polyhedron difference renders ok but bad preview

Hi,
I'm intersecting two polyedrons (I double checked their faces definiftions), both taken singularly are previewed and rendered ok, but when I difference them the preview fails from certain angles, while the render is ok.
Am I missing something or could be a bug?

$vpr = [85, 0, 50];

//-------------------------------------
module single_roof(dim, sp)
   {
    // P2  P8    p2 +-------------+ p3       P9  P3
    //    \\        |      Y+     |           //
    //     \\    p1 +-------------+ p4 ->X   //
    //  P1 //P7     |      Y-     |       P10\\ P4
    //    //     p0 +-------------+ p5        \\
    // P0  P6                               P11  P5
    p0 = [-dim.x/2, -dim.y/2, 0];
    p1 = [p0.x, 0, dim.z];
    p2 = [p0.x, dim.y/2, p0.z];
    p3 = [dim.x/2, p2.y, p0.z];
    p4 = [p3.x, p1.y, p1.z];
    p5 = [p3.x, p0.y, p0.z];
    p6 = [p0.x, p0.y, p0.z+sp];
    p7 = [p1.x, p1.y, p1.z+sp];
    p8 = [p2.x, p2.y, p2.z+sp];
    p9 = [p3.x, p3.y, p3.z+sp];
    p10 = [p4.x, p4.y, p4.z+sp];
    p11 = [p5.x, p5.y, p5.z+sp];
    points = [p0, p1, p2, p3, p4,  p5,
              p6, p7, p8, p9, p10, p11];

    // faces are defined by points indices (CW)
    faces =
       [
        [ 0, 5, 4, 1 ], // Under Y-
        [ 6, 7,10,11 ], // Over Y-
        [ 1, 4, 3, 2 ], // Under Y+
        [ 7, 8, 9,10 ], // Over Y+
        [ 5,11,10, 4 ], // Side Y- X+
        [ 4,10, 9, 3 ], // Side Y+ X+
        [ 1, 7, 6, 0 ], // Side Y- X-
        [ 2, 8, 7, 1 ], // Side Y+ X-
        [ 0, 6,11, 5 ], // Side Y-
        [ 3, 9, 8, 2 ]  // Side Y+
       ];

    polyhedron(points, faces);
   }

//-------------------------------------
module single_roof_vol(dim)
   {
    dz = 0.001;
    //  p2 +-------------+ p3
    //     |      Y+     |
    //  p1 +-------------+ p4 ->X
    //     |      Y-     |
    //  p0 +-------------+ p5
    p0 = [-dim.x/2, -dim.y/2, 0-dz];
    p1 = [p0.x, 0, dim.z-dz];
    p2 = [p0.x, dim.y/2, p0.z];
    p3 = [dim.x/2, p2.y, p0.z];
    p4 = [p3.x, p1.y, p1.z];
    p5 = [p3.x, p0.y, p0.z];
    points = [p0,  p1,  p2,  p3,  p4,  p5];

    faces =
       [
        [ 0, 5, 3, 2 ], // Under
        [ 0, 1, 4, 5 ], // Over Y+
        [ 1, 2, 3, 4 ], // Over Y-
        [ 5, 4, 3 ], // Front X+
        [ 2, 1, 0 ]  // Front X-
       ];

    polyhedron(points, faces);
   }

// Intersection
difference()
   {
    color("DarkRed") single_roof([13,11,3], 0.3);
    rotate([0,0,90]) single_roof_vol([12,6,3]);
   }

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MG wrote
> Am I missing something or could be a bug?

https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#polyhedron

>         convexity
>
>             Integer. The convexity parameter specifies the maximum number
> of faces a ray intersecting the object might penetrate. This parameter is
> needed only for correct display of the object in OpenCSG preview mode. It
> has no effect on the polyhedron rendering. For display problems, setting
> it to 10 should work fine for most cases.





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