Anytime you open up your roof or cut into the structural framing of your house, you'd better have an excellent idea of what you're getting involved with.
When building a dormer, you'll be doing both. Jay gives good, cautious advice.
Here's a link to an on-line article over at Fine Homebuilding. It's a trade magazine, thus it assumes you have some basic construction knowledge.
Read through it several times. Draw many, many scaled drawings. Know what type of framing you have in your house, and what may happen when you cut into your structural framing.
If I recall, JLC.com has an on-line article about building an eyebrow dormer. Some info will be applicable, some over your head.
Rob Thallon (sp?) also wrote a very nice book, "Graphic guide to Frame Construction." Lot of pics...lot of drawings. Many variations. It may be a nice reference book for you.
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