Size of purlin/ridge beam

[Pathfinder85]

I'm renovating my 1850's cottage and in the single story 9" solid brickwork extension I would like to replace the ceiling and vault it. The room is 3.1m long by 3.3m wide with a pitched roof with skielings 1940mm high and a flat ceiling approx 2240mm high. The floor to ridge height is 3110mm.

Currently there are 2 purlins and a thin ridge board supporting the roof with 50x50mm ceiling joists supporting the ceiling and tieing the roof structure. I would like to remove the ties and vault the ceiling but obviously don't want the roof spreading which I guess it would do if I removed the ties. If a ridge beam is the best solution it could span between the gable end and chimney breast.

Can anyone advise on a suitable size for replacement purlins or ridge beam please and if this is the best way forward? Many thanks in advance for any help.

 

[noseall]

What is the pitch of the roof?

What type of roof covering. i.e. plain tiles, concrete tiles etc are currently on the roof?

What is the section size of the existing purlins?

 

[Pathfinder85]

Sorry forgot to include those;

Pitch of the roof is approx 34 degrees

Double Roman tiles

Purlins are 200x50's

 

[Pathfinder85]

*Double Roman Clay tiles

 

[freddiemercurystwin]

You’re gonna need a structural engineer, the works are notifiable so need Building Control sign off, who will expect some engineers calcs.