Gambrel Mansard Truss

A plated timber gambrel (mansard) truss: each side of the roof has a steep lower rafter and a shallower upper rafter that meet at a break, with a ceiling tie, a king post at mid span, a post under each break and a diagonal from each break to the foot of the king post, drawn as members without connector plates. Span, lower pitch and overhang, Member thickness and chord and web depths

Details

Category: Roof Framing › Timber Roof Framing › Gable and Skillion Roofs › Timber Trusses

By Huu Khanh Nguyen · Updated November 4, 2026

Parameters

Name	Default	Unit
Span	8000	mm
Pitch	45	°
Overhang	450	mm
Member Thickness	35	mm
Chord Depth	97	mm
Web Depth	70	mm
Upper Pitch	20	°
Break Fraction	0.25
Slope	Math.tan((pitch * Math.PI / 180))
Upper Slope	Math.tan((upperPitch * Math.PI / 180))
Heel Height	chordDepth / Math.cos((pitch * Math.PI / 180))	mm
Upper Heel Height	chordDepth / Math.cos((upperPitch * Math.PI / 180))	mm
Break Position	span * breakFraction	mm
Break Height	breakX * slope	mm
Apex Height	breakZ + (span / 2 - breakX) * upperSlope	mm
Width	span + 2 * overhang	mm
Depth	chordThickness	mm
Height	apexZ + upperHeelHeight + overhang * slope	mm