Q:

How do you use span tables to size joists and rafters?

A:

Quick Answer

To size joists and rafters using span tables, first find the table that corresponds to your span requirements. The span is the distance between the faces of a joist or a rafter. Select a lumber size from this table, and find the size that corresponds to your required span.

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Full Answer

Next, use the same table to determine the modulus of elasticity, or the E value. Select the appropriate extreme fiber stress in bending or Fb value from this table as well. With the E and Fb values, select a wood species from a design values table that matches these two values. Stiff materials have high E values, and strong materials have high Fb values.

Find the span table that lists the values of the compression strength perpendicular to the grain for your loading conditions. After this, check that for your chosen species, the value of the compression strength perpendicular to the grain matches the value for your loading conditions.

If the two values of the compression strength perpendicular to the grain are the same, then the selected wood species is correct for your span requirements. The chosen lumber size is also correct, and you can use a joist or a rafter of that size for your required loading conditions.

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