Timber Grading C16/24

KerrySurfer

Hi folks, I'm currently in the process of getting quotes for the timber for the roof of my project. I asked my local builders provider to give me a quote for both C16 and C24 although I think my mind is already made up in that I'm going to go with C24. My question is, who stamps the timber with the relative grade?!

The reason I'm asking is that my local provider is questioning my reasons for wanting c24 as he reckons C16 is "fit for purpose" for the Irish market. He also mentioned in passing that he could use his own stamp on any timber I'd like and call it c24. He doesn't normally stock C24 except in extremely long lengths. Can anyone shed any light on the timber grading system?

Also, if anyone knows of any suppliers of C24 I'd appreciate a PM.

Thanks!

con1982

C16 is pretty much the standard structural grade. I occasionally specify C24 if I'm really stuck.

Can you use C16s at closer centres instead of C24?

sydthebeat

It's the strength class

C24 is stronger than C16 and can span longer lengths.

Please note that IS444 has been superseded as the standard for specifying structural timber joists.

KerrySurfer

I'm already planning on using 400mm centres and if I used C16 it would be pushing towards the upper limit of C16 (as per IS444 Span Tables).

Regardless of which grade though I'm assuming that timber should already be stamped before it hits the builders merchants?

Metric Tensor

KerrySurfer wrote: »

Regardless of which grade though I'm assuming that timber should already be stamped before it hits the builders merchants?

100% correct.

As well as machine grading there is a system for visual grading (I can't claim to be familiar with it - maybe someone here might be able to give more info) but I seriously doubt your local builders provider is either qualified or equipped to do this!

Also if you get some proper C24 timber and compare it with C16 the difference between the joists will be quite apparent to any carpenter or trained timber worker.

KerrySurfer

Metric Tensor wrote: »

Also if you get some proper C24 timber and compare it with C16 the difference between the joists will be quite apparent to any carpenter or trained timber worker.

I had heard this alright and it's one of the reasons I'm splashing out on the higher grade. Cheers for the response!

Johnnyhpipe

If calcs deem that you need C24 then you need it.

Amazing that the builders provider who, without design calculation or knowledge of the project is able to tell you that the weaker timber is "fit for purpose" which incidentally is the one he has in stock..

Whats "fit for..Irish market"??? Nonsense- I have designed many 2m wide sheds and a number of 20m spanning glulam timber roofs - both within Ireland. Does his magic stock timber cover all cases equally?

Anyway, to answer your question - the timber should be graded/stamped at the mill. Long before it ever reaches the middle man.

IS 444 is outdated also.

KerrySurfer

Yeah I just wanted to double check that he was talking sh1te!

Under IS444 C16 would have just about fallen within spec at 400mm centres but under Eurocode 5 I need C24.

Thanks for the info! I'm in contact with another provider that's using CE stamped C24 but I wanted to give the local guy an opportunity to quote but he seems to just want to push what he can sell as opposed sourcing what I need!

tedimc

For what its worth:
My engineer specced some C24 9*3 purlins for my build.
Believe it or not - galvanized RSJs worked out cheaper for the lengths required
The quality of the C24 timber did seem to be far superior than C16
It can be difficult to source the correct length and can take a while to deliver. Also - may be difficult to get it pre-treated.

Odelay

The C16 is fit for purpose. You design to C24 and that is fine so long as you are using standard lengths and sizes. But try buying a couple of 4.2m lengths of 175x44 C24 and you will soon find them hard to source where you wouod have the c16 delivered in the morning.

An interesting note is a structure designed and built with C24 is more likely to fail dangerous, where a structure designed and built with c16 is more likely to fail safley. This is because jn the timber population used for c24, strength is the limiting property, in the timber used in c16, stiffness is the limiting property. This does supprise people.

Johnnyhpipe

Odelay wrote: »

The C16 is fit for purpose. You design to C24 and that is fine so long as you are using standard lengths and sizes. But try buying a couple of 4.2m lengths of 175x44 C24 and you will soon find them hard to source where you wouod have the c16 delivered in the morning.

An interesting note is a structure designed and built with C24 is more likely to fail dangerous, where a structure designed and built with c16 is more likely to fail safley. This is because jn the timber population used for c24, strength is the limiting property, in the timber used in c16, stiffness is the limiting property. This does supprise people.

On what basis have you deemed the C16 timber fit for this particular purpose?

Odelay

Johnnyhpipe wrote: »

On what basis have you deemed the C16 timber fit for this particular purpose?

On the basis that it's used in the same maner using span tables.

bertie 56

Odelay wrote: »

An interesting note is a structure designed and built with C24 is more likely to fail dangerous, where a structure designed and built with c16 is more likely to fail safley. This is because jn the timber population used for c24, strength is the limiting property, in the timber used in c16, stiffness is the limiting property. This does supprise people.

Hi there,

Could you explain a bit more this ? Any links ?

Thanks

Lantus

I have a project with long spans. I'm using steico I joists. The on line catalogue has full sizing tables for floors and roofs.

Just an option.

bertie 56

You can use both : C16 or C24 , once the calculations are properly made.

But I'll prefer C24 for few reasons :
- C 16 is the poorest grade, you can have good ones, but you can find lengths that would be C 10 , if this grade existed...
- The amount of unusable lengths in a pile of C16 is greater than in a pile of C 24 : a careful selection in the pile of C16 is compulsory, IMO. But not everybody do this, specially when you get deliver straight from the yard. You can be less picky with C 24 : the wood is already good in general .
- My modest experience showed me that the dimensional stability of C 24 is far better than C 16 . A flat length of C 24 in the yard will stay flat after few months drying, when a length of C 16 could become unusable, unless you want to do a plane propeller !
- The difference of price is not that bad : C 24 is more expensive, but you use less as well .

The only problem is to find a C 24 supplier now, it was easier to find some years ago...

Yes, C16 is " fit for purpose for the Irish market " , since the Irish market customers are looking for " cheap" instead of " good " and don't complain to have sh.tty materials in their houses... And as well because the forestry companies here and in UK had to find a way to get rid of their too-fast growing stuff that is not even good at making heat in a stove !

Last concern with C24, the carbon foot print is higher : it doesn't grow locally so has to be imported ...


If you are still in doubts about which one choosing, go in a yard and compare them both : if you have a bit of experience , you'll surely go out of there with "C 24" written in the docket !!!

And remember you are building a roof that will shelter you for years to come, not a kennel for your dog


Hope you the best for your roof.

Odelay

bertie 56 wrote: »

Hi there,

Could you explain a bit more this ? Any links ?

Thanks

http://blogs.napier.ac.uk/cwst/making-the-grade/



That is a link to a grading blog from Napier University in Glasgow.

Scroll down to "Critical Property". It lists the three properties required, strength, stiffness and density.

Stiffness- The resistance to bending, you will notice this if you were sitting on a chair in a prefab and someone walks past, you can feel the floor move/dip as they walk past. The higher the stiffness the less you will notice this.

Strength- The ability to resist breaking. Glass is stiff but not strong.

Density- This is used to calculate the fixings needed. If the density value is low then more/larger fixings are needed say for example fixing roof sheeting.

Now have a look at the next section "critical property for uk and roi Spruce"

Here you see that spruce in uk and ROI make C20 strength and density values but it is stiffness that limits it. So if using c16 from uk or Ireland in a structure, if its going to fail, it will be for stiffness, but it will not be for breaking strength.
The opposite is true for the critical property for northern European spruce. Their resource meets the stiffness and density requirements for c24 easily but it is the strenght that is the limit, it just makes the c24 requirements but does not pass it. Therefore if your c24 structure is going to fail, it will be stiff enough but may break without warning.

This is all very unlikely but is interesting. It also assumes each grade is used as designed.
If using c24 in a design for c16, then that is overkill, bit like using stainless steel where the design specifies galvanised.

I understand that it is easy to look at a bale of timber and pick out pieces that are not "strong enough" when compared to others, however when you put a board into a machine for testing to destruction you will be stunned by how much force it takes to break weak looking boards.

SC024

can we name suppliers of C24 timber on thread? I'm not sure

bertie 56

Odelay wrote: »

http://blogs.napier.ac.uk/cwst/making-the-grade/



That is a link to a grading blog from Napier University in Glasgow.

Scroll down to "Critical Property". It lists the three properties required, strength, stiffness and density.

Stiffness- The resistance to bending, you will notice this if you were sitting on a chair in a prefab and someone walks past, you can feel the floor move/dip as they walk past. The higher the stiffness the less you will notice this.

Strength- The ability to resist breaking. Glass is stiff but not strong.

Density- This is used to calculate the fixings needed. If the density value is low then more/larger fixings are needed say for example fixing roof sheeting.

Now have a look at the next section "critical property for uk and roi Spruce"

Here you see that spruce in uk and ROI make C20 strength and density values but it is stiffness that limits it. So if using c16 from uk or Ireland in a structure, if its going to fail, it will be for stiffness, but it will not be for breaking strength.
The opposite is true for the critical property for northern European spruce. Their resource meets the stiffness and density requirements for c24 easily but it is the strenght that is the limit, it just makes the c24 requirements but does not pass it. Therefore if your c24 structure is going to fail, it will be stiff enough but may break without warning.

This is all very unlikely but is interesting. It also assumes each grade is used as designed.
If using c24 in a design for c16, then that is overkill, bit like using stainless steel where the design specifies galvanised.

I understand that it is easy to look at a bale of timber and pick out pieces that are not "strong enough" when compared to others, however when you put a board into a machine for testing to destruction you will be stunned by how much force it takes to break weak looking boards.

Thank you for the very interesting document.
I take these informations with care, knowing where they come from.
It's important to note that this document is a presentation made at a Forestry Conference in UK, with sponsors as Scottish Woodlands / Tilhill Forestry / A.W Jenkinson Forest Products, etc..., in a word, a meeting of C 16 producers... The author is surely genuine, but , as we can say, he won't bite the hand of the ones who feeds him...

In terms of numbers, it will be interesting to have stiffness and strenght of Nordic-grown pine too , to compare with the British grown ones.

The poor properties for stiffness are not very reassuring. That means C 16 timber are bending easily under weight. Not the best thing for a roof, over time ! I suppose you minimise this if it's well calculated, but still ...

Strenght is suppose to be good for C16 . Better than C 24 ? I doubt it...
Between knots, pockets of sap and drying cracks...
( I was in a roof today, walking in 2x2's of C 16... Man, talking about strenght of C 16 ? I wasn't stunned by how much force it takes to break weak looking boards anyway...!!! :eek: )

About overkill , it's what cathedrals builders were doing, when using oak instead of pine for their roof. 300 years after, and still standing ! :rolleyes:

I appreciate the efforts of everybody to defend their business, but seriously, when something is sh.te, it's sh.te and it's not a PowerPoint that will change this...

tabby aspreme

I doubt if much timber fails to make the c16 grade as you would see 1' 1/2 knots in some 4 by 2 , and the pith running through planks also .