How to support existing rafters with a new ridge beam?

19 Aug 2021
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United Kingdom
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!
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