Roof Pivot Line Placement

[Anonymous]

Just trying to see where people usually place their roof pivot lines with respect to the walls they are sitting on. Do you place the roof pivot line to the inside or the outside of the wall? Or do you place them elsewhere? The inside would give you a birdsmouth right? But it's not always correct. No method really seems entirely correct.

Any comments?

Hunter.

[Barry Kelly]

Hunter wrote:
Just trying to see where people usually place their roof pivot lines with respect to the walls they are sitting on. Do you place the roof pivot line to the inside or the outside of the wall? Or do you place them elsewhere? The inside would give you a birdsmouth right? But it's not always correct. No method really seems entirely correct.

Any comments?

Hunter.

The best you can do is "Model it as it is built".
Place the pivot line at the point where the rafter will intersect the wall top plate (or beam).
This may be the back (inside) of the wall or may be closer to the centre of the wall.
This will give you the best results if the roof pitch is ever changed - the roof will still be sitting on the wall (beam).
Of course the position of the pivot line (birdsmouth) may change as you alter the pitch so you may wish to relocate them.
It all depends on how accurately you want to model.
Barry.

[Gerald Hoffman]

Hunter I agree with Barry. I decide where to place it depending on how the roof is to constructed. If I are using prefab trusses for eg. I would place it on the outside as that is where the heel height is calculated from. (pivot point also if roof pitch changes ) If I am hand framing the roof and I want the rafters to have a full birdsmouth I would put it on the inside face.

Gerald

[Anonymous]

I have a somewhat different perspective. The roof pivot line should be placed at whatever is the known defining elevation of the roof. In some cases this will be the plate height, in others it will be the dripedge, and in others it may be the ridge. Of course I am referring here mainly to pitched roofs such as gable, hip & shed.

On simple gables plate height generally makes the most sense since this is usually the known value

This can also be true of hip roofs of all equal pitch, but gets complicated when the pitches vary (assuming the eaves are to align) so I usually use the drip edge pivot on these.

Shed roofs are often defined by where the top meets a wall or another roof, in which case I often use the ridge as the pivot. This can also be true for cross gables, especially as infill between other gables.

In really complex cases (I'm sure James Murray knows what I'm talking about) I have used all these methods (and more). Take for example a building with asymmetrical gambrels with gable infill, gable and shed dormers, plus round turrets and hipped porches of varying pitch (and throw in some flared shingle siding for good measure).

[Thomas Holm]

So it all cooks down to what was said before: Model as you build!

[Stephen Dolbee]

Same as Gerald.

[James Murray]

Place it so you, the modeler, has proper control over the roof. If that's the plate, so be it.

The pivot line is an AC thing; it is not 'real' in any way. It will not show in any output. The builder doesn't care, he cares if the geometry is constructible. There are lots of constructible ways to put a roof on a wall.

As the architect, are you going to let a slope change move the soffit up and down? I'm not. I'm going to find a way to get the roof I want and the soffit I want, in a buildable fashion. In the process of design development, you will place a roof, change the slope, elevate it, de-elevate it, round and round etc. It's our responsibility to hand over buildable stuff, regardless of tools.

I place the pivot line at the lower edge of the roof. I don't know what its elevation is, since the edge is often horizontal, or Solid Op'd. The handle just off the corner, to make the roof easier to select.

I'm all about modeling as they will build it, but the pivot line is beside the point.

It's like the ref plane of a mesh. Use Project Zero or Sea Level to set your Zs and don't worry about it.

IMHO,

[__archiben]

Matthew wrote:
The roof pivot line should be placed at whatever is the known defining elevation of the roof. In some cases this will be the plate height, in others it will be the dripedge, and in others it may be the ridge.

that about sums it up. i often find that the pivot line gets all out of shape once some editing has taken place anyway: once they're initially placed i tend to edit them in the 3D window - elevating, stretching and pivoting from there. where the pivot actually ends up is anyone's guess . . . just as long as the roof is in the right place, eh?

~/archiben

[Anonymous]

Thanks for all the answers. So there's no hard and fast rule. Figured as much.

Appreciated.

Hunter.