How to build a mount roof? ...

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The certainty that ou don't know a gabled roof from a hip roof suggests that perhaps you could use for a moment professional help. Hire a framer. It can go and get too complicated to explain in the space allotted here.

How to cut Thermocool sheet ?
Get a book about roof framing.
Cutting out a roof, be it Hip,Gable, or other is a bookish skill that even many house framers do not do very well.

Read as much as you can about it and afterwards practice what you learn on small peices of wood.

A framing square is an big tool for the job.If you can swot up how to read one you should have few problems.
Hairline crack contained by a wound bar?
Answers:    if you mean a peaked ceiling, you inevitability a beam to turn acrross the ridge, and either butt the rafter into it, or cross the rafter over the top of it