We cant just change these things, and it would be totally rediculous to have separate building materials for both inches and centimeters. How can we ever change this? I surely dont want to buy a sheet of plywood or sheetrock that wont fit across my 16" walls or floor studs/joists. And if we keep the size the same, but change to metric measurements, instead of saying a 4 by 8 foot sheet of plywood we would have to say a long decimal equivalant with POINT something at the end.
Something like carpeting or linoleum probably could be changed to the nearest whole metric number because it does not rely on spaced studs or joists, but then if a room was built to be 12 feet, and the flooring ends up being a half inch less because of the conversion, many people would be quite angry. So, they darn well better make it larger, not smaller.....
Then comes the plumbing pipe. It MUST fit the old pipe. We can not just change to the nearest metric number. And I'd sure hate to have to go to the store and ask for a 3.856 by 5.891 CM electrical box, when I can not ask for a 3x5 box.
Then comes dimentional lumber. It's bad enough we now have 2X4's in buildings that are 2" X 4" 1 5/8" X 3 5/8" 1 1/2" X 3 1/2". Lets not make this worse by adding yet another mismatched size, because the number has to be metric.
I think building materials should just be left as they are. Even if they were to leave the size as it is now, but change to metric numbers, do you really thing many people would ask for that "3.856 by 5.891 CM electrical box"? I'd probably not even be able to remember all those numbers.
About the only place in building where I could see the change not being a real problem would be with liquid measurements. For example, a gallon of paint or a quart of roofing cement.
And finally nails, screws, wire, etc..... I am going to turn this into a question.... Is a #16 common nail 16 cm long? Is a #8 screw a metric number? (of course the length is still in inches). Is a #12 guage wire in metric, or what does that "12" mean? I'm just guessing on this, I dont know metric well enough...