Raising the rafter ties in one room but not in an adjacent room

[Mark.alan65]

I am not sure how to frame this, so I thought I would ask for help.
I would like to raise the rafter ties in the master bedroom. Across from the bedroom is the bath, that my wife wants to keep the original ceiling height. I know under normal situations you would lap the rafter ties and rest them on the center loadbearing wall. So would I take 2x4's and go up to the ridge, nail one end of the rafter tie to that, then put an additional cut 2x4 up to the underside of the tie? I could then take another cut 2x4 and go from the top of the tie to the ridge board. It would basically be like building a wall up to the bottom of the ridge board. The rafter ties would be install in the bottom one third.

[Don_P]

That's gonna take a picture 

[Mark.alan65]

I will try and do one and post it.
Let me try again. Lets say I have 2 rooms across from each other, and are separated by a wall. In the current configuration the rafter ties are nailed together and resting on top of the seperation wall. If I wanted to raise the rafter ties in only one of the two rooms how would I frame the center loadbearing wall to support the raised side of the rafter ties. Could I take 2x4's and go from the top plate of the loadbearing wall to the underside of the ridge board and attach the raised rafter tie up on the new 2x4?

[Mark.alan65]

This is a very poor drawing, but maybe it will work.

[NathanS]

I think you have to run the new raised rafter ties from rafter to rafter. The way it is drawn, I would be worried about that wall being under a 'rotation' kind of stress.

[akwoodchuck]

If it's a true load-bearing wall under the peak of the roof, that cancels the thrust....no ties necessary...(although I would want the studs to be continuous from floor to ridge)....hack away...

[ChugiakTinkerer]

I don't interpet it as being load bearing, just a support for the lapped ceiling joists, which also serve as rafter ties.

R802.3.1 in the IRC covers celing joists and rafter ties.  I read the section as requiring a continuous horizontal tie running from rafter to rafter.  To do it like you have drawn, if you extend the raised ceiling joist all the way across to the other wall then you have a rafter tie.  The existing ceiling joist on the room drawn on the left then just becomes a low ceiling that doesn't serve as a rafter tie.

If your rafter tie in the room on the right is raised, it could potentially impact the rafters themselves.  The rafter tie cannot be higher than 1/3 of the height of your roof above the the top plate.  The footnotes in Table R802.5.1 have a factor for how your rafter span is reduced by how high the rafter tie is.  For maximum head room in the bedroom, if you raise the rafter tie up to 1/3 the roof height then your rafter span is reduced by multiplying max span for the existing by 0.67.  This could result in a need to sister in additional rafters if they end up being undersized.

https://codes.iccsafe.org/public/document/IRC2015/chapter-8-roof-ceiling-construction

[Don_P]

I agree with akwoodchuck, if it is a loadbearing wall supporting the ridge then no ties are needed put them wherever you want. The floor needs to be capable of bearing its loads and half the roof in that scenario. Otherwise the tie does need to run across from rafter to rafter and then you can drop another ceiling joist below that on the lower side if desired.

[Mark.alan65]

Thanks everyone. I do know that the wall down the center is tied into a massive beam that runs the length of the house. In fact there are 3 beams that go from the back to the front one on each side of the house and one down the middle. Then cross beams tie the three together. So supporting it should not be an issue.