Quarter Turn Helix



Hello again,

As usual I have run into an issue which is annoying and confuses me greatly! Anyway, I am drawing up a staircase which has quarter turn helical elements at each end (see the excerpt, from the whole model, I have attached!) but despite using the correct (I think!) heights, angles, radii and central axis for all the curves and the helix (orange) the helix still doesn’t seem to sit exactly in the same place on each tread leading me to believe I have calculated something incorrectly. Unfortunately this is my first time dealing with helical shapes and I am having trouble figuring out what I’ve done wrong. Any help would be greatly appreciated. Also I am aware that my description here is quite vague and probably not very easy to follow so if necessary I can try and give more detail!


forum.skp (744.1 KB)


You might try the parametric 3D shapes plugin from sketchucation.com/pluginstore. That has a range of helical shapes.

Try a Helical Ramp with Sides. Make it narrow - just the width you want for the helical stringer, with 90 deg slope for the sides, 0.25 rotations, and height and number of segments to match what you have in mind.

Bottom part of stairs

Thank you for the suggestion, I just downloaded it and had a go but I end up with the same result unfortunately. Also I should have mentioned initially that I used the plugin Curve Maker originally! This leads me to believe that rather than a Sketchup problem, I probably have a problem with my maths and geometry knowledge :open_mouth:



are you trying to follow your floor plan?



Hi, thanks for responding,

I am, roughly yes, I was given the plan as a guide for the staircase though there could be potential for change if it was necessary.



How about this, made using Helical Ramp with sides?

I edited the ramp after drawing it, to make the bottom surface a copy of the top, then deleted the bit from there to the ground. However, it is still a parametric shape, and if you R click and edit it, you can change its parameters to suit you, then re-copy the top surface to the bottom, and delete what’s below it again.

Quarter helix stairs.skp (721.4 KB)

See what you think.

Not sure what version of SU you are using (please complete your profile) so in case it isn’t the latest, I’ve saved it as v2016.

PS. Because the winder treads have a 2" straight section at the front edge, the top helical side (which I made the same as the bottom one) won’t cover the inner edges of the treads there, unless you were to make a horizontal 2" stringer in front of them. I’ve compromised a bit here, but it illustrates the principle.

And you’ll need to trim a bit off the botttom helix where it goes below the floor.

You can’t get a same-slope join between straight stairs and helix unless you considerably flatten the slope of the stairs with more treads and a longer horizontal dimension for the straight flight.

PPS. CORRECTION: And I THOUGHT your winder treads weren’t exactly 30 degrees - I just copied the plan. The angle between the edges is 30 degrees, but neither edge is radial from the centre of the support pillar (quite possibly by design). That means I’ve rotated the treads slightly too far, since I used the back outside corner of the tread to define the rotation angle. So the back of the top tread of the winder isn’t at 90 degrees to the front of the bottom tread, which will cause constructional problems. Sorry. got that slightly wrong but haven’t time at the moment to fix it.


Just re-read your original post. I don’t think you necessarily miscalculated - at least not any more than I did.

The front edges of the winder treads are about 2" in front of a radial line from the centre of the post. The back edges aren’t quite radial. The straight treads in the plan view don’t have any corresponding overlap at the front (though I gave them 1" in the 3D tread). The slope of the straight stairs is steeper than the slope of the helix where they join (as it should be - the slope roughly in the middle or between middle and outside of the winder should be about the same as the straight portion).

You should probably re-work the geometry a bit, make the stringer deeper, and allow properly (as I have not) for the ‘nosing’ (overhang of the front of the tread compared to the back edge of the tread below it) and the slope change in the stringer, at the junction of the top of the straight section with the bottom of the helical section.

Quite tricky to get right. Might return to this later tonight, if it would be helpful.


Wow, thank you for taking so much time to look through this! The file you created looks to follow the path a lot more accurately so I will use that method instead, plus I didn’t know you could edit afterwards, that’s useful to note!! Apologies for not updating my profile, I forgot it existed!! It should be up to date now. As for the transition between the stairs and the helix, that won’t be an issue since the balustrade will cover it completely (I can’t wait to draw that part :stuck_out_tongue:) once the model is complete. Also as you guessed the radial centre is not at the centre of the support because of the design (not mine). Don’t worry about fixing it, you’ve been super helpful already!! The other elements of the construction are a bit beyond me to be honest, I’ve just been tasked with drawing up certain parts of the framework for the staircase.

Thanks again for taking the time to look over this, it has been very helpful! I won’t have access to the file again until Monday so if you do decide to return to it there is certainly no rush, I’m already very grateful at how generous you have been with helping!



This version is a bit tidier - just finished it when I saw your most recent post.

Quarter helix stairs v2.skp (863.2 KB)

Best wishes


PS forgot to put in the risers - but that should be easy for you.


There is a 2D to 3D plugin that works for treads and risers.


Curved treads and volutes are no problem.


Thanks, I’ll take a look at that too, it looks like it could be useful!



Thanks a lot, it looks great! I’ll take a closer look on Monday when I’ve got the original model back but you may have saved my life!! :smiley:





Log on and go to download page.



Hi, just an update.

Thank you so much for all your help last week! I was able to sort it yesterday! It is looking much better now and will be much more accurate so hopefully less chance of an error being made during construction!

Thanks a lot!