Ken wrote,

"All I can see in Mr. Corkhill's illustration is a hipped roof with gablet. This in no way resembles a gambrel roof."

Is this an extract from an American book?"

Yes, it is an illustration from an American book, published by Stein and Day, Scarborough House of New York. The Gambrel IS a hipped gable.

Gambrel is a Norman English word, sometimes spelled gamerel, gamrel, gambril and gameral meaning "a crooked or hooked stick". A Gambrel is a stick or piece of timber used to spread open and hang a slaughtered animal by its hind legs. Gambrel is also a term for the joint in the upper part of a horse’s hind leg, the hock. The shape of the spread legs, with a stick between them, looks like the form of the letter 'A'. The Dutch colonized Indonesia and borrowed this roof form from there, naming it 'Gamberil due to the resemblance to the slaughterhouse equipment - the sill crossing between the hips at the base of the 'gablet', as Corkhill calls it, being the part similar to the stick spread between the animal's legs.

Sometime in the mid to late 1800's the term got misapplied, in a tract housing advertisement I believe, to the Mansard roof form, which comes in a 2-side and 4-side variant. I asked a French campanon his terms for both these roof shapes, and he answered 'Mansard' for both. 'Case closed' as far as I'm concerned.

The Mansard may also be called a curb roof in the US is there is a fascia prominent at the fold of the roof planes.

For other American sources on this, you may wish to consult:

Dictionary of Americanisms by John Russel Bartlett, 1848, pg. 166 (also readable online through Google books - look it up): Gambrel, “A hipped roof of a house, so called from the resemblance to the hind leg of a horse which by farriers is termed the gambrel”.

-Dictionary of Architectural and Building Technology 4th Edition (by Henry J.Cowan and Peter R.Smith, published in the US by Spon Press 2004. Library of Congress ISBN 0-415-31234-5) you will find it correctly illustrated (Google has this book available for online reading too -see pg. 134 in the text), to wit: gambrel roof "A sloping roof similar to a HIP ROOF, but with the addition of small gables part-way up the end sloping portions".

So, the illustration provided by Mr. Corkhill is in fact, in every respect, resembling a Gambrel roof. That's is why the word GAMBREL is appended underneath the illustration.

Personally I just prefer to use the term hipped gable in preference to gambrel, which has become confused in popular idiom.

MTF, as far as the two pitches on a Mansard are concerned, the French terms for those are brisis for the lower steeply-pitched roof section, and terrason for the slacker-pitched upper roof section. FWIW, I think the shape of Mansard you built was handsomely proportioned. The shape of the roof can vary quite a lot, as you noted, and I also don't like the ones where the lower pitch dominates, or those designs where the upper pitch is at all steep. I like to layout the Mansard roof using the half-circle method.

I wrote more extensively on the topic of the word 'gambrel' - my postings are currently found on page 3 of the General forum section, under the title "Gambrel roof design".