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About Nails & Numbers

A construction calculator is only worth as much as the source behind it. This page explains what that source is.

Why this site exists

Search for almost any trade calculation and you will find a dozen sites that give you a number and nothing else. No table, no code section, no statement of what load or service condition the figure assumes. Some of them are wrong. While building the deck beam page here, two of the highest-ranking deck calculators on the web were found to disagree with each other and with the actual values in IRC Table R507.5(1), and the table shows how easy the error is: a 2-ply 2x10 carrying 6 ft joists spans 10 ft 4 in in southern pine and 8 ft 4 in in Douglas fir-larch. Quote the wrong column and you have overstated a span by 24% on a member holding people off the ground.

Nails and Numbers is built on the opposite principle. Every calculator names the standard it comes from, cites the section, prints the actual table, and shows the worked example, so you can check the answer rather than trust it.

The other half of the principle took longer to arrive at. Quite often the honest answer is that no table exists. A roof truss is a sealed engineered design and cannot be sized from a chart by anybody. The IRC contains no steel beam table for dwellings, and no joist spacing for composite decking at all. Several pages here exist to say that clearly and stop, rather than produce a number to fill the box.

How a page here is built

Each page follows the same discipline:

• The source is named. IRC Table R507.6, the CLFMI product manual, ANSI/TPI 1, APA span ratings, a manufacturer's own installation guide. Whatever it is, it is on the page with its section or document number.
• The table is printed in full. Not a fragment, not a paraphrase. If the calculator reads a table, you get the table.
• The assumptions are stated. Live load, dead load, deflection limit, lumber grade, service condition. These are what make a span number mean anything.
• The worked example is checked automatically. Every figure quoted in the prose is executed against the same data the calculator uses, every time the site is built. If an edit to a table contradicts the paragraph beneath it, the page does not publish.
• The edge is stated. Where the table stops applying, the page says so instead of extrapolating.

What this site is not

It is not engineering. These are prescriptive code tables and published formulas presented for planning, and they do not replace a licensed professional or your building department. Building codes are amended locally and adopted at different editions in different places. Every page says this, and it is meant literally.

There are no accounts, no subscriptions, no lead-generation forms and nothing is sold here. Nobody's contact details are collected in exchange for a number.

Corrections

If you find a value here that disagrees with the code in front of you, that is worth knowing about: say which page, which figure, and which edition you are reading. Corrections are the point.