Best way to "Drape" Roads on Contours as Objects? Extension?

I’m familiar with the Drape tool however it only seems to drop an outline onto my geometry which isn’t very useful. Is there a different extension I can use that would let me create an object, drape it onto a contoured terrain, and leave it as that object? Draping lines onto land geometry is fairly useless to me. I know Clothworks exists for a drape function but I have to imagine there is something more specific.

What version of SketchUp are you using? Your profile says Make but you posted in Pro. It might be possible to do what you want but it depends on the version. (Please correct your profile so it is clear.)

Hey Dave I’m using SU Pro '21

Maybe you could fix this?
Screenshot - 7_12_2021 , 6_58_05 AM

Probably the easiest way to do what you suggest is to make a copy of the terrain, drape the road onto the copy and delete everything but the road. No extensions required beyond the native Sandbox tools.


I’ve used Dave’s way, then you can change parts of the road where it leaves the terrain, and your terrain model doesn’t get all messed up with the road. The road model has to be above the terrain a little. I’ve also used joint push-pull to give it a little edge You may need to sculpt the terrain prior to the road draping.

You may want to look at ValiArchitects InstantRoad plugin. Google it as
Chuck has an extensive website demonstrating his plugins.

I definitely understand the approach @DaveR but what happens when I need to move the road later? It feels like this method means you have to get the road right the first time and it’s hard-coded to the terrain object. Am I right in that or is there something else that gives me more flexibility if/when I want to make an edit?

I’ve never seen anything that would make a “flexible” road. It doesn’t take long to do what I did and it would be easy enough to keep a copy of the terrain or make a new road if the road needs to be moved.

OK so in this case if the terrain were modified (smoove tool that perhaps changed some nearby geometry) I’d have to re-buidl the road each time. I get the limitation just wanted to confirm there wasn’t some other trick to this that I was missing.

I guess I’d wait until the terrain was finished before I add the road.

If you want the road to be separate geometry from the rest of the terrain, you need to make it separate. You could just wait to cut the road out later.

@DaveR have you ever worked on a project assuming you had made the “final” edit only to find out later the requirements changed and you needed to make it final again? That’s what I’m worried about my friend! :grinning:

I think you would have to change it. You could drape it directly and it could change with smooving the terrain to some extent. I never found that to be nice since it makes a mess of the terrain geometry. I’d rather redo it.
I think 3ds max has a more flexible road procedure (from videos, I haven’t tried it).

Pretty much every project I’ve ever done goes that way. Very often with more difficult changes than this road would be.


So, somewhat related to this topic is the automation of altering terrain based on draping 2D planes. Roads are a good example (but cut and fill of earthworks for structures applies as well). If one models the starting terrain and then adds a new road / driveway, is there a way of automatically generating the altered terrain? To simply drape a road, will following the contours as they are which means the camber of the road won’t match a real-world scenario. A real-world scenario might have a steep bank or retaining wall to perhaps the outside radius of a road that is on the down-slope of the terrain to give the road a design, real-world surface. Then as the road turns down or up a slope, the retaining walls might be minimised. Up until this point (over almost 20 years of SU usage), I have always had to hack my model and add my own triangulated surfaces manually. (Fortunately I typically need a “look and feel” rather than true accuracy). I have seen the use of smoove but haven’t mastered it yet. So I am wondering if there’s any thoughts regarding automation of proposed contours / levels based on 2D draping.

With the road, the important parts are:

  1. The Centreline (the longitunal spline).
  2. the Cross section

What I do is I model that centreline, using the terrain as a base. I just use the basic Arc tools to make the centreline (and it’s in 3d). I save that as a Group. It’s important to keep the curves smoothly linked together.

Then, I create a 2d face that represents the profile of the road - the cross section. It will have a carriageway, kerbs, footpath, berm, etc.

I use the extension Eneroth Upright Extruder or Followme, to combine the centreline and the cross section into a 3d road.

The benefit of keeping them seperate is that they are easy to adjust…save a group containing just the centreline and cross section geometry and you can come back and edit either of them at any time, and then regenerate the road. The entire road building process should take less than a few minutes.

If you want to merge your road into the terrain then you need to create a border around it (I use an offset, Tools On Surface is a good extension for this). Then you can drape that border onto your terrain, and use Sandbox tools to merge the border edges with the edges of your road, to create a smooth “fill” (or cut) effect.

Once created, use Smoove to adjust the entire road up or down, and even add some camber.

You can then use the Copy Along Path extension (or Profile Builder) to add things like trees, streetlights, signs, at regular intervals.

If you are modelling “real” roads using camber (superelevation) then you need to get a bit more techical (you need two splines; the inner and outer).

Also, intersections (or similar flattish turning areas) are a little bit trickier because they don’t follow the Centreline/Profile method. As with real road construction it is best to start with these; they are somewhat flat, so you can model them in 2d and use the Sandbox tools (Stamp) to place it onto the terrain, then use Smoove to adjust and shape the vertical elements. Your road Centrelines should, of course, match into the centrelines of the intersections.

I should mention that it’s always very helpful to keep an origianl copy of the 3d terrain surface…! This process is designed to be quick, simple, repeatable… so using Smoove to mess around with the roads a lot, adjusting your Centreline, and then going back and creating the road again, is perfectly acceptable! I have designed many real roads in this way and I find that Sketchup with some “intuition” gets a very good result - the actual design of the road is as good as my engineers who have specialised tools. Of course, to get it consructed they need to redraw it using their tools - but they have an excellent base to start from!

( @eneroth3 Upright Extruder doesn’t appear to be compatible with SketchUp2021??)

All my extensions (except Eneroth open Newer Version) should be SketchUp 2021 compatible. Extension Warehouse 2.0 has just made it a lot more cumbersome (in practice impossible) to mark such large quantities of extensions as compatible. You can ignore these warnings.