Rafter and wall plate connection

[Kristofer-O]

Hi guys,

I'm building myself a garage and I wanted to get your opinion on wall thrust and the rafter and wall plate connection. All the dimensions are in mm.
As you see on the picture, I want to connect the rafter to the floor joist using a 45x145 mm material. I was wondering is there a concern that the outward thrust that the rafter exerts wants to bend that piece outward? At the moment I have the collar tie there to prevent the rafters from pushing outward but what would happen if I remove the collar tie? Would then the outward thrust bend the rafters apart?



Appreciate all thoughts

 

[Eng16080]

At the end walls (which is what your detail shows) there would be no outward thrust because the wall studs are directly supporting the rafter.

I assume your question is concerning locations other than the end walls, though. In that case, I'd create a structural model or do some hand calcs. to determine the collar tie force. Assuming the collar tie is properly sized, and the connection between the collar tie and rafters is adequate to transfer the thrust force, and the rafters are adequately sized to resist the concentrated load at that location, then I wouldn't be concerned with the outward thrust at the top of the wall.

Without the collar tie, then yes, the rafters will definitely want to spread outwards and you would need to find a means to restrain them.

I probably wouldn't use a collar tie like you show. The higher up on the rafters the collar tie is located, the less effective it becomes. Ideally, the rafters would be connected directly to the floor joists, although you'll obviously need to modify the geometry to make that work. You could also extend the floor joists to meet the rafters at the overhang as another option.

 

[jayrod12]

You could also extend the floor joists to meet the rafters at the overhang as another option.

To me, this is the best option and the easiest to implement and design around.

 

[jdgengineer]

To me, this is the best option and the easiest to implement and design around.

I don't love the idea of the seat cut in the rafter occuring inside of the "truss". I would make sure to look at those dimensions closely.

 

[Brad805]

I would start with the typical section and not the end walls. If you have overhangs on all sides the top of your roof joists will be lower at the endwalls for your ladder framing unless you notch your roof joists for that framing. Cutting your roof joists for ladder framing on flat is common, but I have never been a fan of that.

What is your code requirement for insulation if heated? The heel height is driven by that requirement. 312 seems large unless you need a lot of insulation. A common detail for rafters like this is a birdsmouth connections.

 

[jayrod12]

I don't love the idea of the seat cut in the rafter occuring inside of the "truss". I would make sure to look at those dimensions closely.

It wouldn't? I would anticipate the ceiling/floor joists and the rafters to be lapped and so it's just nails/screws in shear. I don't see a seat cut happening at all in the proposed detail.

 

[jdgengineer]

It wouldn't? I would anticipate the ceiling/floor joists and the rafters to be lapped and so it's just nails/screws in shear. I don't see a seat cut happening at all in the proposed detail.

Wouldn't there be a seat cut at the exterior wall for bearing support of the rafters? Or do the rafters solely land on the cantilevered floor joist?

 

[Kristofer-O]

The idea was to have no seat cut on the rafters and have the rafter and floor joist on one plane above each other and use the small vertical piece lapped over them to connect the two. So it would be nails in shear.

 

[Ron247]

As mentioned by Eng16080, I am assuming you are talking about away from the gable end of your structure.

Your drawing appears closer to what is called "stick-built" which is very standard method of wood framing. The location and names of some items in your drawing do not match the typical profile of stick-built (conventional construction). Is your "floor joist" a ceiling joist or is there really going to be floored attic area? Joists (ceiling or floor) typically set on the same wall the rafters are notched into. You show rafters completely on top of joists and not notched. This can be done, but it is more work.

What you call a "collar tie" by definition has to be in the upper third of the rafter, not in the middle. I tend to put them directly under the ridge board, which you do not show one in your drawing. It ties the rafters together for wind uplift and is not intended to resist much (if any) thrust. If you lower it as mentioned in another comment by someone, it is a joist, not a collar tie, and it can assist in resisting thrust.