Roof Construction and Roof Plans

Roof Construction and Roof Plans

Grade 12 takes the building to its roof, and roof geometry is where civil drawing meets the solid geometry from earlier years.

Members. A rafter runs from the wall plate to the ridge. The ridge is the horizontal top member. A hip runs from the ridge down to an external corner; a valley runs down an internal corner. Purlins run horizontally to support long rafters. The eaves are the lower edge, usually overhanging.

Pitch is the angle of the roof surface to the horizontal, commonly 26° to 45° in South African housing. It is stated on the drawing and drives every other roof dimension.

The roof plan is a view from directly above showing the ridges, hips and valleys as lines and the roof surfaces as areas. Hips appear at 45° in plan when the two adjacent roof surfaces have equal pitch — that is a geometric consequence, not a drawing convention.

Worked example. A hip rafter's true length cannot be measured from the roof plan: the plan shows only its horizontal run and ignores the rise. It is the same true-length problem from Grade 10, applied to timber someone must actually cut.

Core checkpoint: The roof plan gives the layout. True lengths of sloping members must still be constructed.