How often to a reasonably need to put a ceiling joist to prevent wall blowouts?

[phalynx]

On my man cave build, I am building 2x4 walls that are 10' high.  I plan on having a 4/12 roof pitch and I want to have a cathedral ceiling.  Obviously a normal ceiling would have ceiling joists ever 24" or less to support the sheetrock.  If I am going to sheetrock the rafters directly, how often do I need put a ceiling joist or a beam as it would be in this case.  I figure I could easily double the 2x8's and go every 4'.  Could I also go every 8'?  I would like as few as needed. I don't have any building inspections or permitting so I am asking for what is reasonably safe.  Not wanting to cut corners on safety but would like an open ceiling as much as possible.  Also, in the South, no snow loads.

Thoughts?  I am fishing here.  Hoping I can pull in a PEGG, John or other well experienced quality opinion.  All opinions are appreciated of course.

[Don_P]

If its a shed roof none, if its a gable every 4' unless the ridge is designed as a beam.

[phalynx]

If I put a kingpost on the double 2x8's to support the ridge beam, would I be able to go every 8'?

[Don_P]

We're stepping outside of prescriptive into an engineer's realm. A "normal" kingpost actually hangs down from the ridge and holds the bottom chord up, another name for it which might be more descriptive is a kingrod truss. Doesn't mean your way won't work but let me strip it down first. The triangle formed by the 2 rafters and a tie is rigid. It can support a ridgebeam without a post. If the ridgebeam is sized to carry the load of half the roof width x 8' then this can work. The shear force at the connection between the rafter and ties is going to be high, that connection will have to be well made. When you cut down on redundancy in a frame each member becomes more critical. If one of the ties fails it will throw much more load on its neighbors.

If the double ties are checked as beams with a point load in the middle, the same load as above, you can install them and get some of that backup. If a heeljoint lets go the ridge will land on the kingpost which is adequately supported by the ties.

[phalynx]

Don, Thanks,.  Sounds like I just need to go every 4'.

[phalynx]

How is this supported?  Is it the king post trusses?  I don't see a load bearing ridge beam.  Not sure what to make of it.

[Don_P]

I'm not entirely certain what I'm seeing there so take it with a big grain of salt. The left side can't bow outward because an intersecting roof and walls butresses that side. I'd sight down that right wall and then sight down it in a few years. If it gets much roof load it looks like the ridge would start to sag and the right wall would bow outwards.

What is your building's width (rafter span)?

[phalynx]

My building is 20' wide.

[PEG688]

   #1: How is this supported? 

   #2:  Is it the king post trusses? 

   #3:  I don't see a load bearing ridge beam.

   #4:  Not sure what to make of it.

  #1: There's a lot going on there, using some or parts of all of whats going on for the maximum strenght of the whole unit.

  #2: It could be considered a king post truss.

  #3: No,  it's a common ridge, BUT it has beam like aspects. I suspect it's jointed above the king post trusses and gusseted in a couple of directions. Can't see clearly but there are gussets up there. 

  #4: Here's my take on what I see. All a educated guess with some conjecture thrown in for good measure.

  The "trusses" are doubled with a spacer 2x12's . They project below the "common" rafters. The lighter colored 2by nailed to the sides of the trusses are sheet rock or ceiling board "backing"/ something to nail the finished ceiling to.   



The roof must be metal as they have 2x4 purlins on top of the rafters to create a air space , to screw the metal down to , and to act as roof sheathing.



You can also see some tension cables or rods running diagonally in that created space as well to resist racking of the roof framing. At least they look like rods , to straight to be electrical sub-panel wiring and theres one on each side, so I'm thinking tension rods.



On the right hand side I see two valleys so the roof is a sort of T shape, those valleys oppose each other and stiffen that section of the roof , the main section in the photo uses that  stiffen along with the king post trusses and tension rods.



One other thing  that I assume as I can't tell for sure , but that wall of glass on the far end MUST have a steel beam as it's to small in height to be a LVL or glue lam and span that far ,AND be expected   to pick up the weight of the POST they have under the far end gable ridge board.



It is FAR outside the prescriptive code , it has no shear walls on the far end , and nothing about it looks "simple".



That being said,,  I'd say with all of it combined  as  a unit , it's hell for stout. Remove any of it,,,,,,,  and the overall structural soundness would suffer.



It's not built to the limits but it is a stretch and not at all common.



Where did you find or take this photo?






 

[phalynx]

PEGG, thanks for joining in with Don.  I found it while trying to research the question I asked at the beginning of the post.  "How far apart can I go on the ceiling joists on a 20x32 building with 2x4x10 walls.  The roof is a simple gable roof and the ideal spacing in my mind for "the look" would be every 8'.  I am concerned about the walls blowing out due to the roof load pushing down and out on the un-joisted spans.  It appears that 4' is safe with doubled 2X's.  Wondering what I would have to do to get 8' safely?

[PEG688]

  A timber framed truss every 8' would do it ,

Like this hammer truss.




Or a simpler type , it could be made to plane in with the roof , or really the roof could be made to plane in with the truss.





Or you could do a similar situation as your photo shows every 8'.

Of course if you need a permit , you'll also need a engineer to stamp it.

But I'm sure it would work, but I'm not a engineer. But even with a snow load any of those continuous bottom cord trusses would be strong enought to resist the outward forces, in all but the most far north areas.

Of course with global warming and snow in Florida this may be a coming issue.

Sorry for the politcal aside/ joke wise crack.

     

[phalynx]

So do you feel with 2X framing, 8' is not wise or reasonable?  I don't have inspections or permits here.  But again, I do want to be safe.

[John Raabe]

If you do some of the widely spaced timber or king post trusses - say 8' o/c, then you have to go over the top of that with rafters (purlins?) going the long direction of the building. These will hold the insulation and support the roof sheathing, roofing and interior finish. These will likely be 2x6 or 2x8 (spanning 8') and be spaced 2' o/c since that is standard spans for roof and insulation materials.

Not withstanding the handsome exposed truss you have thus built a more complex and expensive roof.

If you want an inexpensive cathedral ceiling, do standard rafters at 24" o/c and have a built-up rafter tie at every 2nd rafter (4' o/c). The ridge need not be a structural beam. This is a time tested technique that is shown in my 20' plans. The rafter tie should be in the bottom 1/3 of the ceiling height.



Here's an image from this 1-1/2 story cottage http://countryplans.com/smf/index.php?topic=1503.msg13982

[phalynx]

It sounds more and more after each reply that double 2x8's every 4' is the widest I can go on the spacing using 2X material.  The rafters will be 24" o/c.

[John Raabe]

Yes, you can safely double the specified rafter and space it twice as far apart - they will carry about the same load.

But you will want to deck this with something like 2x6 T&G decking to span the 4' and any insulation will go topside.

[Don_P]

I see more responses since I started doodling, but what the hey its all talk at this point.

The tension straps in the building are simpson flat steel strap. Ply would be stronger, more connections.

PEG's last drawing is right on. For educational purposes, the bottom chord of that true truss is in tension. The kingpost is in tension, hanging from the peak. The web struts are in compression, with their feet buried in the dangling kingpost they are pushing back up on the rafters, turning them into 2 shorter spans.

This is another view of that idea. Trusses and purlins across.

Let's take a stab at it and see what happens.
8'x20'=160 sf bearing on each truss. No snow but for wind and dead load I think I'd still figure on 30psf so ~4800 lbs on each truss.

This is a diagram of the truss at 20' wide, 4/12 pitch with the 4800 lb load distributed on the truss (add a zero to all numbers on the drawing) The resulting forces and sense are shown. Checking them against lumber dimensions and connection strength would be the next step if the design is right.

[phalynx]

All, that's great information.  I see where my thought process derailed in the discussion.  I think for my purposes the answer has been given and seconded.  However, for further educational discussion, my thoughts were to do as follows:

Build the roof with rafters 24" o/c (2x6) connected to a ridge beam.  2x8 ceiling joists 24" o/c.  With my question resting only around the ceiling joists changing and not the rafters.  By doubling the 2x8's I can safely put them ever 4' o/c with the rafters still going every 24" o/c.  The question came in as if I could do the ceiling joists every 8' instead while still keeping the rafters 24" o/c.  My thinking was that the ceiling joists would keep the walls from blowing out at the point where the joists are connected.  However, in the distance between the 8' o/c ceiling joists there may be not enough structure to keep the wall from bowing/blowing out.  After further discussion, it appeared that I couldn't go 8' o/c unless I had a structurally supported ridge beam.  This got me thinking that maybe I could build the 8' o/c beams as trusses and it would then support the ridge beam.  I don't believe this would work either as the walls may not be strong enough.

Am I on track thinking it is not reasonable to do the 8' o/c as described above because of the conditions it would create or is it actually possible.  The difference I see is that you 3 are thinking timber frame where, as John describes, the purlins would be large and spanning a large distance.  I am thinking using only trusses to support the ridge beam and transfer the load down on the trusses and the rafters 24" o/c would help support the roof.

Wow,  re-reading that confuses me.  I think I may end up doing it as the picture John posted.  But, for education, I'd like to hear the answer.

[PEG688]

  I figured you'd get confused, lots of info here and it all mixed and mis-matched.

  So legally or  for safely/ better structure ,  John would like to see no MORE than a 4' OC spacing on standard framing lumber, I also think he's missing that part where you'd still has 24"OC rafters.

Don and I, at least I am ,I may be reading Don's input wrong, are suggesting a timber truss every 8' ALONG with a standard 24" OC standard rafter layout.

In all cases the roof would be sheathed with 1/2" CDX  at least / or  OSB (7/16")   (((( Side bar on sheathing::::::I like 5/8 CDX for better roofing nail holding strenght mainly , also on 24" OC I like the feel of the 5/8" as well. Most folks won't spend the added dollars for the 5/8" sheathing. )



  Another thing that could be added to the mix would be interior braces walls inside 90 deg. to the main walls. I would think some where in the interior there would be some partitions that tie to the exterior walls, those could be sheathed with 1/2" CDX or OSB to tie the exterior walls to them they being attached to the deck and the wall would add to you mix of wall spreading resistance items.

Again to be legal / code-able a engineer would have to approve.

But IMO interior brace walls would help do what you'd like to do.


 

   
   

[Don_P]

Don and I, at least I am ,I may be reading Don's input wrong, are suggesting a timber truss every 8' ALONG with a standard 24" OC standard rafter layout.

Nope I was thinking trusses with horizontal purlins (both our pics)

And yup, we're hitting you fast and hard  

Sooo, Half my roof load just dropped onto the walls directly (the bottom half of the common rafters), half bears on the ridge, which bears on the trusses.

The center 10' of the building width X 8' truss spacing=80sf... 2400 lbs bearing on the truss
Working that out, the heeljoint tie strength (the connection between rafter and bottom chord on the truss) needed is still around 3600 lbs. A steeper pitch would lower those forces.
I would be comfortable with southern pine 2 ply 2x8 trusses with 1/4" plate steel in the middle of the sammich at the joints and four 3/4" bolts in each the rafter and the tie. That is not the only connection option, for one move to 2 steel plates on the outsides and two 5/8 bolts into each member works. The "timberframe" truss is made of 3x8's... so not a real massive structure.

The ridge would support 2400 lbs uniformly along it. I'm pushing too hard with a single #2 SYP 2x10, a double passes handily.

2x8 rafters pass easily.

This could actually be done with steel rod for the bottom chord and no king or webs... just another tangent thought.

I agree with PEG above, well mostly.. EVERYONE uses 5/8 ply on the roof. That's a story and I'm sticking to it  .