ridge beam sizing..

[needmorstuff]

what size ridge beam would you guys recommend for for vaulted ceiling? the room plan is 3.5x3.5m and I planned using infills of 150x50mm?

I am being told 12x4? seems crazy big.

also are purlins required for such a modest roof?

tiles are marley mendip concrete - not sure on pitch yet.. maybe 35'

 

[garyo]

Holy cr*p. For a 3.5M span? What's at either end - standard gable or equivalent?

I've seen chaps on here advocate that for small ridge spans, if the pitch is right, you wouldn't need a beam at all - a ridge board would be fine because the roof spread loads are so small. I'm out of my depth on that one though.

 

[needmorstuff]

at other end just standard gable... yes 3.5m.. I'm happy with a beam but just think this is overkill..

 

[needmorstuff]

just spoke to BCO - he didn't say it was a silly idea..

spoke again to the joiner and he has said to get 10x4
bco also wanted 12mm kingspan so 6x2 out for rafters..

going to get 8x2 rafters and put 150 kingspan in roof and use either 25mm or 50mm insulated plasterboard as well.. rather too much insulation than not enough as 2 of the walls are full of glass.

One thing.. BCO said he wanted collar ties? are the needed with a structural ridge beam? didnt think they were as ridghe supports roof and stops horizontal spread.

 

[garyo]

Have you considered getting an SE to design these for you rather than letting the BCO dictate things to you? Was it the BCO that came up with the original 12 x 4 beam spec? If the SE calcs the beam sizes for a complete design then you'll be less subject to the BCO whims.

 

[tony1851]

A 10x4 C16 would be fine for that span and loading. You don't need collars at the top as long as there are good fixings between rafter and ridge, but you are unlikely to persuade the inspector otherwise.

 

[needmorstuff]

well the joiner never put any on so we will see I guess... worst case I'll have to get an SE to ratify the design.

 

[catlad]

Surly you will need some collar ties, if nothing else but to flatten out the
ceiling at the top. Thats pretty standard really.

 

[needmorstuff]

i wasnt going to flatten the ceiling tbh.. have it meet at 30 degrees in a triangle. if building inspector picks it up I can always have joiner put collars in, i did read that unless they are a third down they are useless anyway?

this is the roof - **note the horizontal pieces are now gone - they were there until ridge beam got supported on a padstone at the gable end.

 

[garyo]

When using a structural ridge like that, what would be the advantage of fitting the beam 100mm lower and birdsmouthing the rafters on to it rather than spiking? You see it done both ways - are there particular circumstances where one's more suitable than the other?

 

[needmorstuff]

beam was specified that size for structural reasons.. rafters smaller purely for cost saving and to accommodate 150mm insulation

 

[garyo]

My question relates to the way the rafters are fitted to the beams. In your photo the joiner has used a single plumb cut. The alternative method is to birds mouth them over the top so that the seat cut is taking the load rather than the nails.

 

[needmorstuff]

you got me worried now.. are you saying mines wrong?

 

[garyo]

No not at all - there are two methods, and your picture made me wonder how/why people choose one over the other. I'll be doing something similar in a six months so it's of particular interest.

 

[needmorstuff]

i'd say they go with easiest... shotting a bunch of nails iwth aq gun is easier than cutting birdsmouths?