Rafters between studs?

[Drew]

Hi folks,

I have a question on sizing for framing and roofing.

I am building a straw bale building using non-load bearing bale wrap construction.  That means I will build the frame to support the weight of the roof and wrap the bales around the frame.

The building will be 24' x 18' and have a shed roof with a 2 in 12 pitch.

I plan to frame the walls with 4"x4" studs on 48" centers.  On top of the studs I plan to put in a 4"x6" header the entire length of the building, front and back, to support the weight of the roof.  Windows and doors will be installed in bucks in the bales and won't interfere with the studs.

My roof will be 2"x6" 24' members on 24" centers.  I will be spanning 15 feet between the front and back headers over the floor (the 18' width of the building includes the ~3' (18"x2) for the bales.).  This means that  only every other rafter will land on a 4"x4" stud and the others will land in the middle of the 4"x6" header.

A 4"x6" is good for a 4' span (and how), but am I okay with resting every other rafter on the header between the studs?  It really seems to make intuitive sense that it would, but I'd like to make sure.   

I thought about using 4"x4" for rafters, but a.) I couldn't find a span chart with 4"x4"s on it, and b.) muscling around 24' 4"x4"s might be a little tricky on a ladder.  (They have to be 24' for the 2' overhang we want to protect the plaster walls from rain.).

Thanks for the advice.

Drew

[MountainDon]

As for the rafters sitting on the upper timber in between posts that would be the least of my concerns.

The first thing that occurred to me was "where are the 24 foot 2x6's going to come from?"  Around here the longest I've ever seen are 20 footers.   

The second thing that hit me was that I thought your span, using 2x6's, was too great. My span table agrees with my suspicions. Using doug fir select it looks like a 14 foot span would be a stretch even using the lowest loads that it goes down to (10# live, 5# dead). 

So unless someone else can point out the error of my thinking it would seem that your proposed roof may have some design deficiencies.

span calculator

[glenn kangiser]

Looks right to me, Don.

[MountainDon]

My sinuses are killing me today/tonight.... Does that mean he's okay or I'm okay.... it hurts to think/see/sneeze.... ooooh

[Drew]

Right, Don.  It should be 2"x8".  Ouch.  I wrote it down right but remembered it wrong.

I'd like to go with the shed roof for simplicity's sake.  However, as you point out, 24' members might not be possible to get and a design change may be in order.

Here's what immediately comes to mind:

    a.  Can you splice rafters?  Perhaps I could splice two 12' members together.

    b.  Perhaps I could make 4"x6" rafters out of 2"x6"x8's with a 1/2 ply spacer sandwiched in the middle.  If I did that, would a 4"x6" (Or even a 4"x4") span 15'?  My span chart is not that sophisticated.

    c.  I can knuckle under and make trusses for a gable roof.  That gets to be a lot more complicated, but not impossible.  I'd get good storage loft.   

The county has no snow load requirement at my elevation but a 75 mph wind design speed.  I don't know how that translates into dead load.  Any pointers?

Fortunately it's an ag building and I don't have to get the designs reviewed again by the county.

[glenn kangiser]

I guess I was a bit ambiguous wasn't I.

You're OK as I see it.  I think 24's will be very hard to find and not quite make the span.

With no codes to deal with and no snow they might make it but 2x8's would make the span-- still the problem of finding 24 footers though.  Since the outside 2  feet or so is only roof over the bales maybe a splice on the ends would do it.  Also -- part of the load is cantilevered.

[glenn kangiser]

I wouldn't try to splice the middle.  I'd get at least 16'ers and splice both ends - or maybe longer and splice one end but span the entire room with the solid board some way or another.

Maybe alternate end splices every 2' -- opposite end splice every other one?

[MountainDon]

Just exactly what is the space from the inside edge of one upper beam to the other going to spec out as?

Check and see what lengths are available locally. If you can get 20 footers, it seems that should allow overhang at one end while still allowing the other end to bear on the opposite beam. My calculator at AWC (click here) comes up calling for a minimum bearing length of 1/3 inch (doesn't seem like much but that's what it spit out.)

Then you could add on a section to each rafter to create the overhang on that side. Since there's no snow requirement a good number of nails should hold that okay.

[Drew]

After setting back for the 2" plaster and 4" header, I need to cover 14' from the inside edges of the headers.  I'll have a plywood box beam on top of the bales on the outside of the header that can stabilize and even bear some of the overhang weight.  Keeping the member between the headers continuous and splicing the ends sounds like the right approach.  It even makes for minimal changes to the design.

Thanks for the help guys!

Drew