A roof development drawing is a geometric development, or unfolding, of a roof used to find the true lengths and angles of the roof surfaces, hips and valleys, which appear foreshortened in an ordinary elevation.
Characteristics
On a normal plan or elevation, a sloping roof surface is seen at an angle, so its measurements look shorter than they really are. This foreshortening makes it impossible to read off true sizes directly.
- A development “folds out” each sloping surface into its real, flat shape.
- It reveals the actual length of every rafter, hip and valley.
- It gives the correct angles at which members meet.
By working the geometry through on paper, the draughtsman or carpenter recovers the dimensions that the roof will have once built, rather than the distorted ones an elevation would suggest.
Application
The roof development drawing is the tool that lets a carpenter mark out roof members to exactly the right size and cut them to the correct angle before assembly. It is especially valuable for hipped and complex roofs, where hips and valleys run diagonally and cannot be measured straight off a plan. Used together with the roof plan, it ensures that every timber fits precisely when the roof is put together.
Related terms
- Roof plan
- Hip
- Valley
- Roof structure
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