How to support existing rafters with a new ridge beam?

[minerscottage]

I've got a gabled roof made up of two chunky pairs of rafters, and two dozen much lighter rafters that sit on purlins resting on the larger rafter (and the stone gable-ends of the property).

It looks a bit like the diagram below:

1) Green 'lightweight' rafters
2) Purple 'heavy' rafter (which also has a lateral tie 1/3rd up I've omitted from the diagram)
3) Blue ridge board
4) Pink purlin

The purple rafter is buried into the stone wall, and the green rafters sit on top of the rubble wall, and the purlin. Some of the green rafters are attached to ceiling ties, but most aren't, so the only thing preventing the roof from spreading out is the tie on the purple rafter.

My plan is to turn this roof space into a vaulted ceiling for extra head-space. I've provided detailed measurements to my engineer, and he's produced plans that call for the installation of

1) Extra (large) rafters
2) A ridge beam to hold the weight of the roof, removing the need for ceiling ties. He's happy the span is short enough for a load bearing timber.

I'm currently thinking about how do go about executing this upgrade to the roof, and am looking for insights from those with more experience.

My current thought is to fix collar ties at the top of each existing rafter. This partly to provide extra support to the ridge of the roof, but also to serve as a plate I can rest the new ridge beam under.

Once the existing rafters are supported by the new beam, I would then slide in the new rafters underneath the existing purlin (supported with a birdmouth), and hang them from the new beam with either metal hangars or another birdmouth.

Here's a diagram of what I'm thinking:-

1) Orange collar ties
2) Red ridge beam
3) Yellow new rafter

For further context, here's an image of the inside of the roof colour-coded as above

Thanks in advanced for your thoughts!

 

[blup]

.

 

[Mr Chibs]

I would have thought beam spanning gables, sat on pads, rafters birds mouthed on top of beam or sat in web of beam, then collar ties under beam. My vaulted ceiling was 4.6m long (span around 5m) and from memory was 203mm deep, so you will probably need chunyk timber if you are allowed use this, mine was pocketed into gable walls. I have no purlins.

SE had said if it was longer, it could be broken down into sections and bolted together, although this deflection may be greater as not in one piece, but easier to install (maybe )

Neighbours had a large vaulted ceiling recently, it used a wooden ridge, but it was huge, a glue lam thing, looked about 300mm deep and about 150mm wide, it wasn't even that long, 4-5m.

 

[minerscottage]

I would have thought beam spanning gables, sat on pads, rafters birds mouthed on top of beam or sat in web of beam, then collar ties under beam. My vaulted ceiling was 4.6m long (span around 5m) and from memory was 203mm deep, so you will probably need chunyk timber if you are allowed use this, mine was pocketed into gable walls. I have no purlins.

SE had said if it was longer, it could be broken down into sections and bolted together, although this deflection may be greater as not in one piece, but easier to install (maybe )

My original plan was to birdmouth the existing rafters (small and large) onto the new ridge beam, but the size difference between the "large" and "small" rafter, and the size of the timber beam means one-or-the-other wouldn't be able to touch the other. If I fix a collar to each, I can position them all so they're at equal distance to the beam underneath. There's also the difficulty of trying to pry apart the existing rafters with 100kgs of roof on them.

Seems a lot easier to me to leave them kissing the ridgeboard, and then jack-up the beam underneath them.

Neighbours had a large vaulted ceiling recently, it used a wooden ridge, but it was huge, a glue lam thing, looked about 300mm deep and about 150mm wide, it wasn't even that long, 4-5m.

I've got three spans in total, the longest two are 4m and my engineer has calculated 100mm x 150mm C16/24 timber for the job.