Vaulted ceiling, without purlins, second opinion please

[Mr Chibs]

A friend of mine is adding an extension to his bungalow, he's intending to have a vaulted ceiling on it.
The extension projects around 3m from the back of the house, and he/me will be building up a new gable to rest the ridge beam on.

The bungalow is 7m wide (approx), and a 30° roof pitch, he doesn't want purlins, I did a quick tot-up and the rafter length will be something like 4.3m
Would he be advised to install purlins or not?

Currently he's awaiting the paperwork from the SE, but he's called to say purlins shouldn't be needed, and timber sizing will be around 195x44.

All just seems a little on the thin size to me?

Any advice appreciated.

Chips

 

[freddiemercurystwin]

Are you a structural engineer?

 

[Mr Chibs]

No, just concerned... it seems a lot of roof weight to support that's all, thought you guys may have 'practical' advice/opinions.

 

[freddiemercurystwin]

For that span they'll be fine, rule of thumb is the depth required is about 1/20th of the span.

 

[Mr Chibs]

Sorry to seem like a stuck record would the above seem ok with no purlins? I understand what you are saying regarding the timber sizing.

thanks again

 

[freddiemercurystwin]

yes

 

[oldbutnotdead]

8 x 2 on a 30 degree pitch sound nice and chunky to me if he's slating it- might be a different matter with concrete tiles but that's why you pay the SE

 

[Mr Chibs]

Sadly, It's concrete... more than likely when he buys the wood, he might upsize.

I will get him to give the SE a double check regarding the covering although I've seen a copy of the paperwork: tiles, lath, felt are 2.66kN/m²

Thanks

 

[freddiemercurystwin]

It'll be designed for a 1m thick snow load.

 

[Mr Chibs]

2.1kN/m² for snow loading, nearly as much as the tiles... so what you are saying is panic not!

 

[Dereekoo]

2.1kN/m² for snow loading, nearly as much as the tiles... so what you are saying is panic not!

Snow loading is 0.75 or 1.0kn/m2 depending on location and altitude. Suitability of 44 x 195 depends on selected spacing,wood grade and whether dead load is 0.5-0.75kn or 0.75- 1.0kn

 

[Dereekoo]

Sadly, It's concrete... more than likely when he buys the wood, he might upsize.

I will get him to give the SE a double check regarding the covering although I've seen a copy of the paperwork: tiles, lath, felt are 2.66kN/m²

Thanks

2.66kn?????? I suggest you check that fig, even with max snow loading and max dead load TRADA tables only design to 2kn/m2. Maybe the SE is playing it ultra safe?

 

[Mr Chibs]

OK, these are the figures he has for the beam in question, I mistakenly added the "²" to the previous snow figure.

Snow 2.1kN/m
Tiles, lath, felt 2.66kN/m²
Roof timbers 0.57kN/m²
Plasterboard/Skim 0.57kN/m²

Any better?

 

[Dereekoo]

OK, these are the figures he has for the beam in question, I mistakenly added the "²" to the previous snow figure.

Snow 2.1kN/m
Tiles, lath, felt 2.66kN/m²
Roof timbers 0.57kN/m²
Plasterboard/Skim 0.57kN/

What beam in question, are we discussing rafters or ridge beam?

 

[Mr Chibs]

Those are the specs for the ridge beam, the rafters are set at 450mm (c) and are 195x44 c16 (will be using c24)

Ridge beam is 152 x 89 x 16

I have built a vaulted roof before, but I'm no expert, (I probably up-scaled my timbers etc) but when I saw this spec, I thought it just felt a bit light.