What do these lumber fractions mean?
4/4, 5/4, 6/4, 8/4, 10/4, 12/4
In short, these fractions describe the thickness of hardwood lumber. The thickness is in increments of quarters of an inch and tells you approximately how thick the lumber actually is.
But why? The Big Box stores have “standard” sizes like 1×6, 2x4, 2×8 etc.
While you might be used to seeing sizes like 1×6 or 2×8 in lumberyards or stores that sell softwoods, the hardwood industry takes a different approach.
That’s because the primary users of hardwoods (red oak, walnut, maple, etc.) build custom products – like furniture and cabinetry – where uniform sizes in the raw material is unnecessary and more troublesome. For example, there’s no standard size for kitchen tables, you can make one any size you want. So sawmills cut hardwood logs to get the best yield from a log, which means all boards will be various in width rather than the same width. If sawmills were to cut logs to specific sizes or uniform widths, that process would incur more waste and require more labor. Waste + Labor = More Money.
Softwoods that get sold in “standard” sizes like 1×6 and 2×8 are cut for particular applications that require those sizes. In building construction you’ll find standard and uniform sizes from building to building such as door jambs and wall studs for example.
So, the hardwood industry standard for indicating the size starts with lumber thickness, and it’s expressed as a fraction: 4/4, 5/4, 6/4, 8/4 and so on.
With rough lumber, it’s hard to see the grain and the boards are frequently slightly cupped or warped from the drying process. Surfacing fixes both of these, but it does remove thickness.