Often, when people ask about how to prevent light leaking, the common answer is that your walls must be at least 4 studs thick, as voxels in Roblox are placed on a 4x4x4 grid.
This does not actually stop light leaking; you instead need your walls to be at least 6 studs (just over 1 voxel) thick. See below; on the left is 4 thick walls, and on the right is 6 thick walls (note that this is using Voxel):
Just a snippet of information I discovered while messing around in Studio 
I’m pretty sure (although haven’t tested this specifically) that it may also depend on the position of the walls in relation to the voxel grid.
If you move them smoothly, you should notice the shadow/unlit areas more or less snap from voxel to voxel.
Yup! In some cases you can almost get away with 5 studs if you manipulate it finely - but from my testing 6 is the minimum to work reliably everywhere.
Would it be possible to make it six studs, then union/negate the walls to be smaller, and still be completely dark. Just wondering.
Interesting idea. I don’t think it would work, but I’m not sure. Will have to test tomorrow 
Pretty sure the answer is no, but perhaps you can extend the union’s bounding box (while having the main shape look like it’s supposed to). Maybe it could trick the union into having a larger shadow.
(should work on MeshParts a well)
This is nice to know, but that’d also make for some ridiculously large walls for those who build in a realistic fashion. Roblox should be able to fix this where regardless of brick size, the the corners are closed off, no light should see through.
It’s sad we gotta go this far for the proper result!
Here’s a secret technique:
Put a 45 degree wedge in the corner.
It has to be 2x2 studs (and the length of the wall) to hide the leak completely if the walls are 4x4 aligned.
… and yeah we don’t like the fact that this problem exists. We’ll try to do something about this this year.
Out of curiosity, did the anisotropic occupancy stuff ever get added from that old hack week you did?
Specifically, this: https://youtu.be/z5TmqDtpwSM?t=469
Yeah it was and it’s active in Voxel (& ShadowMap) mode - you can test it yourself on Crossroads!
Unfortunately, it doesn’t fully solve leaking through corners.
I suspect that with the wedge trick you can actually make the walls/ceilings pretty thin with Voxel but the wedge will still have to be pretty large, even larger perhaps to compensate for thin walls.
edit actually the trampoline in Crossroads most likely will not correctly show the effect because in the video the voxels were 1^3 and we still use 4^3, but you can see it on thin objects that are further away from the receiver.
Is this still an issue or has it been fixed? I’m currently in the process of writing a Building Procedure and would like to include only truthful information.
It is definitely still an issue. I hope they fix it soon so we don’t have to use any wonky workarounds 
Its simply not applicable to a lot of builds. For example if you’re making a house with an interior, the walls have to be thin. You cant just extend them to fix the lighting cause it will mess up the visuals of the build and the size.
Is there any new ETA on this? It’s been like 4 years now.
Zeux is no longer with roblox.
I’m aware, I was hoping for another engineer to reply.
Yeah, I’m also awaiting news on this. Light leakage is such a problem in realistic builds, I’m honestly very disappointed it is still a bug.
