Strength of finished verses rough sawn lumber

Just how much stronger is a full dimension of 2"'s (rough cut plank) over the finished dimension of 1.5" of finished planks. Where I priced finished lumber the other day, the finished dimension was 1.5 " verses 1.75 's a few years ago. Have span tables been adjusted down for the reductions in finished lumber dimensions ? How would I calculate the increase in the span of a full 2 X ?

The increase in strength between a 1.5" and 2.0" plank depends on how it is loaded. If the load is parallel to the 2.0" dimension (used as a beam) it will be 1.95 times stronger (1.25 cubed) and if the load is perpendicular to the 2.0"(used like a plank) it will be 1.25 times as strong.

In practice, rough saw lumber needs to be degraded for defects, so the above is true only if both pieces have the same defect density.

Rafter and joist tables take the actual vs. nominal dimensions into account.

Thanks again, Brian -

How would one determine what increase in span these increases in strength allow.

Tedd

The increase in permissible spans depends upon the loading situation and the relevant wood parameter that is constraining the situation. Usually, the later is either bending (expressed as a fraction of total length -- like 1/360) or extreme fiber stress (Fb). The numbers I mentioned above are for the condition of uniform loading and total bending. Thus, for the two loading conditions the increases in span would be 25% or about 40%(1.25**3/2) with the 25% increase in thickness.

Thanks Brian, you have been a big help. You might or might not be suprised as to how many other replies on other forums told me that there is no appreciable difference.

Tedd