Principes du dévers

[roberts]

I have been studying the video series "Principes du dévers: la sauterelle (partie 1)"
I can follow all except the last few minutes of the 5th part: coupe de niveau, where last herse is constructed.

Where do the lines I have marked 9 and 10 come from ? (No.11 I do understand, as a rabattement from 9 and 10)
See this link:
https://i.servimg.com/u/f59/19/07/18/10/ddep_s10.jpg

I could not follow the video commentary at that point !
Grateful for any help . . .
Best regards

[MichelV]

Rob,

the DP line is a projection on the ground of the hip rafter face. Knowing this you can imagine that the dévers de pas is a plane (see attachment: delimited on the plan view by the three points 1-2-3 (in red).
To find the angle of the seat cut on this face of the hip rafter you need to do the herse of the dévers de pas. The profile rafter of the dévers de pas is on the plan between points 2 and 3 (in red).
The line 9 is the lattis of this profile rafter and the line 10 is the axis of the same elevation. If you do the rabattement of this elevation you get the herse of the dévers de pas (delimited by the points 1-2-3' (in red).
The angle of the seat cut of the hip rafter is delimited by the two lines: 1-2 and 1-3' (in red).



For a better understanding I would say: The DP line on the ground is a sablière (in this case we say sablière d'emprunt meaning "that has no existence" and a profile rafter (here line 2-3) is ALWAYS perpendicular to the sablière. According to this when you draw the elevation of the profile rafter his axis line is parallel to the sablière ( here the DP line).

Am I clear enough ?

[roberts]

Thanks Michel, that was a great help and allowed me to complete the "exercise".
I have written the part of the development discussed here "in my own words" and this follows:

Let me call your 3' on the line 2-3-3'  = 3'' (three double prime to distinguish from 3').

We start with the dévers de pas (DP), line 1-2 extended and the centerpoint in plan of the roof (here point 3 but often labelled as K).
Draw line 2-3 extended, through centerpoint 3 and perpendicular to DP.
  Then 2-3 is the run of the DP profile rafter.
Draw 3-3' through 3 and perpendicular to 2-3 (we will use line 2-3 as a fold line; called sablière d’emprunt in French, very literally “virtual eave”).
  Then 3-3' is the rise line of the DP profile rafter (but whose length is not yet known).
Mark length 3-3' on 3-3'extended line as equal to actual height of roof (e.g. taken from hip rise or profile rafter rise)
  Then 3-3' is the rise line of the DP profile rafter in true length.
Join point 2 to point 3'.
  Then 2-3' is the lattis (line) of the DP profile rafter.
Draw an arc, center 2, radius 2-3' to cut the line 2-3 extended at 3''.

The plan of the DP plane (1-2-3) has been transformed into the herse of the DP plane (1-2-3''), via
the profile of the DP plane (2-3-3').

I now recognise this construction as a standard technique which occurs repeatedly in Le Trait.
In the above text I have tried to give a sort of recipe for it, but with some explanation of what is going on.
May be together our posts will help others . . . ?

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