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.

[saveasteading]

Here's an offcut from a 6 x 2.

As is my wont I rescued this from the waste pile and asked why it wasn't kept for noggins.

Because of the big split.

 

So I'll ask for the opinion of anyone who is interested. My decision won't change.

 

@ab12   you first?

 

[ab12]

On 27/05/2026 at 17:15, saveasteading said:

Here's an offcut from a 6 x 2.

As is my wont I rescued this from the waste pile and asked why it wasn't kept for noggins.

Because of the big split.

 

So I'll ask for the opinion of anyone who is interested. My decision won't change.

 

@ab12   you first?

 

 

I hate wasting, in actual fact I'm a bit of a hoarder. Would definitely make use of the timber in some meaningful way, maybe to use as noggin between adjacent floor joists etc.

 

[saveasteading]

Wastage costs an absolute fortune. 10% often gets added as default, and trades are very happy to not use offcuts or measure (plan) in advance.

Materials are typically 40% of a project.

 So imagine a 500k project x 0.4 x 0.1. That is 20k of wasted material.

It then goes in the skips, typically thrown in loose , costing  £4k?

 

The only argument some will understand is: if this was your project, how much waste would there be?  With your skills, let's work together on this.

 

That timber will be a noggin. It remains much stronger than it looks. For its'  like,  I may have to collect/denail/ clean and carry indoors myself and instruct firmly.

Really bad bits will become raised beds or nature piles.