Opening up a vaulted ceiling

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My house had a flat roof extension which was then changed to a vaulted ceiling at some point. The flat roof was removed but the joists were left in place holding the plasterboard for the room below.

The pitch of the roof is 24 degrees.

I would like to remove the ceiling and joists and then install velux windows in the roof, but am I ok to remove the joists?

I've attached photos to hopefully clarify things.

Thanks in advance for any help.
 

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Looking at the photos, yes, can't see a problem.
 
I'd say you'd need some sort of wall plate to birds mouth the vaulted timbers on as currently I presume they're fixed to your flat joists. Remove the flat joists alone and what's stopping the roof from pushing out.
 
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Hmmm, I’m not sure that the rafters aren’t being held together by the joists. Cut the joists and the roof may spread and pancake.

But what do I know? Set up a video camera and saw the joists out. Post it on here. Wear a hard hat.
 
If I interpret the photos correctly, there are two sets of joists... 1 original set for the flat roof and one new set that tie the feet of the new rafters. Is that right Dave?

If so and you only want to remove the lower original set then you should be fine. You mention veluxes and a vaulted ceiling though, so do you want to remove both? If so it'll depend on the construction of the new roof, specifically whether you have a structural ridge beam. Unfortunately this is pretty unlikely. Can you take a wider photo of the new roof in this area? Does the extension have a gable end?
 
There is only 1 set of joists.

The extension is single storey lean-to type, the wall plate is attached to the house with the rafters on top and is shown in the first photo.

Yes, there is a gable end.

I've attached a photo of the gable end hopefully this should provide a better view.
 

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The rafters are fixed to the joists and in turn the joists are stopping the roof spreading. Remove the joists and your roof pushes out as you have no wall plate or seat cut. You need a wall plate on the internal skin along with straps then the rafters need birds mouthing onto this to stop the spread of the roof. Whether that can be done in situ or maybe in stages I don't know.
 
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Thanks for the help all. Could I replace the joists with collar ties higher up in the roof or is that not safe?
 
As CJ has said, you just need to ensure that the forces at either end of the rafter are downward onto a wall plate at either end, rather than allowing the weight on the rafter to push the top of the wall out. This has been debated on here before and some would argue that seat cuts aren't strictly necessary as long as the fixings are good enough. It's hard to see from your photo whether you have a small wall plate, or just a batten that was nailed in place for convenience of assembly.

What I'd do is take a few bricks out of the gable and slide a wall plate in (say 150mm x 50mm) just below the top of the rafters and bolt it in to the wall, then to avoid having to take the first few rows of tiles off for nailing, fix the rafters to the wall plates using 300mm long noggins screwed sideways in to both.
 
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Something else to ponder is whether you may need to double up your rafters around the velux windows once your openings are formed and how you plan on doing that.

It'll also need insulating and a good vapour barrier with ventilation above insulation as it doesn't look like you have a breather membrane.

It's certainly not as easy as just removing the joists and voilà.
 

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