there is a choice in the joist spacing (12", 16" or 14"). The floor joist spacing is the distance between the centers of any two installed joists. See the image below for an example of joists spaced 16" on center (16" o.c.).

Since the example house we are designing for is 12 feet wide, we need to find, in the floor joist span table, a joist size and centering that can span 12' or wider. Reading the table, you'll see that 2 X 8s installed 16" apart on center can span 12'4".

So our house would look like the picture below (with all floor joists 2" X 8"). The picture below shows a plan view, that means looking straight down from above the house. The outer gray walls are the concrete foundation walls which support the floor joists. The narrow double lines show the floor joist themselves.

From a three-dimensional perspective, the floor joist framing would look like this:

Keep in mind the span table above shows just the allowable spans for Douglas fir or larch, there are different tables for all the construction lumber species.