C16 vs oak rafters?

[imsparticus]

Hi,
I'm the process of building a new conservatory and C16 rafters have been specified but I would like to consider having exposed rafter tails, in which case I thought of using oak rafters instead.
I would keep to the same size of rafters but if I do does the oak have to be graded and if so what is the equivalent grade or would it just be assumed that size for size oak rafters would outperform C16 softwood?

Thanks.

 

[AronSearle]

If the rafters are exposed, then they would need to be oversized for charring, for half an hour its +17mm on each exposed face (34mm thicker).

Also some of the oak grades are only a little bit stronger in bending than C16, so you will end up with larger section sizes.

 

[imsparticus]

Hi I have been told that the rafters are already oversized for the application they are 50 x 150 at 400m centres at 30 deg for just over a 1M span. Would they need to be over sized further?

Also if oak is even slightly stronger would that not mean that I could at least keep the section sizes as it wouldn't be worse? Or do you mean because over the charring issue only?

PS the exposed rafters would only be exposed on the outside not on the ceiling ie just the tails if that makes any difference?

 

[AronSearle]

I assumed you meant exposed internally, if the ends are just exposed outside, then no you don't need to oversize for charring.

That size of oak can be graded to TH1 and TH2 (TH2 is the lower grade), both grades are stronger than C16.

 

[imsparticus]

Just found this in building regulations for fire safety.

Anyone know if this would apply to a conservatory roof?

"Exclusions from the provisions for elements of structure

4.4 The following are excluded from the definition of element of structure for the purposes of these provisions:

a. structure that only supports a roof, unless:

i. the roof performs the function of a floor, such as a roof terrace, or as a means of escape (see Section 2), or

ii. the structure is essential for the stability of an external wall which needs to have fire resistance; and

b. the lowest floor of the building."

 

[imsparticus]

I assumed you meant exposed internally, if the ends are just exposed outside, then no you don't need to oversize for charring.

That size of oak can be graded to TH1 and TH2 (TH2 is the lower grade), both grades are stronger than C16.

Thanks Aron that's what I wanted to hear.