On 2015-10-09 9:36 AM, Gary wrote:
> Dave Smith wrote:
>>
>> On 2015-10-08 7:56 AM, Gary wrote:
>>> Cindy Hamilton wrote:
>>>>
>>>> No, you're not. I'd love to see the U.S. abandon the old system,
>>>> even if it were just a soft conversion.
>>>>
>>>> Isn't plywood thickness actually sized in mm now?
>>>
>>> Not in my area. Still see 1/4", 1/2", 3/4" plywood. I agree that the
>>> metric is more efficient but it would take several generations to
>>> change to that in the US.
>>>
>>
>> How wide are those panels? Are they really 1/4", 1/2" and 3/4". Let me
>> know if they 2x4s are 2" by 4" because I have not seen a real 2x4 in
>> years. I guess the story is that they are nominally 2x4 and then
>> supposedly planed down a bit.
>
> Without looking it up, I do believe that the size difference is
> because of sawing the boards to size. You might measure 2X4 but each
> cut on every side takes away a saw-blade width. What you end up with
> is slightly less than 2X4.


The official story is that the lumber starts off at 2x4 but after drying
and planing they are 1 -1/2 by 3-1/2..... a half inch lost in the
processing.


> Same with all lumber. It's just accepted.

I think most people who work with lumber realize it. Some do not. Think
of some guy wanting to put up a solid fence around his yard. He sets the
posts at 8 foot centre to centre intervals and uses 1x6 boards hung
vertically. He is likely to base his materials supplies on 16 boards
per section. Then he realizes that each board is 1/2 inch narrower than
he had figured and he is 8" short.

It gets worse when you get into even wider boards. They end up 3/4" less
than the nominal sizes.

Think of it in relation to the discussion about metric vs imperial. It
is difficult to seriously consider concerns about exactness of lumber
measurement when imperial measurements are nominal and not exact
measurements.