Wednesday, March 15, 2006

More on Mansard Roofs

About a month ago, I posted a lecture on the historic origins and merits of the Mansard Roof. In ways most subtle, it appears to have been my greatest success so far. Although if we're measuring by the number of comments posted, then my pre-Oscar silliness on my love of The Movies and growing giddiness over the film, V for Vengeance, which is being released this Friday, was my chef-d'oeuvre. Though to be technical about it, a post about movies on a blog dedicated to architecture is a bit outside my "oeuvre", if I do say so myself. That's why I consider my Mansard post to truly be my greatest Why Howard Laughed blog effort so far. As a matter of fact, at least half a dozen people and counting have found this blog by Googling "Mansard Roofs". Whatever gets them in the door...

Which brings me, in typical indirect fashion, around to the point of this post. In the comments of the Mansard post, someone recently posed an interesting question, which inspired my usual long-winded, curiously unresponsive, and know-it-all answer. I re-post both question and answer prominently for all to see. Here goes:
Luke S.: A little Q&A (I've got the Q). Is it correct as Second Empire Mansard roofs go, to have any section of the upper hip roof flat? I'm adding on to my house but local codes wont let me go high enough to raise the ridge when I make the house wider. To keep the same angles everywhere I would need a 6' wide section at the top to be flat. Any advice would be appreicated.

Howard, your modest moderator: (I'll take the A). To be technically correct, there ain't no such thing as a flat roof, or at least there shouldn't be. So while the upper facet of Mansard roofs often may appear to be flat, they really aren't. Low-slope roofs--where the slope is 1/4"/foot minimum, enabling water to run off quickly rather than linger around and find a way inside your house--are still very doable and possible with house projects. I myself designed an addition to a turn-of-the-century Victorian house three years ago that had a low-slope roof with a single-ply membrane system over a few inches of fire-proofed polyisocyanurate insulation. However, it wasn't building codes that I had to wrestle with, but zoning laws. In this particular case, the neighborhood was zoned with an historic character in mind, necessitating minimum slopes of something like 5:12 (5" rise per 12" run -- see? high school geometry does have its applications). To keep from blocking the one good window on the second floor that looked out over the nicely landscaped backyard, I recommended and designed an addition with a low-sloped roof and deep overhangs for sun control and general aesthetics. This meant a visit to my clients' zoning board, which was meeting to hear monthly zoning variance appeals. They looked at my design, listened to my rationale for the low-slope roof, pronounced it an excellent solution, and granted the variance. Piece of pie. Of course, your zoning requirements may vary. Three last caveats: While single-ply membrane roofing systems are just about the best low-slope roofing system going, they may not be recommended in your area for residential projects, and the warranty may not apply for this reason. Second, usable deck space over an addition is typically not permitted by many zoning laws in residential neighborhoods, except possibly over porches and sometimes garages. And even then, don't be walking on a singly-ply roofing system if you can help it. While the rubber membrane is tough as nails, it can still be punctured. And then all is lost.
As you might notice, I went off on a tangent about low-slope roofs, completely overlooking Luke's concerns about designing a roof that didn't get too tall. To keep it reeelly brief, a Mansard Roof might work handily for what Luke has in mind -- assuming that the "Mansard look" works.


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