C16 when SE has specified C24

[Conor]

My SE has specified 150x50mm C24 joists for our balcony. I can't find any treated C24 timber locally, I'm sure I could get it as a custom order from a mill, but I've neither the time nor money.

 

I can't find a definitive strength difference or loading table to compare C16 vs C24... and I've not heard back from the SE. Is there any general rule I can follow when substituting C16 for C24? I can get 200x50mm C16 no bother....

 

Thanks

[MikeSharp01]

I found this: https://armstrongsupplies.co.uk/blogs/faqs/c16-c24-timber#:~:text=C24 timber is roughly 50,as a C24 timber joist. which says that C16 is 50% of the strength (technical term) than C24 so if you want to do it in C16 all you need to do is double them up and bolt them together.

[JohnMo]

I recently went through the same thing.  Also when I questioned the supplier about suitability for external use they said the C16 was not treated correctly anyway.  In then the end anything suitable for external use was not in stock and had the be treated on request.  So ended up with the specified C24 which was a 1 week lead time.

[markc]

The difference in ‘strength’ isn’t as easy as double etc. you need to look at the loads being imposed.

Strength And Properties Classifications	C16	C24
Tension parallel to grain (N/mm2)	3.2	4.5
Bending parallel to grain (N/mm2)	5.3	7.5
Compression parallel to grain (N/mm2)	1.8	7.9
Compression perpendicular to the grain (N/mm2)	2.2-1.7	2.4-1.9
Shear parallel to grain (N/mm2)	0.67	0.71
Modulus of elasticity mean (N/mm2)	8,800	10,800
Modulus of elasticity minimum (N/mm2)	5,800	7,200
Characteristic density (kg/m5)	310	350
Average density (kg/m5)	370	420

 

[Redbeard]

I ordered a whole load of C24, and when the delivery came some was C16. Took that part of the order back to the yard and out comes the bloke with an ink-pad and a stamp... Oh look! C24! The official explanation was that all their timber could be graded C24 but if they stamped it all thus some people would think they were paying over the odds, when all they wanted was C16. So they stamp some of it C16 and some C24. It must be true.

[Conor]

Just got a quote back from Haldane fisher (lbm) for a one off, special order of C24 treated 6x2s... Comes in exactly the same price as their c16. Same stuff, different stamp???!!!!

[saveasteading]

34 minutes ago, Conor said:

Same stuff, different stamp

Yes, very likely.

 

I looked into this recently on the basis of designing a timber building with both options. 

Timber produced in the Highlands is only C16 at best, because it grows quickly in the great heat of a Highland summer.

(that is the case when compared with Siberia/Finland/Canada)

It is a little bit cheaper than C24 for structural timbers, and a fair bit cheaper for big sections.

But for stud, the guy at the timber mill told me they couldn't compete with imported. 

 

Since then perhaps Siberian is off limits, or perhaps it is coming a roundabout route. Whatever, I don't think they have much, or bother to transport C16.

 

8 hours ago, Conor said:

I can't find any treated C24 timber locally

First ask the merchants  to check as discussed above. It is probably sitting there with the wrong stamp on it. 

Not the yard man, you may need the manager.

 

Secondly the SE can change the design easily, if a little tediously. 

They will either simply change the input to their design programme, or look at span tables. 

 

Somewhere I have a book of tables of every possible section in the 2 grades, simply supported or fixed, and they should have this too.

 

The building regs even used to have it, but I'm not sure about now. It was very conservative.

 

[Gus Potter]

11 hours ago, Conor said:

My SE has specified 150x50mm C24 joists for our balcony.

Good question here @Conor

 

Lot's of good info from other members.

 

2 hours ago, Conor said:

Just got a quote back from Haldane fisher (lbm) for a one off, special order of C24 treated 6x2s... Comes in exactly the same price as their c16. Same stuff, different stamp???!!!!

Make no mistake here folks if the mill start forging stamps or anyone they are in deep trouble if caught.. and they will be.

 

With a bit of experience you can tell the difference between C16 and C24 pretty quicky The same price is probably more due to price point and stocking levels. C24 is denser.. has more cellulose and less air.

 

I can see how folk may think they are buying the same thing just with a different stamp. For a while you had to pay more for S355 steel than S275.. but the same steel meet the requirement for strength and ductility in both grades.. so the same material could often be dual certified but with the different stamps.. S355 commanding a premium... cynisism developed! But there is definitly a difference between C16 and C24 that you can pick up on as a lay person.

 

2 hours ago, saveasteading said:

They will either simply change the input to their design programme, or look at span tables.

Sometimes its not that simple as the connections stop working. If you take a Simpson Strong Tie connection the declared capacity is often based on C24.. reduce that to C16 and the connection can then fail at the nails.

 

11 hours ago, Conor said:

for our balcony.

Please be careful here.. especially if it has any form of cantilever..

 

If you want what about posting a few drawings and you'll maybe get some targeted advice?

 

 

 

[Conor]

11 hours ago, Gus Potter said:

Please be careful here.. especially if it has any form of cantilever..

 

If you want what about posting a few drawings and you'll maybe get some targeted advice?

 

 

 

When I say balcony, it's actually a beast that's going to drain the last bit of cash we have out of us!!! I'm getting the C24 and not deviating from the drawings... after all they cost me £1500.

 

It's all essential as we've no access to the garden from our ground floor, and I eliminated the cheaper option of an exposed timber balcony a long time ago.