framing an eyebrow dormer

I am an architect and have been asked to design the framing of an eyebrow dormer. Any diagrams or descriptions on how to do this?

Are you looking for pictures of framing methods or descriptive geometrical methods, or both?

The Journal of Light Construction did a good article on eyebrow dormers (I think) within the last year. I'm pretty sure that Will Holliday's book on roof framing (also available from JLC) covers the framing of eyebrows. Lastly, the JLC online forum is often is good source for all sorts of strange framing details.
Brad

Hey SAA (Daniel),

I tried replying to your e-mail today, but the response couldn't be delivered to you for some reason - "couldn't find server" or something like that. So, I'll paste below what I wrote to you:

"The framing of an eyebrow dowmer need not be a whole lot different, for the most part, than a shed/pent roof dormer. One of two methods is usually employed: either the roof is sheathed and the dormer scabbed on to the top of the sheathing - a common enough solution in light framing. Alternatively, the opening is completely framed in timber. I have attached a picture of one solution to that framing from a German layout book - F. Kramer's "Grundwissen Des Zimmerers". I believe it will answer your question on how the main roof rafters can be supported as they intersect the eyebrow framing elements.

The real difficulty of the matter, it seems to me, is developing the profiles for the series of curved eyebrow joists as they reduce in size, pair by pair, going toward the plane of the main roof. There are some tricks for that, but it is really an exercise in descriptive geometry. There are also several different eyebrow profiles to consider, and the matter of whether the eyebrow projects in a straight line from the main roof plane, in a curve, or even askew to the roof plane.

Chris

If anyone is still following this topic, here's how I implemented an eyebrow dormer (at least the inside part so far!)...

pics and brief description of eyebrow dormer

Thomas, awesome job! Especially that tongue-and-groove.
I've never had the opportunity to build an eyebrow dormer. Some framers use plywood ribs for the arcs. The framing can also be achieved with purlins. The compound angles may be determined by projection or development as you did.
Here are some links to Excel and Javascript "cheat sheets".
Purlin/Square Fascia intersects Main Roof (Excel)
Purlin/Square Fascia intersects Main Roof (Javascript) . For an eyebrow domer the main roof pitch remains fixed while the Dormer pitch is assigned the slope of the curve at the point of interest. (Use MSIE4 or later for the web based calculator. It seems my code gives Firefox indigestion).

Again, I'm impressed by how clean your cuts are on the T&G. In theory the saw blade bevel would constantly have to change as it follows the miter of the arc (the curves would be similar to the sinusoids in the link below):
Development of Circular or Elliptic Dormer intersects Main Roof

If anyone is interested here is how I resolved the compound angles on the purlins. These actually were the solutions of the miter and bevel angles on the faces of square tail fascia perpendicular to the roof of the dormer. (Square fascia, purlins, who cares? Same thing). The method shows the use of ratio and proportion for those who don’t feel comfortable with trig and the angles can of course be solved by development.
This first link is the Purlin/Main Roof Intersection: General Solution ; the intersection of the ridge lines in plan may be any value.
Most ridge lines will intersect at 90° in plan. Here’s a link to an easier solution of the Purlin/Main Roof Intersection: Angle between Ridges equals 90 Degrees .
The cut the compound angle from the face of the purlin set in the plane of the dormer roof the angle on the purlin will follow angle 90° – P2a (i.e., the Main Roof/Dormer intersection follows the Valley trough line). The saw miter will be P2a, the saw blade bevel equals 90° – (C5m + C5a).
The roof ridge lines for these solutions are assumed to lie on a level plane. But many eyebrow dormers have sloping ridge lines ...

Thanks for the links to the compound angle calculators... those angles made my head spin (it's still spinning) back when I was doing the hip roof. Looks like you have it mastered. The spreadsheets available here on the guild website are handy too, but the hardest thing with using those is figuring out which of the fifty calculated angles you need. It produces lots of angles - which one do you set your skil saw to? Ah, that's the trick.

The trick with the tongue and groove miter joint, for me, was how to hide the inaccuracies. The hole in the main roof came out spot on, and then I sanded it with a belt sander and put a chamfer on the end of each main-roof board to cover up any mistakes. But each of the dormer boards has a gap of up to 1/16th of an inch, and you can see this if you stand in the dormer and look up and backwards. I could take a picture of that too... but who wants to see the ugly view? My original plan was to come up with a piece of compound trim to cover up the joint, but the joint came out OK, so I'm not even going to put trim on it. Solid wood is a wonderful medium.

I know this is a long-dead thread resurrection, but I came across a couple of great posts about drawing eyebrow dormer geometry on Sim Ayers' "Roof Framing Geometry" blog, which has all kinds of great stuff. Here are the links:

http://sbebuilders.blogspot.ca/2014/10/fledermausgaube-mit-gerader-kehle.html
http://sbebuilders.blogspot.ca/2014/12/the-eyebrow-roof-dormer-with-straight.html

Both of them describe the process for an eyebrow dormer with a straight valley in order to have greater flexibility with roofing materials, at no great loss to the overall appearance in my opinion.

That thread does go way back. Since then, Billy Dillon has been teaching a 3 day workshop, mainly at the Heartwood School. He also taught it out west, where Sim took the class. The German way(s) are really the best. Most stick framers just wing it, with generally poor results.

I'm available to pre-fab eyebrows and ship them out. Any size or shape.

I agree with Brad...this is an old one...as threads go...

Further, in concurrence with the comparison of "stick building" the entire general modalities of timber framing an "eyebrow" is more what you would find in shipbuilding or fine furniture making, even if the 50% or more of it is hidden...It's a different world to "general contracting."

I would also add, that much of it can and is done without a lot contractor type "measuring" (per se) as it would follow one of the lofting, templating, and/or "story poling" methods, reflecting levels of accuracy that you just do not find on most "modern buildings" built today in domestic architecture...

Yup, Billy/Sim's method is lofted/laid out full scale. So they (can be) fully prefabricated on the ground and lifted up as a unit.

And easiest way to frame is boatbuilding-style, with ribs. Mainly because the sheathing works far better. In contrast, winging it with stick framing with rafters is easy, but then sheathing that surface is ridiculous. Especially with plywood.

...from 14 to 23, I never touched a "tape measure," and lived in the world of templates, scribing, story poles and "lofting" from floor or wall.

First "eyebrow" was more "art and template" than ever the idea of measure and fitting studs and plywood. I so often watch in amazement (and amusement ) wrestl studs and sheet goods into place...Most of what the contractor does today is more out of "habit" than logic and/or "good practice."

These ancient traditions are simply brilliant in there simplicity, ease, and logic in acute accuracy...even with "green wood."