Can I raise my joists?

Yes, like making sure you don't remove the old ones before enough of the new support is in! Also, planning how you are going to get the old joists out of the roof space and the new joists in. You may need to remove some roof covering and use some sort of crane or hoist, unless you are planning to reuse the shortened old joists.

Christian.

I think you could be spot on there. The removed ceiling definitely sagged.

OK

That's what I was thinking/hoping.

Thanks again for your help,

John

What is holding the purlins up at the inside ends? You should have posted that. The only reason you have purlins on a 10 ft span is that they are doing something interesting with the dogleg in the roof.

He could take out 3 or 4 and put them up cleaned. Putting 3/4 ply on them means he won't need insulation. Take out another set and start loading out. If he puts a strip of 1" ply under the purlin tight to both the wall plate and the purlin he could get all his sheets up on a calm, dry day without sagging the roof or losing slates and then bobs your uncle.

I'd use screws as they will cause less jarring than nails. If I didn't have them already, I'd buy a cheap circular saw and a second battery screwdriver. One for predrilling and one for screwdriving. Paint or stain the boards before sending them up as this will save half the job later and prevent them getting dirty.

They go straight into the wall.

You mean the hip extension? Again I think you could be right.

I've posted more photos showing the purlins going into the wall and views of the purlins on both sides of the extension;