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xkcd #3138: Dimensional Lumber Tape Measure
Title text:
A person with two watches is never sure what time it is, especially if I got them one of the watches.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3138/
Its 2x4 before drying and planing.
ITT: people arguing about how to measure their wood
I've always chalked it up to shrinkage of the product during storage and shipping
You don't even need a custom one. Readily available is the universal tape measure where the relationship is marked as 2.54 units, equals 1 inch.
It means that a 2 x 4 is actually 5.08 x 10.16, which leaves you with a margin for surfacing and sanding.
american lumber doesn't use standard imperial units. an "inch" isn't the standard you'd expect on a so-called "nominally" 2 inch by 4 inch board. just attempting to use metric without accounting for the extra wackiness added by lumber 'measurement' on top of standard imperial silliness, you'd end up with a bunch of errors.
tl;dr: a 2" x 4" board actually measures something like 3.81cm x 7.62cm.
I understand.
I was making a joke about secretly introducing a metric tape measure.
I realise that my sense of humour is not universal, like the use of SAE .. or Fahrenheit 😇
At one time a 2 x 4 really was 2" x 4". Very old houses will have these in the walls, not planed and quite rough and splintery. I think I still have splinters from the 1913 bungalow I renovated more than 30 years ago.
I've worked on a lot of pre-1900 houses (I even grew up in one) and the 2x4s from back then really were 2" x 4" instead of the modern 1.5" x 3.5". Two years ago I bought a house built in 1942 and I demolished one interior wall and re-used the studs from it to build some new walls. I kept building these walls 1/2" too tall even though I measured and re-measured the spaces I was putting them into very carefully. I eventually realized that these 1942 studs were not in fact 1.5" x 3.5" like I had been assuming, but were actually 1.75" x 3.75" (the extra 1/4" in width of the top and bottom plates of my walls is where the phantom extra 1/2" was coming from). So apparently there was a transitional period between the real 2" x 4" 2x4s and the 1.5" x 3.5" ones.
I discovered another weird transitional thing in this house. The old houses I worked on all had lath-and-plaster walls, with strips of rough wood lath covered with a thick rough plaster layer which was in turn covered with a thin smooth plaster layer. Modern houses of course use sheetrock, but my 1942 house covered the bare studs with 16" x 16" pre-formed interlocking blocks of 1" thick rough plaster, and then smooth plaster was laid over these blocks. I first encountered these when tearing down the ceiling in my kitchen, and these things were unbelievably fucking heavy. They basically weighed as much as solid stone of these dimensions, and I can't imagine what it must have been like to install them initially. It surely must have been a two-man job.
Edit: another fun experience I had was renovating an Atlanta house that had been built in 1843. When we tore down the original lath-and-plaster walls, we found embedded in every single wall and ceiling a single dead, flattened rat. That house must have stunk to high fucking hell when they first moved into it. I like to imagine that it had been built with slave labor and this was some well-deserved payback.
This is how you become a character in House of Leaves.
pretty sure its been modified for one as an excuse to cut costs but the excuse is that it allows clearance for remodeling since in the 1900s a 2x4 WAS a 2x4 but with wear and tear smaller size 2x4s were easier for renovations.
I thought it was rough-cut dimensions.
Just when I thought I managed to contain my metric arrogance...
harrumph..
Those nonsensical rules exercise the brain more. Helps stave off mental deterioration.
Carpenters in the USA have a higher mental acuity at advanced ages than scientists
Maybe they just measure their age wrong.
I don’t think so, they are all 6 doggers old or more
Frankly, using a base 12 measurement system solves more problems for a woodworker than a decimal system does. It works very well for the task of woodworking. I'm familiar with and use the metric system for other things but I'm never building furniture in centimeters.
Genuinely curious. Mind giving an example?
Okay, a simple mortise and tenon joint. If I cut my board to 3/4" wide, if I want a tenon that is half the width of the board, it is 3/8" with 3/16" on either side. All my tools have these markings, I have router bits and such that are these sizes, easy. If I want a tenon that is 1/3 the width of the board, that's 1/4" with 1/4" on each side. Also quite easy to find tools for.
In metric land, they often mill wood, or manufacture plywood, to 19mm. Because that's quite close to 3/4". Show me a half, or a third, of 19mm on a metric tape measure.
You've got a 4 foot cabinet with 3 doors in it. How wide is each door? 1 foot, 4 inches. You've got a 400cm wide cabinet with three doors, how wide is each door? 133.3333cm.
Now do it with 5 doors.
You can always make up examples where one is easier. The truth is the easiest one is the one you're used to.
48" / 5 = 9 3/5" = 9 9/15" ~= 9 9/16" or 9 5/8". Dividing by five gets a little messy, but I divide by 2, 3 and 4 a lot more often than I divide by five. Thing is, that works out to be some pretty narrow doors, like, middle school locker narrow. You can indeed contrive scenarios where the math is ugly, but inevitably the cabinet you'd make would also be ugly. In actual scenarios you face in the real world it has a way of working out.
I'll give you a real world example. I recently built this dining room cupboard and hutch. The absolute overall width of the cabinet is 4 feet at the tabletops. The tabletops overhang the edges of the carcass 7/8", and the legs are 1 3/4" thick. So the area between the legs that the doors fill is 3' 6 3/4" (4' minus a total of 5 1/4"). The upper doors are 1' 2 1/4" and the lower doors are 1' 9 3/8". In reality each is 1/16" narrower than that to allow for some space for the doors to swing open and closed. The drawers have a 3/4" thick bulkhead between them, so each opening is 1' 9", and the drawers are 1/8" narrower than that to allow a 1/16" gap on either side so each drawer is 1' 8 7/8".
The leg dimension was chosen so I could have two layers of 3/4" boards, one for internal structure one for the outer rails, doors etc. and still have the legs stand 1/4" proud to make the legs look like legs (which they are; they're genuine posts) and to hide any impreciseness in fitment or milling of the rails, doors, drawers etc. The top overhang on each side is half of the leg's thickness, and then every dimension after that comes from the plan of the cabinet.
Tell me that wouldn't have been a pain in the ass to do in metric.
Interestingly enough, this concept was used in pattern making for casting machine parts back before modern machining and parts manufacturing.
They were colloquially called shrink rulers, and looked like a standard ruler, but were actually longer to account for the shrinkage of the material being cast.
For example, say you're casting a part from iron, which shrinks 1% as it cools, which amounts to 1/8 inch per foot.
An iron shrink rule would look standard, but actually measure a foot as 1 foot 1/8 inches to account for the shrinkage (this is an example and not meant to be actually accurate).
Source: am historian that interviewed pattern makers that used shrink rulers in their work.
Edit: spelling
I need to get a shrink tape ruler like this. I own a skoolie (used school bus converted to a motorhome) which is 35' 4" long from bumper-to-bumper. A lot of campsites have rules where RVs can't be longer than 35'. My thought was to get a tape measure with feet just slightly longer than normal and use it to make my bus appear to be shorter than 35'.
It shrinks? Like a frightened turtle!
That’s what happens when things cool down… It is the cold, that why it looks small 😳
Huh, the explain link says the dimensional sizes originated from the wood being cut at the listed size while green, then shrinking as it dried. I was told that it was done for construction purposes, where the wood would likely be covered by plywood or drywall that would bring the dimension up to size. I never questioned it before; that always seemed plausible enough.
I don't think this is true. There was a transitional period around the 1940s where 2x4s were 1.75" x 3.75", and that wasn't because wood shrunk half as much as it does today.
that's one of the common excuses that the mills quote. It's bullshit of course, but it sounds plausible so they continue to get away with it.
Another bullshit excuse is that they're providing an additional service by milling and planing the lumber for you, and that the nominal measurement is before that process.
It's all just greed. If they could get away with selling a 2x4 that was half an inch thick, they would. At least it's all standardized now.
Wood does shrink while it dries, and that is the reason why a "2x4" is actually 1.5x3.5, but you're missing a lot of detail in there.
To make a tree into boards, first a lumberjack fells the tree and bucks off the branches. It is then taken to a sawmill where a big clumsy saw slices it into kind of rectangular shapes. I will gloss over some nuance here about the algorithm chosen to do that and how it relates to the growth rings to produce boards of different qualities. You now have green boards, VERY wet. If you were to build something from these boards, as they dried they would warp and twist and pull the assembly apart. So it has to be dried.
At commercial scale, this is done by stacking the boards with spacers in between so air can circulate through the pile and letting it sit outside for a few months, and then the piles are taken into a kiln and heated for a couple weeks. The weight of the stack, or perhaps straps holding the stack together, has kept the boards relatively straight, but they will have warped a little. Before they're used for much, they have to be more precisely cut so they are straight, square and true.
Woodworkers making fine furniture tend to buy their lumber rough cut and mill it themselves, so that the lumber is as flat, straight and square as possible. Let all warping happen while the board is rough and mill it as a first step in building so that the pieces are very precise, and then the finished assembly holds itself true. Woodworkers buy wood per unit volume; it's usually priced per board foot. A board foot is 1 foot long, 1 foot wide and 1 inch thick, or 144 cubic inches. A 1 inch thick, 6 inch wide, 8 foot long board is 4 board feet. Checking out at the lumber yard requires a bit of middle school geometry homework.
Carpenters building houses or sheds used to do the same. In the early 20th century, lumber companies shipping lumber long distances by rail started shipping wood pre-milled. It doesn't really matter if the boards are a little warped; the worst boards in a stack can be cut into the smaller pieces you need, the better ones used whole for studs or plates. Pre-milling the boards at the sawmill means that it's cheaper and more efficient to ship the lumber, the sawmill now has the sawdust/shavings/chips to make particle board, OSB and other engineered lumber products out of, and the carpenter gets a commodity product he can buy and use rather than a raw material that needs further processing. Because they're all the same size, they're sold at a price each. It was a true 2 inches by 4 inches in the kiln, and the milling has been done for you.
Before they're used for much, they have to be more precisely cut so they are straight, square and true.
One trip to home depot determined that was a lie.
There is, now, a separate problem where 2x4s specifically are made of very inferior lumber. most will have either pith or wane, and I've seen them have both, which means the tree they harvested is maybe 5 inches in diameter and they might have gotten 3 2x4s out of one log. Even compared to when I was in carpentry class in high school the quality of construction lumber has decreased. Larger boards like 2x8s or 2x12s don't similarly suffer because it's impossible to make them out of saplings. But still.
I own a house built in 1942 and it's insane how good-quality the 2x4s are -- perfectly straight and true and no knots on them anywhere. They're so good that I've reused them for railings on my stairs. And this house was built as very cheap temporary worker housing during the war! I find it hilarious how much better the construction quality is on my house than on these million-dollar cardboard mcmansions they're building these days.
Unfinished 2x4s are actually 2in x 4in, which originally you would trim on all sides to get a nicer surface getting down to the "modern" dimensions. Nowadays, you pretty much only see pre-trimmed lumber.
None of the above is true, or at least isn't the full answer for why today a "2x4" is missing an entire half an inch all the way around. The shrinkage due to drying is around 5% (and the real math there is more complicated, as wood shrinks different amounts in different directions relative to the grain), which would only account for 1/10" of difference in the thickness of a 2x4. With some species of pine it's as low as 2%.
No, the lumber industry has consistently shaved boards in order to fit more into rail cars for transport and make more money and spend less per plank on transportation costs. Various lumber consortiums determined via internal testing that the smaller board sizes are still "sufficient" for their intended purpose vis-a-vis structural integrity of stick framed residential buildings.
Of course the answer is "capitalism". It always is.
There is no sensible reason to sell something as a dimension before it's ready to sell, a 2"x4" should be so when sold, not when curing, it takes nothing to cut oversize to accomodate for shrinkage, or to cure and cut later at the right dimension
The carpenter or woodworker cares about how big the board is. The sawyer cares about how much of the tree it took to make that board. The lumber yard has to make those two ends meet at a price point.
Modern lumber is planed, so some of that difference is because of losses from that. If you open up the walls of a house built 100+ years ago, you see these thick rough wall studs that never went through a planer. Even with shrinking, it's close to being actual 2" x 4".