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Author Topic: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling  (Read 1020 times)

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Offline Spruce_Goose

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Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« on: January 04, 2018, 01:02:22 PM »
Hi all,

I am seeking advice on the 'do-ability' of taking a standard trussed roof and converting it into a cathedral ceiling using timber frame trusses. (possibly just 1 TF truss)

I have attached a sketchup model that shows to some degree what I am thinking (half model is hidden for clarity. Also end posts are omitted, and could themselves be trusses).

This is a retrofit. What is existing is an attached garage with exposed conventional trusses 24" OC. Wall framing is 2x4 and is exposed. 8' high walls/bottom chord of existing trusses.

My plan is to insulate, but also gain head room. One idea was to remove the conventional trusses (carefully), install deep rafters and a supported ridge beam to counter-act the thrust. The length of ridge is 28' so it would be a mighty long clear span. I do not particularly want a post in the middle, nor is there a footing there to receive it.

That is where the TF truss comes in. I suppose I am viewing this from the perspective of a conventional (non TF) framing system and simply viewing the truss as a way to displace the location of the center ridge beam post to the outside walls (two posts). That said, I am considering this also for the aesthetic and because of an interest in timber framing.



If the general plan does make sense, I would then need to decide on timber sizing and joinery. My inclination is to use joinery for the truss, but perhaps attach the truss to the posts with metal straps, fasteners, etc.

Additional insulation info if interested: I plan to place the bottom of the rafters flush with the TOP of the truss, so that the truss was completely inside the envelop and the rafters were filled with dense pack cellulose, shooting for as close to R-60 as possible with good air sealing. Additionally, the wall R-value is going to be boosted by building a secondary interior wall (double stud). The posts will complicate this detail, but I don't see it being a show stopper. The floor is a slab, but the perimeter is a 7.5" wide footing that goes (I think) 4' down.

Offline Don P

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Re: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« Reply #1 on: January 04, 2018, 07:24:59 PM »
Sure, this is a pic of some kingpost trusses installed underneath a roof to support the structural ridge. I dropped them about an inch under the rafters so I could pass 3/4 T&G over them and then  made small trim strips to cover the gap against the ceiling. The ridgebeam bears on the top of the kingposts. I made the post pockets out of built up framing in the walls.

Offline Spruce_Goose

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Re: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« Reply #2 on: January 04, 2018, 08:52:26 PM »
Ooh nice. Thanks Don. That appears to be what I am looking for.

Couple questions, if I may.

Is the ridge beam continuous or does it lap/splice above the king post truss? Fastening there?


What is your span between trusses?

and did you use trusses at the ends (gables) or just a single straight post down?


In thinking about trusses I realized I don't fully understand King Post Trusses. All the literature I can find indicates that the post is in tension and will aid in supporting the tie beam. I do not get how the post is in tension if the tie beam is not being loaded as it would be if it were a floor. In other words, if used in the scenario we are discussing, how is the post in tension (and how is it helping any more than just a simple triangle)?

Offline Don P

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Re: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« Reply #3 on: January 04, 2018, 10:52:22 PM »
Think of it as a simple triangle, delivering the load at the peak down the upper chords to the posts below. To keep them from being pushed outward tie a rope across the room from the foot of one upper chord to the foot of the other. Now the triangle is locked and stable. However the rope is prone to sagging at that span so tie a rope at the peak and dangle it down to the tying rope lift it back up into plane and lash it there. That is the purpose of the kingpost, or kingrod as it is also known and is probably a better term. Hopefully that became clear in the minds eye and it can be appreciated that since one cannot push a rope, those members described as rope are in tension.

The simplest form of truss is the triangle, the first progression as spans increase or member sizes decrease is to support the bottom chord by way of the kingpost, thus cutting the span of the bottom chord in half. The next place to halve the span would be to support the top chords at midspan with a compression member. From low on the dangling kingrod rope secure a pair of rigid struts the run back up to the underside of the top chords and wedge them in place. They bear that load in compression. We could cut the rope immediately beneath those struts releasing the bottom chord and those struts would still perform their function.

The ridgebeam in this case is a pair of 2x12's with splices alternating and occurring over the trusses. IIRC the trusses were 5 or 6' apart and so that size ridge was adequate for the span and load. That is something to consider in your design, the loads and reactions will be lower if you put more trusses up there. Not saying its necessary just another path to explore.

At one end the ridgebeam landed on a gable wall at the other it crossed a loadbearing wall and then spanned to a gable end wall. This was a stick framed home with timberframe elements in the public areas.

Offline woodworker9

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Re: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« Reply #4 on: January 05, 2018, 07:41:44 AM »
Really good and clear explanation, Don.  Thanks.
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Offline Spruce_Goose

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Re: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« Reply #5 on: January 05, 2018, 10:49:05 AM »
Yes thank you Don that was very helpful. I appreciate the design insights.


As for the king post truss: I like the rope analogy and your explanation of the progression from simple triangle, to king post, to king post with compression intermediaries (or struts?)

I have to admit though, that I still don't really get how the bottom chord (or tie beam) needs additional support beyond sag from its own dead load (or is that what its for; dead load?). You said, "the first progression as spans increase or member sizes decrease is to support the bottom chord by way of the kingpost, thus cutting the span of the bottom chord in half."

I get how the bottom chord is in tension from the top chords wanting to spread but I don't get how any live load would benefit from a mid-span support (king post). I'm sure your explanation is sufficient, I can just be a bit dense on stuff like this sometimes.

The only way I can make sense of it is that the king post is really only needed if one of the following three scenarios exist:
1) the bottom chord is loaded such as with a floor
2) the dead load of bottom chord is so great that it would sag too much over time or
3) there are other intermediary struts / compression members that are loading the king post directly, or indirectly by way bottom chord.

or also perhaps some overall stability in an off plane axis... and aesthetics.

In the end that probably leaves few reasons not to have the king post, but I am just curious from a theoretical live loading scenario if it would actually be beneficial if used in the ridge beam support scenario. Or if there is a better truss given the point loading directly on its peak...

Offline Don P

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Re: Hybrid Retrofit: Timber trusses to convert to cathedral ceiling
« Reply #6 on: January 05, 2018, 05:45:51 PM »
I think you are understanding it. Typically there was a ceiling and perhaps an attic loading the bottom chord. In the pic I posted we had a temporary plank floor on the bottom chord as we installed the T&G and I was happy the kingpost was there. The bottom chord would have probably sagged over time at that span. Aesthetics is also a big driver in many designs you see.

Another thing to consider when designing the truss is resisting the thrust at the heeljoint where the top chord drives into the bottom chord which is resisting the horizontal spreading force. This is typically the highest stress joint in the truss, begin joint design there. Usually if you can make that work the truss is buildable.

This is what it looked like on those trusses. The shear area beyond the notch at the white pine's allowable shear stress was checked against the stress at design load.


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