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I am building a small outbuilding (wood-fired sauna) that is 8 feet by 16 feet. The design is based off a friend who made a similar structure, just four feet shorter.

For the roof trusses, he used 2x6s for the top chords cut at 45 degrees (12:12 slope) and then a 2x4 braced to one side. This 2x4 sits on top of the walls' top plates. The nine total trusses will be at 24 inches OC. We live in southern Wisconsin in the US, so we see a fair share of snow but nothing like what they see even an hour north.

Truss

This avoids the birds mouth cuts, which is great, but I haven't seen anything like this in the internet research I've done -- most of what I've found uses birds mouths or something similar to the below picture (minus the king post), and always 2x4s for the top chords. Is my friend's design good enough to roll with or should I reconsider?

Internet truss

calcium3000
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    Technically speaking, a king post does more-or-less nothing, from what I recall of truss theory. For a sauna, I'd suggest doing a "high-heel" or "raised heel" truss that adds depth at the wall so you can put lots of insulation in - or else build the walls a foot higher and drop the ceiling joists down in the walls for the same purpose. You don't want that thin spot in the insulation. – Ecnerwal Apr 22 '21 at 13:17
  • Good point, but I'm not sure I'd want a ton of insulation in a wood-fired building of that size. You'll have an oven. – isherwood Apr 22 '21 at 13:18
  • @Ecnerwal my friend said that the steep roof gave him enough room to crawl in and lay insulation. He made a small hatch that he could crawl into on one of the outer trusses. Says he needs to watch his beer intake or else one day he won't be able to fit :) – calcium3000 Apr 22 '21 at 13:23
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    The point wasn't about access--you can insulate from above before you close the sheathing--but about heel height. if you want R38 you won't fit it with a basic rafter situation. – isherwood Apr 22 '21 at 13:24
  • @isherwood gotcha. I'll probably just put up 3.5" of mineral wool (R-15) and however much else I can fit on top of that without sacrificing airflow. And a big oven is basically when I'm striving for :) – calcium3000 Apr 22 '21 at 14:09
  • Yeah, "crawl in" to insulate is nuts! Insulate before you put the finished ceiling on – FreeMan Apr 22 '21 at 15:43
  • If the king post is not used, you might consider adding a collar tie near the top. – r13 Apr 22 '21 at 16:27
  • No collar tie is needed for such a tiny building. The rafter tie is more than adequate. – isherwood Apr 22 '21 at 18:40
  • @isherwood: "I'm not sure I'd want a ton of insulation in a wood-fired building of that size" -- sure you would. More insulation is always better. No reason to let the building get too hot (though, for a sauna you're going to want it pretty warm). Just means you get to burn less wood for the same temperature. – Peter Duniho Apr 23 '21 at 05:49

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For small buildings like this I'm not a load calculations guy. I'm a seat-of-the-pants experience guy. This is because lumber is usually grossly oversized for such short spans anyway, so it comes down to your carpentry techniques.

Yes, that design is plenty stout from a lumber standpoint (even 2x4 rafters are plenty), but you must fasten the joints together very well. Due to the steep nature of your roof and the lack of direct rafter bearing (without a bird's mouth cut) there's a lot of shear force on that connection. Nearly all the roof load gets carried by the fastening.

I might use three 2-1/2" construction screws from each side of each joint. For a lesser sloped roof with 2x4 rafters I've even used timber screws through from the top into the wall plate (with countersunk heads). Ecnerwal rightly points out that construction adhesive is a good idea here. I've used it for OSB gussets. It'll add a lot of rigidity to the connection.

Whatever you decide, it needs to be stout. If you bounce your weight on the peak and there's any flex at all in the joints it's not good enough. Do that right and you'll be golden.

One other tip: Build all trusses from the same prototype (jig) and mark the ends so they stay oriented. Trusses are rarely perfect and if you swap ends with any you'll have a wavy roof and fascia.

isherwood
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