A gambrel has two pitches per side: a steep lower slope and a shallow upper slope. The classic barn proportion is a 20/12 lower and a 7/12 upper, with the break, or knuckle, at the midpoint of the half span.
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What are the standard gambrel angles?
The traditional layout inscribes the truss in a semicircle, which gives four equal 22.5 degree cuts and slopes of roughly 60 degrees lower and 30 degrees upper. Many modern sheds use a steeper lower and shallower upper for more loft headroom.
Where do gambrel roofs fail?
At the knuckle. Water concentrates at the pitch transition and the flashing there is the first thing to go. Detail and maintain that joint carefully.
Does a gambrel need engineering?
For anything beyond a small shed, yes. The break point creates unbalanced snow loads and the tall gable ends usually need engineered bracing.
