Gable Dormer Design Principles - Where to find?

I'm adding a third story to a two story row house. The third floor will be a mansard roof so I'll be putting in two gable dormers. Other than walking around and looking at other houses in the area, are they any other resources (books or websites) that I could use as a guide in my dormer design?
The house is located in the Annex of Toronto and was built in 1884.
Thanks in advance.
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snipped-for-privacy@gmail.com wrote:

http://www.google.com/search?hl=en&lr=&q=dormer+roof+design+mansard
R
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on an existing 122 year old townhouse can be dangerously surprising. I have seen owner rehabbed buildings collapse or lose their back walls as work was being done. I assume new floor framing rather than reusing the roof framing. Here in Boston I added floors and penthouses to several old buildings and ALWAYS consulted a structural engineer. Dormers are the least of your worries. EDS
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eds wrote:

An old school chum of mine was renovating a huge 3 family in Cambridge, MA. Really taking the thing to town, and he was opening up a two story space on the first floor and putting in some old barn beam framing. He was up on the roof cutting some ceiling ties and the whole exterior wall peeled off and slammed into the street. He nearly had a heart attack. Amazingly luck, though - the wall missed hitting anything. No cars were parked below (oddly enough there almost always are) and no one was walking by. Seems those ceiling ties were holding the wall in place.
R
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