Traditional Roof Framing Geometry Layout Techniques 1

The traditional roof framing geometry used by the French and Germans carpenters of the past , represents the timber in three-dimensional visualizations( Stereotomy). This allows the carpenter to layout complex cuts from 2 dimensional plans that represent the true shape of the rafter or timber framing joint.

There are no English translations of the traditional roof framing geometry used by the French and Germans and this is best illustrated in the French book Traité Théorique et Pratique de Charpente, by Louis Mazerolle in 1866. The techniques used by Louis Mazerolle are vague and need a lot of research to follow his techniques and the written words in his book are hard to translate from French to English. The books I've boughten on Traditional Layout  have helped me to understand some of the techniques used by Louis Mazerolle, even though their written in French or German.

This article will explain what I believe, to be the basics of understanding the traditional layout methods. These 12 basic techniques must be completely understood to produce drawings that represent the true shape of  the timber on 2 dimensional plans.

Traditional Layout Basics 

1. Hip Rafter Offset -->Hip Rafter Shift --> Gratsparren Grundverschiebung
2. Hip Rafter Dihedral Angle for Hip Rafter Backing Angle --> Diederwinkel
3. Rafter Block Rotated Plumb To Roof Surface
4. Rafter Block Rotated Perpendicular to Hip Rafter Run Line
5. Rotating the Roof Surface From The Ground Line
6. Hip Rafter Internal Edge On Roof Surface
7. Jack Rafter Bevel Angles
8. Jack Rafter Miter Angles
9. Hip Rafter Level Line
10. Hip Rafter Backing Depth
11. Lip Angle Line (barbe line, beard line)
12. Lip Length Line-> Claw line-->Klaue 

1: Hip Rafter Offset

See my article on Hip Rafter Offset for examples of drawing the hip rafter offset. The hip rafter offset will align the edges of the hip rafter with the two roof planes on each side of the hip rafter. Each and every drawing should start with the hip rafter offset lines to establish the planning points of the hip rafter. 

2: Hip Rafter Dihedral Angle for Hip Rafter Backing Angle

The dihedral angle triangle establishes the hip rafter backing angles and it is used to draw out the rafter blocks rotated plumb to the roof surface and the rafter block that is drawn perpendicular to the hip rafter run line that establishes the dimension used to draw the hip rafter internal edge on the real roof surface that in turn establishes the jack rafter miter angles.

3: Rafter Block Rotated Plumb To Roof Surface

This rafter block is titled at the slope of the hip rafter backing angle in plan view. It is used to draw the 3 lines that represent the rafter being rotated into the roof surface.

4: Rafter Block Rotated Perpendicular to Hip Rafter Run Line

There different ways to establish the internal edge of the hip rafter on the roof surface, but I think this is the easiest way to remember and to establish this line. 

Rafter Block Rotated Perpendicular to the hip rafter run line. The 1.6105" dimension in this drawing  is used to draw the hip rafter internal edge on the roof surface. You can not draw out the jack rafter miter angle correctly without this dimension.

This next drawing shows the rafter block rotated on the other side of the hip rafter. The dimension used for the hip rafter internal edge on the roof surface is different for unequal pitched roofs.

5: Rotating the Roof Surface From The Ground Line

This should be a no brainer, but it's added here for clarity. The true shape of the rafter is developed from the roof surface.

6: Hip Rafter Internal Edge On Roof Surface

In this next drawing the hip rafter internal edge was drawn, on the roof surface, parallel to the hip rafter offset line using the dimension found from the rafter block rotated perpendicular to the hip rafter run line at the end of the dihedral angle triangle. The hip rafter internal edge establishes a line that determines the miter angle on the sides of the jack rafters.  

7:Jack Rafter Bevel Angles

The jack rafter bevel angle on the top and bottom of the jack rafter is always the same angle that is found on the roof surface drawing. You should always check these two angles to make sure your drawing the true shape of the jack rafter correctly.

8: Jack Rafter Miter Angles

The jack rafter miter angle on the side of the jack rafter is drawn using points 2&4 or 1&3. Points 3 and 4 are always on the hip rafter internal edge line.

9: Hip Rafter Level Line
The hip rafter level line is used to determine the backing depth on the side of an un-backed hip rafter.

10: Hip Rafter Backing Depth
The hip rafter level line and backing depth line is used to determine the placement of the plumb lines on an un-backed hip rafter.

11: Lip Angle Line
The lip angle line on the roof surface determines the points of intersection on jack rafters and purlins to draw out the lip cut angle on the jack rafters or purlins.

12: Lip Length Line

The lip line length determines the actual length of the lip cut on jack rafters and purlins.The lip line length line is determined by the point of intersection of the seat line in plan view and the seat line on the roof surface.