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MntGoat
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# Posted: 15 May 2017 08:41pm
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Hi all,
I've got 2x10 rafters I will be installing. The ridge beam is two 14" LVLs lagged together.
The rafters are notched (think birdsmouth) at the top to sit on the ridge beam. I am using rafter ties on the bottom but I'm wondering how to fasten these at the ridge.
Thanks for the help
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ICC
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# Posted: 15 May 2017 09:25pm - Edited by: ICC
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Are the rafter ends already notched? Hope not, as there is good reason that rafters are usually cut at an angle and then nailed to the ridge. A notch where the upper edge of the rafter is fitted over the ridge creates a stress point where the rafter may under the right, or wrong, circumstances, split. Perhaps if most of the rafter width is above the notch the potential trouble is lessened. The method in the top of the image shows a notch that could be okay, but if notched higher s splitting point may be created.
The lower image shows the better solution, straps over the top help hold the roof assembly together against uplift forces.
Disregard the dimension on the image as they are just some example
Simpson makes some adjustable angle rafter hangers that could be used LSSU210
Couldn't find an image showing the don't do this way I mentioned
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razmichael
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# Posted: 15 May 2017 10:21pm
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I used appropriate Simpson rafter hangers (think they were the ones mentioned above) attached with the structural screws to the the sides of the LVL Ridge Beam and strapping over the top as per the diagram ICC included. Notched at the bottom along with rafter ties. No notching at the top.
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MntGoat
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# Posted: 15 May 2017 10:54pm - Edited by: MntGoat
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The rafters are oversized to begin with so I am not concerned about a notch. I am taking 4" x 1.75 out of each.
Wondering how to fasten. May end up using the same rafter clips.
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MntGoat
Member
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# Posted: 15 May 2017 10:59pm
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Also, if you are saying not to notch the top what's the difference doing it at the bottom where it meets the wall top? There's no difference.
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ICC
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# Posted: 16 May 2017 01:18am
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Notching in at the bottom means there is more material above the cut. So maybe if a split starts there is less likelihood of loosing enough strength to cause a big problem. That is conjecture, I am not an engineer. In all probability any notching in the upper end of a rafter may be bad practice.
However, the fact remains that rafters are never cut or notched at the upper end in normal construction, as was taught to us Seabees. In fact, it is important that the entire cut face, the face that is against the ridge board or beam, be fully supported.
I hope that is clear, in other words the entire cut face of the upper end of the rafter should be against the ridge board or ridge beam. If the lower part of the angled cut is not against the ridge, not supported by the ridge, there is danger of splitting from potential load forces.
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razmichael
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# Posted: 16 May 2017 07:09am
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i was not stating you cannot notch the top - not an expert either but I do think proper notching on the top for over the beam rafters is acceptable. A google search comes up with many examples of how to do it - including this diagram provided by "MountainDon" from back in 2008 (6th in the thread) diagram. I used the hangers to avoid lots of notching (I was making gambrel rafters which were a pain anyway).
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bldginsp
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# Posted: 16 May 2017 09:07am - Edited by: bldginsp
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I agree with ICC that notching the top less desireable. The bottom has to be notched but the bottom of the notch is supported by the top plate. Not so at the top connection.
That said, you can connect a notched rafter at the top with twist hurricane clips, and/or drill thru the edge of the rafter from the top and install timber screws or long SDS screws thru the rafter into the beam. For my woodshed (unengineered, simple, down and dirty) I cut small notches in the top of log rafters just as a reference point to locate them on the 4x6 ridge, then lagged them thru.
In any case, straps across the top are essential.
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MntGoat
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# Posted: 16 May 2017 09:54am
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Thanks for all the info.
I guess I will agree to disagree about the top notch. The top notch is supported by the ridge beam no different than the bottom sitting on the top plate.
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bldginsp
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# Posted: 16 May 2017 02:28pm
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Well just to split hairs on a gnats rear, the bottom portion of the rafter at the top plate rests on top of the top plate- so it can't split away. But the bottom portion of the rafter at the top, if notched for a beam, does not rest on a flat surface, it rests on a vertical surface, and it may not even be in contact with that, so under load it can split downward. If the top rafter is hung from the beam, it can't split because it is supported from the bottom by the hanger. The most common means of attachment is the top of the rafter cut at a vertical angle which lies against the ridge board or beam. If the heel of the cut contacts the board, it cannot split. If the heel does not contact the board but the top does, high loading can split the rafter. Of all these methods, the one most likely to cause splitting would be to cut a deep notch at the top to rest on a beam. The deeper the notch, the worse the problem. If you use this method, best thing to do is cut the notch very small, just enough to seat it on the beam.
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ICC
Member
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# Posted: 16 May 2017 04:33pm - Edited by: ICC
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Well said, that is more clear than the way I was trying to get it across.
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Borrego
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# Posted: 16 May 2017 09:31pm
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Yeah, bldginsp, that's a good explanation...I like to think of it (simply) as the rafters push against the ridge which pushes out and down against the top of the walls, which if framed correctly,  will push back and apply pressure against the ridge beam...keeps everything in place...
I know, kinda simple but......
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NorthRick
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# Posted: 17 May 2017 12:09am - Edited by: NorthRick
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Quoting: MntGoat I guess I will agree to disagree about the top notch. The top notch is supported by the ridge beam no different than the bottom sitting on the top plate.
I'm with MntGoat. A birdmouth cut at the top (ridge beam) isn't any worse than a birdmouth cut at the bottom (top plate). Both will weaken the rafter. However, in small structures the rafter size is often based on insulation and not structural.
If you go with rafters, and not trusses, it is most likely because you are after a vaulted ceiling or heated attic/loft. The rafters need to be wide enough to allow a reasonable amount of insulation to go in. With a relatively small span, this is usually much more width than needed to support the roof loads. So, small notches at the top or bottom are not going to be a problem.
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fastsled
Member
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# Posted: 17 May 2017 10:23pm
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Could you build this without a ridge beam? I have seen it done both ways?
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bldginsp
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# Posted: 17 May 2017 10:41pm
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Yes it can be built without a ridge beam. The issue of the rafters splitting at notches remains the same. The difference between the a ridge beam and a ridge board is how the roof is supported. With a beam, the beam is designed to take 1/2 the roof weight, which it transfers to the footings below the ends of the beam. With a ridge board, the board does not support any weight at all. The roof weight all goes to the walls via the rafters. This requires that the rafters form a triangle with the ceiling joists- a simple truss, so to speak. In this case the rafters just push against each other on opposite sides of the ridge board, which serves the sole purpose of aligning the rafters, and in some cases is eliminated altogether. A ridge beam doesn't need the triangle so is used for vaulted ceilings. In the following photo, what appears to be a ridge beam is really just a ridge board providing no (or limited) support, and the black steel rods supply the triangulation which supports the rafters.
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MntGoat
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# Posted: 19 May 2017 11:19pm - Edited by: MntGoat
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The top cut rests on either the top of the beam, or the verticle face of the beam. If it splits, the top will hold or the verticle face due to inward pressure. I'm not sure how you can lose. The beam is larger than the rafter and therefore has plenty of surface area to land against.
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bldginsp
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# Posted: 20 May 2017 09:28am
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Once the rafter splits, even if supported, it's ability to sustain loads is compromised.
North Rick is right- keep notches small and you'll probably be fine.
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MntGoat
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# Posted: 20 May 2017 11:27pm
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Sometimes when you write a post the person's tone isn't accurately portrayed so i wanted to say thank you to everyone who offered input on this topic.
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