Can I move the datum plane of a roof face, without moving the roof face object itself?

[line-weight]

 

I want to redefine where the datum is within a roof face, but I want the roof face to remain exactly where it is.

 

So, previously the datum was on the bottom surface of the ceiling, but I want it to be in between two internal layers.

 

The dialogue above initially looks a bit like the one you get when you replace or edit a wall style, which gives you a lot of options of what component to align in the old wall, with what component in the new wall. But actually it doesn't give you any of those options. When I click OK, the red line will stay in the same place in the model, and then the roof face will shift relative to it. So my ceiling will drop.

 

Am I right to think, there's not a way to do what I want? I want to redefine the red datum line from where it is in the left hand diagram, to where it is in the right hand one. But I want my ceiling to stay exactly where it is.

[Tom W.]

I don't think there's anything you can do about this. You just need to move the roof object afterwards. I often place 3D loci to mark the ridge/eaves before I perform anything like this so it's clear the roof has moved afterwards + where it needs to be moved back to.

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1 hour ago, Tom W. said:

I don't think there's anything you can do about this. You just need to move the roof object afterwards. I often place 3D loci to mark the ridge/eaves before I perform anything like this so it's clear the roof has moved afterwards + where it needs to be moved back to.

 

Ok, thanks. That's basically what I have done.

 

It would be good if roof face styles could be replaced/edited with the same level of control as is possible for walls.

 

Looks like the same applies to slabs.

[Tom W.]

1 hour ago, line-weight said:

 

Ok, thanks. That's basically what I have done.

 

It would be good if roof face styles could be replaced/edited with the same level of control as is possible for walls.

 

Looks like the same applies to slabs.

 

I'm not sure how you'd go about achieving this. For Slabs + Roofs the Datum marks where the Z-height of the object is measured to so it is not the same thing as the red lines in the Wall Replacement dialog, which are only there temporarily to aid alignment during replacement. I think it's correct that the Datum is the point of alignment when you are replacing one Slab or Roof for another but what you are doing in your example is slightly different: you are changing the point in the same object that the Z-height is measured to so it's only natural that doing this will affect the elevation of the object in the model after the change has been made. To include a control whereby you could do move the datum but effectively change the Z-height of the object as part of the same operation would I think introduce unnecessary complexity: wouldn't that be like asking Vectorworks, 'I am moving the point on the object that you are measuring its elevation to but for the purposes of this operation only, please measure to the top or bottom of the object instead, then go back to measuring from the datum'...?

 

Generally you want the Datum to have a fixed Z-height because it allows you to add material to the Roof above or below that point without it moving the Roof from where you want it in the model + in this respect it's doing the same thing as the red line in the Wall Replacement dialog.

 

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1 hour ago, Tom W. said:

To include a control whereby you could do move the datum but effectively change the Z-height of the object as part of the same operation would I think introduce unnecessary complexity: wouldn't that be like asking Vectorworks, 'I am moving the point on the object that you are measuring its elevation to but for the purposes of this operation only, please measure to the top or bottom of the object instead, then go back to measuring from the datum'...?

The datum is effectively a plane within the roof buildup. Say I have my datum set at bottom of joists but I want to change it to top of joists. The joists are 150mm deep, so I just want VW to offset that plane by 150mm but keep the roof object in the same place.

 

The reason I might want to do that is top keep the top-of-joists on the same plane and then increase or decrease the joist depth.

 

With something similar to what you can do with walls, I'd just ask for the old version of the roof face to be replaced with the new one such that the tops of the joists stay in the same place. That operation could be done without changing the datum within the roof object of course but it would be useful if I could then also move the datum within the object, because maybe I've decided that from now onwards that's where it's most useful to have the point of reference.

 

1 hour ago, Tom W. said:

For Slabs + Roofs the Datum marks where the Z-height of the object is measured to so it is not the same thing as the red lines in the Wall Replacement dialog, which are only there temporarily to aid alignment during replacement.

This is true yes - the diagrams look similar but they are showing different things. It might be helpful if they didn't look so similar.

[Tom W.]

24 minutes ago, line-weight said:

The datum is effectively a plane within the roof buildup. Say I have my datum set at bottom of joists but I want to change it to top of joists. The joists are 150mm deep, so I just want VW to offset that plane by 150mm but keep the roof object in the same place.

 

The reason I might want to do that is top keep the top-of-joists on the same plane and then increase or decrease the joist depth.

 

Sure I understand that. It's just that the Datum is the fixed point within the Roof build-up that the Bearing Height is measured to so I'm just not sure how it would work practically: we want the Datum to be fixed so that the Roof moves up + down based on the Bearing Height value yet we also want to be able to do the opposite + move the Datum without it affecting the height of the Roof? i.e. the Bearing Height value would automatically adjust by the same amount that the Datum was moved by: perhaps an 'Unlock Datum' check box?

 

I have to admit that I never actually use the Bearing Height parameter: I just ignore it + move the Roof up + down manually. It's way too complicated to figure out where it coincides with the Bearing Inset to be useful. But that's the way it's been designed.

[line-weight]

On 3/6/2025 at 5:40 PM, Tom W. said:

 

Sure I understand that. It's just that the Datum is the fixed point within the Roof build-up that the Bearing Height is measured to so I'm just not sure how it would work practically: we want the Datum to be fixed so that the Roof moves up + down based on the Bearing Height value yet we also want to be able to do the opposite + move the Datum without it affecting the height of the Roof? i.e. the Bearing Height value would automatically adjust by the same amount that the Datum was moved by: perhaps an 'Unlock Datum' check box?

 

I have to admit that I never actually use the Bearing Height parameter: I just ignore it + move the Roof up + down manually. It's way too complicated to figure out where it coincides with the Bearing Inset to be useful. But that's the way it's been designed.

 

Yes I see. I also never use the "bearing height". It's something to do with how roofs meet walls?

 

It would perhaps be interesting to know most people actually use roof face objects and also how they approach the setting-out of roof heights when it comes to construction drawings.

[Tom W.]

5 minutes ago, line-weight said:

It's something to do with how roofs meet walls?

 

Yes in theory you'd have a Wall style + a Roof style + they'd be set up to work together from the off. But I think in practise this would only work if you were creating multiple identical houses which I'm not. So I ignore the Bearing Height + Bearing Inset parameters in my Roof styles + focus only on the build-up/materials: I set up the Roof elevation + the interface with the Walls from scratch each time.