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Offset Rafter Tie/Ceiling Joist

Offset Rafter Tie/Ceiling Joist

(OP)
I've got an existing building, with unreinforced multi wythe brick walls and a gable rafter roof. Client wants to raise the roof to increase the usable attic space.

We could take down the existing roof and replace with an increased pitch, giving us some more space. A different roof shape, such as a gambrel is another option.

Another option I'm considering is taking off the roof, building up new masonry knee walls, say 2-3 feet tall. and then replacing the roof. This would increase the space considerably by the time you are a couple feet out from the wall. In this case the attic floor joists would be offset from the rafters and so the roof kick out forces would have to be taken by the knee wall in bending. If I can get the capacity out of the wall section, rafters and bearing place connection, anyone see a specific issue with this sort of detail? It seems like it is kind of like a gambrel roof, just using masonry for the lower portion instead of more wood truss.

Thanks guys.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

RE: Offset Rafter Tie/Ceiling Joist

the roof kick out forces would have to be taken by the knee wall in bending

I would be real concerned about this detail and developin g ny tension in the existing brick wall.

I would try to go with collar ties to limit any lateral kick to the existing wall and the need for the detail. The lower the better...

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Offset Rafter Tie/Ceiling Joist

(OP)
Im concerned too, just putting out some ideas at this stage. the problem with the ties obviously is they intrude in the space, which is the whole point of the exercise. My thinking was if I could dowel into the existing wall and keep the tension stresses low, I may be able to bring the load down to the level of the joists.

How else is this sort of thing commonly accomplished? just trying to get more space in the attic. Dormers in the existing roof?

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

RE: Offset Rafter Tie/Ceiling Joist

Could you install a ridge beam and raise the walls?
Or possibly put a kicker (tension) in at the top of knee wall down to the floor at some angle?

EIT
www.HowToEngineer.com

RE: Offset Rafter Tie/Ceiling Joist

(OP)
A ridge beam is tough because the roof runs the length of the building, so the span would be too long. A kicker or some kind of vertical wood truss inside the knee wall is a possibility, though it does eat into the floor space a bit.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

RE: Offset Rafter Tie/Ceiling Joist

Regarding the ridge beam - Possibly add some interior columns and 'transfer' beams in the ceiling?

EIT
www.HowToEngineer.com

RE: Offset Rafter Tie/Ceiling Joist

(OP)
That is a good suggestion. I'll take some of these ideas back to the architect and see what their inclination is.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

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