How to use span tables

By Martin Zibauer

Numerically dense, but handy nonetheless, span tables can help you look up the right joist size and spacing for a given span—essential knowledge when you’re designing a new deck or just wondering if the floor in your old cottage would meet today’s standards. Although the numbers can be organized in different ways, they all give essentially the same information: the distance a piece of lumber can bridge and still be strong and stiff enough to meet the Building Code. Here's a basic span table, for floor joists in No. 1 or No. 2 grade spruce, pine, or fir.

 

 

with strapping 

with bridging

with bridging & strapping

Space between joists>

12" o.c.

16" o.c.

24" o.c.

12" o.c.

16" o.c.

24" o.c.

12" o.c.

16" o.c.

24"o.c.

 

Joist size 

 

 

 

 

 

 

 

 

 

2x4 

6.10' 

5.64' 

5.18' 

6.53' 

5.94' 

5.18' 

6.53' 

5.94' 

5.18'

2x6 

9.58' 

8.89' 

8.17' 

10.30' 

9.35' 

8.17' 

10.30' 

9.35' 

8.17'

2x8 

11.61' 

11.02' 

10.5' 

12.5' 

11.74' 

10.73' 

13.09' 

12.20' 

10.73'

2x10 

13.68' 

12.99' 

12.37' 

14.57' 

13.68' 

12.86' 

15.09' 

14.07' 

13.12'

2x12 

15.58' 

14.83' 

14.11' 

16.44' 

15.45' 

14.50' 

16.96' 

15.81' 

14.73'

 

When you’re planning a building project, you can use the table to help design a floor structure that will meet Code, which will make getting your building permit an easier process. Let’s say you want to build a deck using No. 1 grade spruce. One end of the deck’s floor joists will be securely bolted to your cottage and the other will be supported by a beam, 13' out from the wall. You’re planning to build with cross bridging between the joists. Following the numbers across for 2x10s, the table shows the joists can span 13.68' (that's about 13'8") if they’re spaced 16" on centre (o.c.)—you’d be OK with that set-up. Now, if you wanted to space the joists 24" on centre, the table shows you could either build with 2x12s or use both bridging and strapping on 2x10s.

A more advanced table will parse the information more finely, showing values for different wood species under various loads. Or it might show span distances that will provide a specified stiffness. A table for deflections of less than 1/720, for example, is handy if you want a very stiff floor.

Just need a quick check of some span values? Try using an online span calculator —just plug in the numbers you know, and the calculator will spit out the values you’re missing. Keep in mind that online calculators often give spans that are longer than the more conservative Building Code, so it’s always a good idea to cross check with Code or your local building inspector.