A roof plan is a view from the top of the house that shows the geometry in flat view of a roof. Drawing a roof plan is not a difficult task if you already have a grasp of basic drafting. A roof plan is derived from a floor plan and the exterior (elevations) views of the house.
On a drafting board, it is a simple matter of placing the floor plan under a transparent sheet, tracing the perimeter exterior walls, and then measuring beyond those walls using an architectural scale to where the overhang (fascia) of the house will be. And then projecting lines from the exterior views you can determine where one part of the roof comes together with the other in peaks and valleys.
In a CAD system, we would do the same thing. A line would be drawn around the parameter of the home, and then offset the distance of the overhang.
You would then measure to each individual peak of the roof and draw a line representing it. Then, if the roof has more complexity, valleys, crickets (little roofs that shed water from a possible water or snow trap), and shed roofs would be drawn using the same method. Once this drawing is completed, it can be used to draw a rafter plan showing how the roof system goes together.
There is a roof plan tutorial at http://internetarticles.net/drawing-an-architectural-roof-plan/
Other related searches would be: how to draw roof plan, How to Draw Architectural Plans, drawing roof plans, drawing your own house plans, roof detail plan, how to draw house blueprints
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If you want to learn to draw complete House Plans, go to http://residentialdrafting.net, which is the website for House Plan Drafting 101.