I would love to see the ability to add spot heights to slabs so you can create more accurate paving and outdoor surfaces. Slab needs to maintain same thickness but would allow you to create accurate surface levels for paved areas outdoors. Mesh tool is only way currently to do this but you can only create the surface profile not the full profile of the slab. If not doable for slab tool, then is there a way to develop a constant thickness profile for the mesh tool (eg a concrete slab that is always 100mm thick but has spot heights so you can show accurate levels in Section & Elevation etc)
The thing is, if you could and nodes to the top of a slab and adjust the heights, it would no longer be a slab (no parallel surface top and bottom no longer horizontal - which I know is what you are asking for).
That means every time you add a node the slab would have to automatically split into separates sections for each plane (surface) - I am not sure that would be easy to program.
However, use a mesh as that is the best tool to adjust the top surface.
Now you have 2 choices.
1. use the 'Create Roofs from Mesh' add-on (part of the 'Goodies' add-ons) and that will create separate roofs for each slope - exactly what you want.
2. Copy and paste the mesh in exactly the same position.
Select one of the meshes and elevate it down the distance you want.
Perform a Solid Element Operation to subtract the lower mesh from the upper mesh.
You will have a mesh with parallel surfaces.
The advantage of 2 is it will still be a mesh, so is easy to edit further.
You just have to delete the lower mesh, copy & paste, lower and SEO again if you do make adjustments.
If using method one, you need to keep a copy of the original mesh in a hidden layer if you want to edit it again.
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I've personally used option nr 2 a few times but it can be confusing in floor play when trying to select one of them. It works very well for the advanced user who can keep track of 3D geometry but the reality in collaboration is that many don't care/accidentally change the wrong mesh.
It would be useful for meshes if they can have a new type - composite of which there would be two subtypes: 1. composite underside follows oveside surface and 2. composite underside has a different surface or is flat (like in Revit). In that case, you'd need to edit which composite materials have variable thickness. Would also be useful for flat roof drainage.