Why are woodworking measurements always in Imperial and not Metric?
April 7, 2016 2:59 PM   Subscribe

My kid is taking Woodshop in high school and the class is taught in Imperial units in Canada. Why? Why are inches still the defacto measuring system for woodworking/home improvement? I'm old, so I have an excuse for buying 8' 2"x4"'s, kids don't. Plus wouldn't removing the fractions make it easier for everyone?
posted by Keith Talent to Home & Garden (10 answers total) 1 user marked this as a favorite
I would guess because we sell so much lumber to the US that it makes more sense for the manufacturers to keep lumber metric so they don't have to make separate products or use equivalent measurements that are weird numbers, although I think the building code has measurements in metric so I guess contractors need to know how to convert between the two.
posted by any portmanteau in a storm at 3:12 PM on April 7, 2016 [1 favorite]

Good question. In general it would not make sense for Canadian construction sizes to insist on metric when so much of what you use comes from, or is manufactured for, the US market in imperial units. (Maybe one way to get the US to think about going metric is to remind them that what they are using is called imperial. Or not.) It's also useful to stay with imperial because there is so much old building stock constructed on the old system, with studs on 16" centers, etc.

Even in fully metric countries, the lumber dimensions tend to follow the imperial dimensions fairly closely, rather than, for example, going to nice round metric numbers. The reason for this is with 16-inch spacing of joists, it turns out, sizing the joists (to avoid excessive bounce) works out nicely: if your are spanning 8 feet, you need a 2x8; 10 feet, a 2x10; 12 feet, a 2x12, etc. So it may be that the rules of thumb are ruling the choice of measurement system.
posted by beagle at 3:25 PM on April 7, 2016 [1 favorite]

Residential carpentry (and too a lesser extent cabinet making depending on the shop) are still dominated by imperial measurements because the materials are only nominally metric. You still by 4x8 sheet goods, 2x solid material etc. As a vocational goal the curriculum probably has use of imperial as a goal.

And fractions are a very useful thing to be good at even if you are using metric units. I find it easier to convert a fraction to a decimal at the final step when dividing up a measurement. EG: If I want to place 4 pot lights in a 3 meter wide ceiling. I'll separate them by 3/5ths of a meter and then measure 60cm.

Finally a lot of the plans they may be working with are designed to maximize material usage and minimize waste. 60 years ago someone developed a plan for a bird house and there isn't any budget to have someone design a new bird house with metric measurements. Because the source material is imperial even divisions in metric is going to end up with a bunch of weird measurements. eg 23" = 58.4 mm. and the thickness of planed material is imperial.

PS: BC makes tonnes of actually metric plywood and solid material but it's all for export. My father used to resell mill seconds cut for export and it was all stuff like 1x4s 3m long and 100mmx20mm in cross section. Sheet goods 1.5m X 3m.
posted by Mitheral at 3:26 PM on April 7, 2016 [1 favorite]

As a side note that may be of interest, as someone that did a lot of DIY in Australia & NZ all the timber is in metric measurements (mm) so I suspect a lot of it is an effect of rubbing shoulders with the USA.
posted by wwax at 4:34 PM on April 7, 2016 [2 favorites]

Heck, 2x4s are not even 2" x 4"-- they're 1½" × 3½".

The simple answer is that changing the system involves high costs (redoing plans, retooling factories, incompatibility with existing stock) while the benefits are fairly limited (easier international trade, easier calculations).

Compare with car manufacturing, where metric has come to dominate, even in the US-- possibly because the benefits were larger (more international suppliers and customers, less of a need to own and maintain two sets of tools).
posted by alexei at 4:49 PM on April 7, 2016 [2 favorites]

The answer is pretty clearly our imperial neighbours to the south.

It isn't just the lumber (as Mitheral points out, we produce plenty of metric lumber in Canada), but so many other parts of a house that would either have to be different for Canada versus the USA or would have weirdly specific millimetre measurements that would be difficult to work with. Some examples: window and door frames (depth of the jamb must match thickness of the wall), insulation (fits between imperial-spaced studs, thickness is imperial), sheet goods (length must relate to spacing between studs or else you'll end up with more waste and more cuts), countertops, cabinets and kitchen appliances (depth is standard to match them all together), plastic sheeting (roll width matches wall height), drywall (matches wall height), plumbing (pipe sizes must be standardized to fit together), and so on.

I think it would be very difficult, if not impossible to force the switch if we all just had to use the millimetre equivalent. If you go to order a window, they aren't going to ask you if you want the jambs for a 89mm or a 140mm wall, but a 2x4 or 2x6 wall, no matter what the official rules are.
posted by ssg at 5:57 PM on April 7, 2016 [2 favorites]

The answer is pretty clearly our imperial neighbours to the south.

My cousin is a handtool woodworker and I asked him and he said its because most of the tools are US still.
posted by jessamyn at 6:35 PM on April 7, 2016 [1 favorite]

I've been told that imperial is still used in woodworking because it's just easier to do on the fly calculations. Unlike the Metric system, Imperial measurement was created to fit human needs. A foot is more or less the same size as the average human foot, the width of a thumb on average is an inch (rule of thumb). 8 feet is the average height of a wall in a house which is easily divisible by 2, 4, 8, 12, 16 and 24 without the need for a calculator, therefore less figuring. Cutting a piece of wood that measures 12 5/8 into 2 even pieces is as easy as halving the 12 and doubling the denominator - 6 5/16, a lot easier that halving 12.625.
posted by any major dude at 7:41 PM on April 7, 2016 [2 favorites]

One advantage of Imperial that I've always liked is the foot is easily divisible by half, thirds, and fourths. 6 inches, 4 inches, 3 inches. A system based on 12 is a detriment in science but a boon in carpentry.
posted by Foam Pants at 8:06 PM on April 7, 2016 [1 favorite]

From what I've seen woodworking in Europe is in metric, so not "always".
posted by humboldt32 at 1:07 AM on April 8, 2016 [1 favorite]

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