Framing Calculators

Framing is the trade where the prescriptive tables are shortest and the things they refuse to cover are easiest to miss.

The calculators

• Board Foot CalculatorA board foot is nominal volume. A third of it is not there
• Sheathing and Subfloor CalculatorIRC Table R503.2.1.1(1) and the APA span rating: two numbers, one sheet
• Shed Framing CalculatorFraming arithmetic, and the size threshold that is not framing at all
• Wall Framing CalculatorIRC Table R602.3(5). A 2x4 under two floors is a dash, not a spacing

Three tables govern most of what happens between the foundation and the roof, and each one has a cell or a footnote that changes the answer.

Table R602.3(5) prints dashes. A dash is not a tighter spacing you can reach by adding studs, it is the table declining to cover the condition. A 2 x 4 bearing wall carrying two floors plus a roof-ceiling assembly is a dash, and so is every bearing column for a 2 x 3. Those walls exist, they are framed every day, and they are outside the prescriptive path.

The stud height in that table is not floor to ceiling. Footnote c calls it the distance between points of lateral support, which is why R602.3.1 wants bearing walls sheathed on one side or bridged at 4 ft before the tabulated height means anything at all.

The span rating on a sheet is two numbers. Roof span, then subfloor span, and they are far apart: a 32/16 panel spans 32 in over rafters and 16 in over joists. It only holds with the strength axis across three or more supports, so a sheet turned the other way to save cutting has no rating, and the roof figure drops again without edge support.

Nominal, actual and what you are billed for

Lumber is sold on a nominal size and delivered at an actual one. A 2 x 4 measures 1 1/2 by 3 1/2 in, so 34% of every board foot on the invoice is not physically present. That is the convention rather than a trick, but it means board feet and cubic feet of wood are different quantities and the calculators here keep them apart.

Panel thickness is the same story from the other direction: a sheet sold as 1/2 in is a 15/32 in performance category. Specify by span rating, not by the fraction somebody says at the counter.

Where framing stops and design begins

Headers over openings are Table R602.7(1) and live with the structural calculators, as do rafters, beams and trusses. What is here is the wall itself, the panels that brace it, and the arithmetic that turns a drawing into a lumber order.