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King post truss calculations?

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#1 Scott123

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Posted 13 May 2021 - 08:40 AM

I'm building a 10'x14' foot observatory this summer, and am working through the materials list. I live in snow country (Maine), so I need some input on sizing up truss chords and rafters.

 

  • The open span will be ten feet.
  • Rafter spacing will be 24".
  • All joints will be reinforced with 1/2" plywood plates.
  • Roof will be metal.
  • Purlins will be let into the rafters
  • Building will be unheated.

 

Questions:

 

1: Can I use 2x4s? I know, bigger is better, but do I need 2x6 construction?

 

2: Roof pitch. I am planning on a metal roof, with either a 5/12 or 6/12 pitch. Thoughts?

 

Attached is a back of the envelop sketch of trusses. Top truss is king post, bottom, king post with webs (or struts).

 

Thank you for your help,

 

Scott

king truss.png


Edited by Scott123, 13 May 2021 - 08:42 AM.


#2 Couder

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Posted 13 May 2021 - 08:51 AM

We don't have any zoning restrictions here, I guess you don't either. Even so, assuming this is a roll off roof I would go with the extra weight and use 2x6. Even here in the Missouri Ozarks we sometimes get heavy snow, and it doesn't take but one roof collapse from the weight of snow to be a catastrophic event.  


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#3 GaryShaw

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Posted 13 May 2021 - 09:08 AM

The rafters, center post and diagonals could be 2x6 but the bottom cord of the truss should be 2x8 since it is picking up the loads above at it’s midpoint. I think you could delete the center post if you use the diagonals. Plywood gussets at the joints would be good. You should confirm with a structural engineer or competent builder in your area. With an unheated building, you’ll need to consider the static snow load. Here in Mass, I believe it’s 40 psf. 
enjoy

Gary


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#4 Scott123

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Posted 13 May 2021 - 09:26 AM

We don't have any zoning restrictions here, I guess you don't either. Even so, assuming this is a roll off roof I would go with the extra weight and use 2x6. Even here in the Missouri Ozarks we sometimes get heavy snow, and it doesn't take but one roof collapse from the weight of snow to be a catastrophic event.  

We have a code enforcement officer, but I don't know who it is, or even if he cares. The last time I saw one in this town, I got a building permit from him, never saw him again.smile.gif



#5 GaryShaw

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Posted 13 May 2021 - 09:45 AM

You’ll want a building permit if for nothing else than to protect you if there’s a fire and insurance claim. You don’t want the insurer refusing to cover the loss because they had the excuse that there was no permit. Still, the major issue is having someone competent check your framing concept.

Gary


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#6 Couder

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Posted 13 May 2021 - 09:52 AM

You’ll want a building permit if for nothing else than to protect you if there’s a fire and insurance claim. You don’t want the insurer refusing to cover the loss because they had the excuse that there was no permit. Still, the major issue is having someone competent check your framing concept.

Gary

There are no permits required here, you can't get one even if you ask. You can build, add, wire, roof, and put in plumbing by yourself or get help, does not matter in the least who does it. You can start a junkyard, as people around here have. Insurance still pays. I'm assuming it is the same where he is.  


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#7 lee14

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Posted 13 May 2021 - 10:12 AM

2x4's are perfectly fine for the rafters. That's a relatively short span. Collar ties will give better support against the roof sagging or spreading under a heavy snow load. I would space the rafters at 16 inches though, not 24. I used this arrangement on my 30 year old ROR in Central NY, and several feet of snow have never proved to be an issue.

 

Lee


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#8 HunterofPhotons

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Posted 13 May 2021 - 10:12 AM

2 x 4s are more than strong enough for a roof this small.  It's common to have them on trusses three times this size.

The bottom drawing is much stronger.  

If I were you I would leave a space at the rafter tops to fit in a ridge beam.  The ridge beam ties all of the trusses together into one rigid structure which is important in a moving roof.  In that layout only the gable ends need a 'king post'.  The ridge beam is the only member than needs to be larger than a 2 x 4.  In essence, you are hanging the rafters off of the ridge beam.  

You might want to raise the lower chord on the truss if you need clearance issues.  It doesn't have to be all the way to the bottom.

Even though this is a small project you might want to make a jig for the truss construction.  Some simple alignment blocks would suffice.

If you let in purlins to a rafter you weaken the rafter.  Take that into consideration.  

In addition to nailing or screwing your plywood plates add a bit of construction adhesive.

 

dan k.

 

P.S.

I almost forgot.  Since you will have no roof sheathing you need something to resist racking.  The simplest way is to add diagonals from the top of a gable end rafter to the bottom of the rafter at the other end.  Add them to the bottom of the rafters where they won't interfere with anything.  You don't have to go wild with these components, 1 x 3s or stiff metal strapping will suffice.


Edited by HunterofPhotons, 13 May 2021 - 10:23 AM.

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#9 lee14

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Posted 13 May 2021 - 11:39 AM

 

  

If I were you I would leave a space at the rafter tops to fit in a ridge beam.  The ridge beam ties all of the trusses together into one rigid structure which is important in a moving roof. 

dan k.

 

P.S.

I almost forgot.  Since you will have no roof sheathing you need something to resist racking.  The simplest way is to add diagonals from the top of a gable end rafter to the bottom of the rafter at the other end.  Add them to the bottom of the rafters where they won't interfere with anything.  You don't have to go wild with these components, 1 x 3s or stiff metal strapping will suffice.

Yes, indeed. A ridge beam should be used with rafters and collar ties. A 2x6 is sufficient, certainly not just a 2x4.

 

Sheathing should definitely be used. 3/8 plywood should be fine, 1/2" is better (but a bit heavier).

 

Lee


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#10 Scott123

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Posted 14 May 2021 - 08:52 AM

Going by the consensus, I'll go with the 2x4 construction, and change the spacing to sixteen inch on center.

 

Sheathing, some of the observatories I have seen on this website just go with a metal roof over purlins, no sheathing was seen on them. I wonder if any of them have racking problems?

 

Thank you all for your input, it is appreciated!

Scott



#11 speedster

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Posted 15 May 2021 - 02:31 AM

If your connections are rigid, the king post has no load.  Neither tension or compression.  If you use 2x4 top chords (rafters) and let the purlins into them, you now have 1-1/2" x 2" top chords.  Better to put purlins on top and leave the full 3-1/2" top chord depth.  Something has to resist racking.  That can be plywood sheathing or diagonal bracing in the plane of the roof sections.  If you really want to do a structural truss, you can also to a scissor truss and get some more head room.  The simplest thing may be to not do trusses and instead do 2x6 horizontal pieces, which are now ceiling joists, and frame whatever slopes you like on top with 2x4.  The 2x6's are then doing all the work.  The thing I'd be more concerned about is sagging between wheels since all you have there is a 2x4 laid flat.  2x6 ceiling joist with 2x6 rim joists and diagonal bracing in the plane of the ceiling joists would solve the racking issue as well as the sagging issue and you'd only need 4 wheels.


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#12 Scott123

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Posted 15 May 2021 - 11:01 AM

If you really want to do a structural truss, you can also to a scissor truss and get some more head room.  The simplest thing may be to not do trusses and instead do 2x6 horizontal pieces, which are now ceiling joists, and frame whatever slopes you like on top with 2x4.  The 2x6's are then doing all the work.  The thing I'd be more concerned about is sagging between wheels since all you have there is a 2x4 laid flat.  2x6 ceiling joist with 2x6 rim joists and diagonal bracing in the plane of the ceiling joists would solve the racking issue as well as the sagging issue and you'd only need 4 wheels.

I had considered a scissors truss, but worried that if it wasn't done correctly, the downward force from the roof loads would spread it, racking the walls out of place.

 

The flat is actually going to be a 2x6, and I was going to add a girder plate. Also, I was considering a roller wheel under each rafter. This would increase the expense, and the rolling friction, but eliminate any sagging. I've added a picture (not to scale) with the girder plate.

wheel detail.png


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#13 555aaa

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Posted 15 May 2021 - 12:03 PM

A rolling roof has to be stronger on shear than a fixed roof because there is nothing constraining the two sides of the roof from moving. In my ROR the metal roof takes all the shear load but I notice the movement when opening. Also I used 2”x3” thin section steel tube instead of wooden rafters with the purlins bolted to them.

#14 555aaa

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Posted 15 May 2021 - 12:05 PM

My other suggestion is to put at least some rollers on springs. You otherwise never have all the wheels working.

#15 lee14

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Posted 15 May 2021 - 01:20 PM

There's no need to go to extremes here, or make the roof structure more complicated than it needs to be. The simplest effective approach is a few sheets of plywood over 2x4 rafters with a ridge beam. That eliminates the possibility of racking, and provides a nailing surface regardless of where you need to fasten the metal roofing. It's a relatively small structure and will remain rigid. 

The header that the wheels exert downward force is fine, the wall studs will keep it from ever sagging. 

 

The roof imaged here has been in service for three decades, unaffected by heavy snow loads, even without the use of collar ties.

 

Lee

Attached Thumbnails

  • roof detail.jpg

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#16 Scott123

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Posted 15 May 2021 - 03:52 PM

Lee, that looks like a solid setup! What is the span? And the roof pitch? 

 

Thank you,

 

Scott



#17 lee14

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Posted 16 May 2021 - 06:48 AM

The pitch is about 4/12. The observatory is 10 x 12 overall, the rafters are 7 feet long. They extend a foot or so past the horizontal plates (that the wheels are attached to), so there's an overhang past the walls to keep moisture away from the foundation. They're fastened with screws, not nails. The unsupported span is about 6 feet, short enough to never sag under any conceivable snow load. The ridge beam is a 2x6.

 

Lee


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#18 t-ara-fan

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Posted 19 May 2021 - 09:20 PM

I know a guy who had trusses built by a lumber yard.  They were engineered for the snow and wind loads expected at the location. It wasn't that expensive, they were all exactly the same, and run true to this day.


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#19 Scott123

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Posted 20 May 2021 - 08:45 AM

I know a guy who had trusses built by a lumber yard.  They were engineered for the snow and wind loads expected at the location. It wasn't that expensive, they were all exactly the same, and run true to this day.

I emailed my local lumberyard, they work with a truss company. Lead time may be prohibitive, we'll see how it goes.

 

Thank you,

Scott



#20 HunterofPhotons

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Posted 20 May 2021 - 09:45 AM

Scott,

You're making this much more complicated than it needs to be for such a small, simple structure.

Just build what Lee14 shows in his above image.  

Use some metal connectors for fastening the pieces together and use a master pattern piece for marking the rafters (take the line).

Any kind of plywood will do for the roof sheathing.

 

dan k.

 

P.S.

Don't forget to 'crown' your ridge beam, i.e. any curve in the beam should point up and you might want to use a 4 x 4 (or doubled 2 x 4s on edge) for the lower cross member for the rafter tails.

Four wheel units are plenty for a roof this small.


Edited by HunterofPhotons, 20 May 2021 - 11:45 AM.

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