Retrofit Roof window - attic truss

[TicTac]

I think you meant 'rafters' and not 'joists'

Yup, did; sorry for that noseall. How about calling a truce.

The attic trusses are above the ceiling (hence the initial confusion) and the rafters in your roof, are at 400mm centres, so as you want a bigger window, you need to hack off the plaster where you want the window, double up the rafters either side of the new window size, and then put in trimmers above and below where the windows going. Easily doable for any competent roofer or builder. All the work can be done from inside the attic, and as the windows already there, the job should be even easier than normal.

 

[Sunshine12]

So the wood that slopes down the roof/the rafters - they’re ok to cut so long as doubled up, and they don’t affect the structure of the truss if cut? I imagined it as a big triangle with a ceiling tie, none of which could be cut. I’m confused!!

 

[TicTac]

It is a big triangle, but you're only messing with 3 rafters, and 2 get doubled, and the middle 1 gets removed, and then supported across the doubled rafters.

Have a read of this and look at picture 16 to see if it makes sense, although they don't seem to double the side rafters. And have a look at these pictures

 

[noseall]

I imagined it as a big triangle with a ceiling tie, none of which could be cut. I’m confused!!

Correct. Do not arbitrarily cut loose with your axe on a truss rafter roof.
Get a designer to look at what you can or can not do regards removing any of the roof elements.

 

[noseall]

It is a big triangle, but you're only messing with 3 rafters, and 2 get doubled, and the middle 1 gets removed, and then supported across the doubled rafters.

Have a read of this and look at picture 16 to see if it makes sense, although they don't seem to double the side rafters. And have a look at these pictures

FFS Tit Tac, that is a cut roof! Please stop giving advice about which you know zero.

 

[TicTac]

FFS Noseall, get real, we don't know what type of roof he's got, as even he's not sure. What I posted was for illustrations purposes to try and give the bloke an idea, that's all.

 

[noseall]

we don't know what type of roof he's got

My question is - is this possible with a truss roof (attic/room in the roof trusses)

How do you know Sunshine is a 'he'?​

 

[TicTac]

How do you know Sunshine is a 'he

And how do you she isn't?

 

[noseall]

And how do you she isn't?

Exactly.
But we did know it was a truss roof because the OP said so in their opening post. Don't give conflicting advice about a roof which does not apply. You only seek to confuse those that know as little as you do.