My roof truss timbers are 38mm x 72mm on 600mm centres with a 6m span. What's that in good old inches?

It seems a little skimpy to me; is this normal for a modern house? I am trying to do a rough dead load calculation. All the information I can find is for 4"x2" lumber. Mine are more like 1 3/4" x 3".

I think I read that 4"x2"s are actually smaller due to shrinkage so I have even less clue as to what my size is!


1 Answer 1


Using Google you can do conversions. Type:

38mm in inches

into Google and it will give you the conversion as the first result.

Your sizes come out at

1.5" x 2.8" with just under 2' centres with a 20' span

Now, the reason these are smaller than the quoted sizes is because wood is not measured in the finished size but in the original size. So a piece of wood that's quoted at 4" x 2" will actually be something like 3.5" x 1.5". This isn't a problem because everybody* knows this.

The calculations should take this into account.

* for some value of "everybody".

  • Gets event more fun with 2by2 ex 2by4 etc, in the UK it is common to ask for 3m of 2by2 as if you ask for the real size in mm no one at the builders yard knows what you are talking about.
    – Walker
    Commented Sep 9, 2011 at 22:30

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