What do these lumber fractions mean? 4/4, 5/4, 6/4, 8/4, 10/4, 12/4
In short, these fractions refer to the thickness of hardwood lumber. The thickness is read in increments of quarters of an inch.
Why those funny fractions? I’m used to standard sizes like 1×6, 1×12, 2×8 etc.
While you might be used to seeing sizes like 1×6 or 2×8 in lumberyards for softwoods like fir, pine and cedar, the hardwood industry takes a different approach. That’s because the primary users of hardwoods (red oak, cherry, maple, etc.) build custom or made-to-fit products – like furniture and cabinetry – where uniform sizes in the raw material is unnecessary. 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 they were cut for specific sizes or uniform widths, there would be more waste, more time, and therefore the resulting boards would cost you 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. 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.
This system of naming lumber thickness by quarters was established by the National Hardwood Lumber Association. The organization was founded in 1898 to establish a uniform system of grading rules for the measurement and inspection of hardwood lumber.
Let’s explain more, starting with the most common, 4/4
If we were talking you’d hear me say it as “four quarters,” which is short for four quarters of an inch. If you’re pretty snappy with numbers, you’ve already put together that 4/4 is equal to one inch.
4/4 = 1″ thick
5/4 = 1-1/4″ thick
6/4 = 1-1/2″ thick
8/4 = 2″ thick
12/4 = 3″ thick
There’s another designation that might confuse you: S2S. That means “surfaced two sides.” A saw that cuts lumber from a log is very large and aggressive, so the resulting lumber is known as “rough sawn” and the surfaces of the boards are . . . yes, rough.
Enter S2S. Lumber that is S2S has been planed smooth and flat. Naturally, that process removes some thickness and answers the age old question, “Why isn’t 4/4 lumber a full one inch thick?”
Actual measured thickness on 4/4 lumber that’s S2S is 13/16″. According to the NHLA rules, 13/16″ is the standard acceptable thickness of surfaced 4/4 lumber. (Note: you can view the current edition of the NHLA rule book here.)
Rough lumber needs to be dressed with a planer and/or jointer and this is where a lumber dealer like ourselves has to make a tough decision. Have the lumber surfaced smooth and clean, or sell it rough? There are benefits to both, but it’s impractical to stock both.
Rough Vs. Surfaced Lumber: What To Know
Pros Cons Rough Lumber
- Usually costs less
- Sometimes you can yield more thickness than the standard surfaced thickness
- Difficult to inspect grain, figure, and color.
- Requires planing and flattening prior to sizing
- Heavier, therefore costs more per board foot to ship
- Usually easier for the less-equipped woodworker to use and get started
- Easy to see the grain, figure and color
- Uniform in thickness
- Lighter, therefore costs less per board foot to ship
- Less control over final thickness
- Surfacing adds cost
One difference between rough and surfaced lumber might not be very obvious, which is the weight. It’s commonly thought that rough lumber is cheaper because it doesn’t have the added cost of surfacing. But that’s not the end of the story. While there is a charge for surfacing lumber, surfaced lumber weighs a lot less, and this plays a big role when we transport truckloads or container loads from suppliers.
For example, a flat bed truck will haul 45,000 lbs. The charge to move that flat bed doesn’t change if load of lumber is rough or surfaced – the charge is the charge. But what does change is how much lumber will fit on the truck. You can fit about 11,000 board feet of rough lumber or 14,000 board feet of surfaced lumber. In the end, it’s more economical to pay the marginal fee for surfacing (which is about $0.10 per board foot) in favor of fitting more lumber on the truck or container.
Therefore we’ve decided to stock surfaced lumber.
Here are pictures of each lumber thickness to help you out: