Attaching 2x8 Rafters vertical through screwing into header/top plate

[Cob31]

What are your thoughts on screwing through a 2x8 rafter (bird mouth notched) on edge vertically down the entire length with a 3/16" pilot hole, 10" structural screw? Top down to connect to header, beam or top plate? Will this connection weaken the strength of the rafter? I'm using FastenMaster 10" Flat Head screws

My application is for a porch (where all timber will remain visible) similar to the one specified in this closed thread: thread507-349500
And as mentioned in that thread there is very little about this online.
I've seen wood framing guys do this often but they are typically using 4x timber or larger. I'm wondering if a 2x can support this connection type.

There are a lot of demonstrations going around of the Simpson SDWC truss screw being used to replace hurricane ties, but they are all screwed into the rafter from below on an angle upwards starting in the header or top plate screwed into the rafter. Is there a reason we should be screwing upwards into the rafter and not down through the length of it?

 

[phamENG]

Ease of construction. That's the big reason. And it's a good retrofit method, too.

Coming down from the top is doable, but it'll be a lot harder to build. Aligning the holes through the depth of the rafter will require a jig and take more time. Just a matter of what the fastener free look is worth to the owner.

Make sure you provide lateral resistance through another means - a screw won't be enough.

 

[dauwerda]

It's also going to be pretty tough for the building inspector to see that they are installed once the roof sheathing goes on. If they can't see it, they don't know it's there and will probably make you add additional fastening that they can see. Maybe discussing with them before hand and taking some pictures would also be a solution.

 

[DoubleStud]

I personally would only do that if I use thicker rafters like a 4x. I would just toe screw it (4, 2 on each side). You wont really see it because they will be close to the dropped beam below it.

 

[Cob31]

phamENG The porch roof is connected to the home on two sides by 2x10 ledger boards (90 degree corner of the house) which we're relying on to anchor the structure's lateral loads. I haven't questioned the lateral resistance because of this, is this a reasonable assumption? Rafters at 16" OC, purlins at 24" OC and 16 Guage metal roofing. Your comment makes me second guess the design and whether I should suggest knee bracing (which is not currently in the design).

 

[phamENG]

Lateral support of the rafter, not the entire structure. What's keeping the rafter from rolling over at the support?

Of course, lateral design of the structure does have to be considered. A ledger can certainly be part of a competent load path.

 

[DoubleStud]

PhamEng, if it is just a porch with lower roof where this roof is not really going to resist load from upper floor diaphragm, how much load are we talking about here? Just lateral load from wind on this porch roof? Kind of small dont you think? If the rafters are only 2x, I would probably put blocking between rafters at the support. I have used thicker 4X rafters where I dont put blocking in between. It will be pretty hard to roll 4X rafters.

 

[XR250]

I have done it before using SDWC. Might have to pre-drill the rafter the diameter of the head if it is not long enough. If I was using a timber screw, I might pre-drill the entire depth for the shank.

 

[phamENG]

Small yes, but I have pried a connection like this apart with my hands before. Unless the rafter is sufficiently wide, code requires rafters and girders to be held in place at the support such that rotation is prevented:

Where the depth of a bending member exceeds its breadth, d > b, lateral support shall be provided at points of bearing to prevent rotation.

I don't think a single screw is sufficient to manage this. If your rafter is flat and you have a 75plf shear load in your diaphragm, without blocking you have a 100lb load at the top of your rafter (assuming 16" o/c) with a 7.25" moment arm. The resisting couple will be a screw in the middle of the rafter and compression on it's edge, so a resisting arm less than 0.75". That's a tension load in the screw of more than 967lbs. Add that to concurrent uplift on the rafter, and I seriously doubt the screw can handle it. So if it fails like that, you have to rely the screw in bending, cantilevering out of the support (assuming the failure mode was not thread tear out). I have doubts about that working.

 

[Cob31]

Got it, I was having a moment's doubt, but the structure's lateral support is sound. The rollover may be an issue that I need to add blocking for. The only design elements that are keeping the rafter from rolling other than the through-rafter-screw is a 2x6 fascia (18" from the connection point) and purlins (closest one is 6" up the rafter) , It sounds like blocking is a good idea for these 2x8 rafters. I'll just have to ask whether fasteners or blocking are more visually appealing to the owner, I would use those diamond shaped hurricane ties.