Alternate method instead of ridge beam and rafter ties...

[Jmeng1026]

See the attached drawing.

A client wants to remove the existing attic trusses over her garage and install a 2x12 floor system and 4' knee walls and then rafters to get more room above the garage. The contractor wants to use rafters instead of trusses. I know typically rafter ties or a ridge beam is installed but in this case rafter ties won't work because the knee wall is only 4' tall. A ridge beam could be installed but the columns supporting the end of the ridge beam would be right above the overhead door and would have a very large load on it.

I thought about installing a couple columns splitting the ridge beam span in thirds but that put a pretty large load on the floor joists. The floor joists with the columns above them would have to be 14" lvl's instead of regular 2x12's. This may be the way it has to be done but I am wondering if there are any other options.

Thank you.

 

[CANPRO]

Putting a beam in the floor system to pick up a post from the ridge beam isn’t a bad option. You could also put the beam up in the ceiling and have a short post to the ridge beam.

I built a shed a while back and I wanted the open space in the ceiling for storage and didn’t want to block access with a bunch of collar ties. I used a few beams to pick up the ridge beam. Worked pretty good, easy to build and left me with a pretty nice open ceiling to store my lumber. Picture is below...I may have gone a little overkill, but as a structural engineer I couldn’t risk having a sagging roof line

 

[oldestguy]

I know a structural engineer may not want to think this way, but take a look at a typical roof system made of rafters and plywood sheathing placed in "brick-work" fashion. Given good nailing of the plywood, that roof side is darn near as stiff,in the plane of the roof,as if the whole roof side was one sheet of plywood. Then at each gable end place sufficient ties, such as using the same staggered plywood sheets, that the eaves of the roof are tied to keep from spreading. Heck for a garage, I'd sure give it a try. It may take a little more in the way of longitudinal strength from bowing in the top of that knee wall, but I doubt it.

 

[oldestguy]

Let's say you don't trust the co0nnections from the corners of the roof eves to the end wall or to the other eave. Nothing in this layout stops you from running a few straps from each eave corner to the opposite to take tension A little thinking and the connection to that eave corner can be made to work.

 

[4thorns]

How deep is the garage? (floor joist span) You also have to bear in mind that you are putting floor load on the garage
door header as well.

Doug

 

[WARose]

I may be missing something.....but if you don't want to send a heavy (vertical) concentrated load to the joists.....why not just omit the posts and let the double top plate take the vertical load? (Assuming the headers and studs can take it.) You'll need to account for the horizontal load at the ends by connecting the rafter ends. (Possibly via the floor joists.)

If memory serves, the IRC doesn't even require a ridge beam design if the slope is 3:12 or greater.....just a certain thickness.

 

[BridgeSmith]

How much headroom is needed/wanted for the bonus room? From the diagram, it appears to be room for collar ties about halfway up the slope while still providing more than 8' of vertical clearance. They may need to be doubled up and/or deeper than a typical rafter tie, but an option to look into.

Another option that may be feasible is connecting or extending the knee wall studs down into the garage wall below to create a moment-resisting knee wall, similar to the older style balloon framing.

I'm a bridge guy, so I'm not familiar with the connection strengths, framing details, etc. of residential construction, so if I'm out in left field, please forgive my ignorant ramblings.

 

[XR250]

Another option that may be feasible is connecting or extending the knee wall studs down into the garage wall below to create a moment-resisting knee wall, similar to the older style balloon framing.

In my experience, this detail almost never works as a retrofit - maybe as new construction with 2x8 or 2x10 balloon framing. The connections to the floor system are difficult and the kneewalls will splay outwards regardless. Go with the full-span ridge beam and swap out the garage door header, jacks and kings as needed.

 

[CANPRO]

You can avoid replacing/reinforcing the existing header beam if you put a new beam in at the roof level to support the ridge beam. New beam would be just to the inside of the exterior wall.

 

[Jmeng1026]

Can I call for 20' long 2x12's to be installed at the 8' ceiling height? I know rafter ties are typically installed at top bottom of the rafter where the rafter meets the top plate but in this case the 2x12's would be about 4' above the top plate and about 4'-9" from the peak. See picture.

 

[CANPRO]

You can do that, but the higher your collar tie, the less effective it is. Play around with different elevations of your tie and look at how it changes the bending in the rafter, the tension in the tie (close look at the connections) and the horizontal movement of the walls.

 

[BridgeSmith]

Jmeng, that's what I was envisioning as the proposed collar tie. I just didn't know what size they would have to be, or if the rafters would need to be larger than typical to handle the moments generated by restraining them in the middle.