stumped on rafters

[atomicskier]

Hi all,
   I've just got my plans for my 20x30 cabin and am stumped in two places (one dumb, one not so). I checked with the county and while we only have a 30lb/sf live load for snow we have a 100mph 3 second wind gust rating. my plans are for a shed style roof (one single slope) that's about a 3/12 pitch. our house will face into the dominant winds which means there will be plenty of chances for the wind to get under the lip of the roof. I assume I at least need hurricane ties, but I don't know whether I should consider also going from the 2x8 specs to 2x10? I don't want to put anything closer together because the modern design of the house means that my only insulation is between those 2x8's (and I know that's not a lot of insulation).
   Also, I'm a little stuck in finding how I should be laying out my rafters. the plans show a 22'8" total roof and the way the bearing wall sits there's a 10' span and a 12' span. forgive the newbie question, but can I simply do this by overlaping my 2x8's similarly to how I'd do my floor joists, or do I need to go find an engineered 22'8" long rafter? every time I search shed roof rafter layout I get articles for garden sheds not a shed style roof. I'd love a link if anyone has one.

thanks!
Mark

[alex trent]

No answers to your questions but here is a link to a simple, but educational presentation on did and calculating pressure on a house.  It is missing note or narration so up front is lacks a bit, but as you get into the calculations it makes sense and is practical.  there are other more "mathematical" and complicated ones, but this is a good start and give you just about all you need to know.

http://www.eng.auburn.edu/users/staylor/wind_loads_SBC_pt1.PDF

[Don_P]

If the bearing wall load continues vertically down through some type of support to the ground then yes you can lap the rafters. I did that on the current project the rafters would have been over 30' long. We did put hurricane ties on the bearing wall as well.

[atomicskier]

Thanks Don. The plans call for a 2x4 12o.c. bearing wall and it's directly above the beam which is supported by posts so the load would carry right through. would I do the standard 2' overlap like I would in floor joists?

[Don_P]

I really wouldn't have more than about 6" hanging over the support on either side of a lap. I understand the thinking, you are trying to weave the two simple beams into one continuous beam.  What often happens is that it still really is two simple beams pivoting over the center wall. The overhanging end kicks up when the main span deflects down under full load. In a roof I doubt you'd know and I can't really see it being a problem but in a floor it can create a squeak.

[atomicskier]

thanks for the info. now you have me curious -  are you saying that in a floor you'd only go about 6" over the beam? I always thought floor joists were to have 2' of overlap (not hanging over the beam, but 2' of contact)

[MountainDon]

Here's how to think of the overlap.  When a load is applied to a joist the joist deflects downwards, more in the center than at the support of course. When there is a large amount of overhang (cantilever), as in a situation where there is several feet of overlap over a central beam, the end of that joist will move up. If there was 12 feet of joist between the two supports and 6 feet of overhang, we would expect the end to move up half the amount that the center, between supports, moved down. Attempting to stop that movement by nailing the overlaps together can achieve only so much. It does not take much movement between two members to create a squeak. Hence you really only need sufficient overlap to provide sufficient bearing surface. Three inches is all that is called for, but a little bit more won't hurt. But not several feet.

Hope that makes sense.

Refer to R502.2 Design and construction.   scroll down to drawing.

[atomicskier]

great info, thanks!

[JRR]

Joists could possibly have a different additional requirement if they support an interior load bearing wall ... in which the joists may be doubled and overlapped in the shear planes to help resist the downward thrust.  (Doing this calculation and design can not be done at my pay scale.  LOL!)