rafter check using the AWC Span Calculator

Now let’s check our roof rafter design. Recall that we (I) came up with 2×6 rough sawn (actual dims) stressed to 900 psi, which is probably doable in SP lumber, and likewise doable with 2×6 dimension lumber, but because the dim lum has less section, it will be stressed to 1428 psi, which is still likely doable, but using a higher `grade’ of lumber.

The American Wood Council (AWC) has put together a free, online, span calculator. I know a bunch of the folks at the AWC, and they are some of my favorite people. I also like the AWC `stuff’ … (use it all the time).

The Span Calculator is available here:


I’m used to using the `2012 Version’ (the one on the left) …

Here goes …

Species: Southern Pine

Required Horizontal Span: 12 ft 0 in.

Member Type: Rafter (Roof Live Load)

Deflection Limit: L/180 … we haven’t talked about deflection yet … I will set it to the least stringent condition … L/180 means that the calculator will select a rafter size/grade that deflects (bends, bows) not more than the length/180 under the design roof load … (L/180 = 12 ft x 12 in./ft / 180 = … in this case … 0.80 in.)

Spacing: 24 (in.)

Live Load: 20 psf

Dead Load: 5 psf

Wet Service: No (we’ll talk about later)

Incised: No

Calculate Span Options:

Gives … SP Select Structural 2×6, No. 1 same 2×6, and No. 2 same 2×6.

That’s marvelous!

That’s what I (we) came up with earlier … the Select Structural (SS), No. 1, and No. 2 are relatively high grades, which is what I (we) expected with the dim lum sections.

NOTE: it does not appear that the Span Calculator took into account the 300-lb concentrated load, which, we showed earlier, `governs’ the design. It could be argued that the 300-lb load need not be entirely carried by a single rafter. I could also argue that the structure in question is `unregulated’, so why the worry??? I’ll tell you the worry! … I’m gonna be up there building the thing, and at times I will be sitting, or standing on, a single rafter!!! I’ll snoop around and find some other (independent, and free, online, for kicks and giggles) means of checking the concentrated load condition (other post).