Roof Framing Geometry: Saint Andrews Cross

In this post I'll show you how to develop the geometry for the Saint Andrews Cross using the Folding Roof Surface and the Draw Down Roof Surface. The draw down roof surface should be used in the WorldSkills Carpentry Competition or when your drawing board is not big enough to draw out the folding roof surface. Using the draw down method you can fit most task models on a 4' x 8' sheet of plywood. It will require two 4' x 8' sheets of plywood for the drawing board to use the folding roof surface method.

This task model has a hexagonal ground plan.

The Saint Andrews Cross rafters are skewed at an angle of 45° from the ground line in plan view.

Develop the Profile Rafter and the the Folding Profile Rafter. Using the radius RP-GL - T1 swing an arc to F for the location of the seat line on the roof surface. Draw a line parallel to the ground line from point T for the seat line in plan view.

Draw the hip rafter in Profile and draw the hip rafter seat line back to the ground line.

Using radius RP-GL - S swing an arc back up to point S1 that is on the level line of the profile rafter.

Draw a line from S1 parallel to the ground line. This line represents the Internal hip rafter edge development line.

Draw a line parallel to the hip rafter on the roof surface that starts at the intersection of the Internal hip rafter edge development line at E1.

Draw the claw lines on the roof surface parallel to the hip rafter on the roof surface starting at the intersection of the hip rafter seat line and the ground line.

Draw perpendicular lines from the Saint Andrews Cross lines in plan view to the hip rafter on the roof surface.

Draw a perpendicular line from the seat line in plan view where it intersects the hip rafter seat line. This perpendicular line will intersect the seat line on the roof surface that establishes the claw line on the hip rafter on the folding roof surface.

Draw in the width of the Saint Andrews Cross on the folding roof surface.

Place the Saint Andrews Cross Timbers on the folding roof surface and mark the miter angles and claw angles on the side of the timber. Mark the back bevel angles on the top and bottom edges of the timbers.

Drawing showing how the timbers will look after cutting on the lines.

Saint Andrews Cross timbers with the crossing bevels cut.

Hip Rafter developed from profile drawing.

Hip rafter and Saint Andrews Cross rotated into the roof surface.

Bottom view of the Saint Andrews Cross rafter claws.

Here I've moved the ground plan off to the side of the drawing for clarity to show the draw down roof surface. To use the draw down roof surface you'll have to add two lines. One line is for the development of the claw line on the draw down roof surface and the other line is for the development of the Internal Edge of the Hip Rafter on the draw down roof surface.