Joist and Rafters - actual vs nominal values.

[ab12]

I'm a little bit confused. Have had a new extension built with pitched roof and 7 by 2 joists were use.

When I measured the joist the other day to work out what insulation will fully fit the joits depth they weren't fully measuring 175mm but about 165mm.  So technically theyre not truely 7 inches.

Is this normal?

Looks like real value is different from the nominal stated value.

Does the same sort of things go for other timber e.g. studs. 

Bit confused

[SteamyTea]

3 hours ago, ab12 said:

Is this normal

Yes. I think the sizes are based on the rough sawn size, not the finished size.

 

https://www.ryedaletimber.co.uk/blog/regularised-timber-sizes-uk-guide-to-nominal-vs-finished-dimensions/

[saveasteading]

We all use inches on conversation for convenience. But these dims don't exist.

The sizes are sold as advertised in mm.

I phone the B M and ask for 20 lengths of 4.8m of 6x2. They don't ask if I mean 145 x 45.

 

For heavy structural timber it will come in sawn finish.

When dressed to avoid splinters  it reduces in size but not a lot.

 

There was a time when most timbers went through a bending machine which translated approximately to strength. Then this was samples, then by sight of grain and knots. 

Now we just sort of trust it. But C16 and C24 are visibly different.

 

I would think that an average timber selected at random then tested for deflection and to failure,  would exceed the rated strength by a distance because of various factors of safety which combine.

And then we allow safety factors in the design loading.

 

[Mike]

6 hours ago, ab12 said:

Does the same sort of things go for other timber e.g. studs. 

Yes, mostly. The Building Regulations went metric in 1972 and the construction industry followed by 1975. However, to add confusion, some products haven't actually change dimensions while others have.

[saveasteading]

2 hours ago, Mike said:

other timber e.g. studs. 

Exactly the same logic. They are generally in compression from the floor above, or just supporting cupboards, but the strength thing is the same: there is plenty if hollowing the regulations.