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I need to specify some framing details for a dormer. It's a bit unusual: it's in unconditioned space. The outside dimensions are 8' wide, with a total span depth of 11' (9' to the nearest support). The total inside height will be just 6'. Above will be plywood and a metal roof (very lightweight) at 6-12 pitch. No insulation or inside finish is needed.

There's no room for a parallel chord truss, or really much of anything. The finish height of the roof above is fixed.

Framing it like drawing A with a tension member in the truss would be great, except I need headroom.

Detail B shows the entire thing hung off a ridge beam, but then it's a pretty big beam for such a small space. The ridge beam must be supported on posts at each end.

Is detail C done, and if so where would I find documents on the approach and prescriptive beam sizes? Detail C imagines a steel gusset at the ridge to prevent outward forces on the walls. All the forces take up by approach A's tension member must be taken by the rafters and the steel strapping.

cathedral ceiling framing

No snow load. Under 4 psf dead load including ply on roof (metal roofing).

The host roof itself matches 3.12a from the 2018 American Wood Council WFCM https://www.awc.org/pdf/codes-standards/publications/wfcm/AWC-WFCM2018-ViewOnly-1904.pdf The question relates to detail C: is it done, are there off the shelf metal brackets, and pre-engineered details?

  • Put the brace between the walls to absorb the load. You say the roof is lightweight - what about snow? – Solar Mike Jul 31 at 7:46
  • I've seen sheds done to detail C in folded steel except the joint of the rafters to the wall is also reinforced against bending – Jasen Jul 31 at 9:24
  • Not sure what an 80s TV star has to do with this question. – JPhi1618 Jul 31 at 13:25
  • @JPhi1618 the dormer needs maximum headroom, to avoid the problem that befell (or created) the 80's TV star :-). – Bryce Aug 1 at 7:16
  • @SolarMike added snow load details (no snow load) – Bryce Aug 1 at 8:04
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If it's not in the framing manual, what you do is go and talk to a structural engineer, they'll find a design that gets you the shape you need.

Achieving your goal may involve bespoke welded steel portals in addition to the engineer's fees. or may get away with a wooden frame with just steel reinforcing plates.

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