|
| Author |
Message |
Josh8880
Member
|
# Posted: 11 Jun 2014 06:57
Reply
Hi all,
I'm building a 10' by 14' shed style cabin in NY. The high wall is 10 1/2' tall, and over the span of 10' it slopes down to the low wall which is 7'. It is all 2' by 6' construction on 2' centers. I will be using 2' by 8' rafters. I planned on notching them, but have seen several low slope applications like this where hurricane rafter ties are used, and the rafters are not notched. Is that a short cut worth taking? Thanks!
|
|
toyota_mdt_tech
Member
|
# Posted: 11 Jun 2014 09:05 - Edited by: toyota_mdt_tech
Reply
Well, I know a 2X8 can span 12 feet, on 16" OC, not sure on 24" OC. I would think notching the rafters would somewhat reduce the amount of wood and reduce its span capability? I wonder if just anchoring them down with hurricane straps then adding a wedge to spread all the wood load from just a single small point. This may be a question for Mt Don. 
IMHO, I wouldn't notch it unless I went to a 2X10, unless you do 16" OC, then I think you would be fine on the notched 2X8. But either way, I'd alwasy anchor each rafter down at both ends with the hurricane strap.
|
|
MtnDon
Member
|
# Posted: 11 Jun 2014 15:19
Reply
Snow load is the question?? See MAP
AWC Rafter calculator for spans and different species, grades of lumber, spacing, etc. ......
Use the map link to find the snow load for your location. Use that as a minimum Live/Snow load in the calculator. Dead load = 10 psf as a minimum. Rafter deflection is usually L/240. Take the calc result as a minimum rafter size or the absolute maximum span however you look at it. I might be tempted to go up a size just to be sure to cover some of the possible crazy weather stuff that we seem to be getting into.
Think high winds too; including how the floor structure is to be held down to the ground. 90 mph is design load in most parts of the country You can find that in the NYS IRC Chapter 3.
If the rafters are not notched to fit over the upper plates, the roof load is then going to be concentrated on those narrow points of contact, where the sloped rafter contacts the upper plate. That is not good as the wood fibers will likely be crushed and they will not crush evenly.
Hurricane ties are still a good idea to help make the rafter to wall connection as solid as possible.
|
|
bldginsp
Member
|
# Posted: 11 Jun 2014 16:34
Reply
Notching the rafters does not reduce their span capability, since it is the width of the rafter at its center of span that takes the most strain. Don is correct that unnotched rafters concentrate the load on a corner, but also they have nothing to prevent them from literally sliding down, which hurricane clips are not designed for. A notch gives the rafter a nice level spot to sit on and stay put. They are easy to cut by ganging all the rafters together at once and making a few careful cuts across them all with a circ saw, then clean up with chisel. I suggest you don't cut this corner, it won't save you much. IMHO
|
|
Don_P
Member
|
# Posted: 11 Jun 2014 21:40
Reply
I agree with all the above. The birdsmouth "notch" is not actually a notch in the sense of what the notching rules describe and limit. There are some codes that do apply to that detail though. The bearing area created by the birdsmouth needs to have a bearing area (the level seat cut) of at least 1-1/2", this is to prevent the crushing MtDon mentioned. The birdsmouth notch should never extend inboard of the wall. There needs to be at least 3-1/2" of wood remaining above the notch extending out to support an overhange of 2' or less. In other words you need at least a 2x4's worth of wood cantilevering over the wall. A notch on the upper end of the rafter should be the minimum 1-1/2" seat cut and no more as it does induce splitting. Do not overcut these notches with a skillsaw, finish them out with a handsaw or jig saw.
On a shed roofed structure you can avoid notching by cutting the tops of the studs at the roof angle, ripping the corners of the top plates and nailing the unnotched rafters to the angled top plates.
Hurricane ties top and bottom either way you go to restrain uplift in the wind. A poorly tied 8x12 shed roof can travel over 100' down a pasture in a 70mph wind... or so I've heard 
|
|
Josh8880
Member
|
# Posted: 12 Jun 2014 06:30
Reply
Very helpful. Thanks everyone. That tells me what I need to know! Josh
|
|
|
