If you're diving into building a sandbox game, getting a roblox custom block system script up and running is likely at the top of your to-do list. It's one of those core mechanics that looks simple from the outside—you click, a block appears—but once you start digging into the code, you realize there's a lot of math and logic working behind the scenes. Whether you're trying to recreate a Minecraft-style builder or something more unique like a base-building survival game, you need a system that feels snappy and doesn't lag the server to death.
Why a Custom System Matters
You might wonder why you can't just use a basic "click to spawn" tool. The problem is that Roblox's default physics and placement can be a bit chaotic. If you don't have a structured script, players will end up overlapping blocks, floating them in mid-air, or clipping them through walls. A custom script gives you control over grid snapping, orientation, and validation.
Plus, building a system from scratch means you can define exactly what a "block" is. It doesn't have to be a cube. It could be a triangle, a furniture item, or a complex machine. When you write the script yourself, you aren't fighting against someone else's messy code; you're setting the rules for your own game world.
The Foundation: Raycasting and the Mouse
The heartbeat of any roblox custom block system script is raycasting. If you aren't familiar, think of a raycast like an invisible laser beam firing from the player's camera or mouse toward the 3D world. When that laser hits something, it sends back a bunch of data: where it hit, what object it hit, and the "normal" (the direction the surface is facing).
In your LocalScript, you'll want to get the player's mouse position constantly. You can use UserInputService or the Player:GetMouse() method. Personally, I prefer using Camera:ViewportPointToRay() because it's more modern and gives you a bit more control. Once you have that ray, you fire it out into the workspace. If it hits a surface, that's your target destination for the block.
Making it Snap to the Grid
Nobody wants to play a building game where blocks are placed at random coordinates like 12.348, 5.01, -99.2. It looks messy and makes building impossible. This is where the grid snapping logic comes in.
To make this work, you take the hit position from your raycast and round it to the nearest increment. If your blocks are 4x4x4 studs, you'd do something like this:
pos = math.floor(hitPosition / 4 + 0.5) * 4
By applying this to the X, Y, and Z coordinates, your block will always "jump" to the nearest 4-stud slot. It makes the placement feel professional and ensures everything lines up perfectly. You can even add a "ghost block" (a semi-transparent preview) that follows the mouse using this same math so the player knows exactly where the block will land before they click.
Handling the Client-Server Relationship
This is where most beginners trip up. You can't just spawn a block in a LocalScript and expect other players to see it. If you do that, the block only exists for the person who placed it. To make it "real" for everyone, you have to use RemoteEvents.
The flow usually looks like this: 1. The player clicks their mouse. 2. The LocalScript calculates the grid position and sends it to the server via a RemoteEvent. 3. The ServerScript receives that position, checks if it's a valid spot, and then creates the actual part.
Wait, why the validation check? Well, if you just let the server place a block wherever the client says, a script-kiddie could fire that RemoteEvent with coordinates at the other side of the map or inside another player's base. Always have the server double-check that the placement position is within a reasonable distance from the player.
Choosing the Placement Logic
One of the trickiest parts of a roblox custom block system script is figuring out how to stack blocks. If you just snap the mouse position to the grid, the block might try to spawn inside the floor.
To fix this, you use the "Surface Normal" I mentioned earlier. If the raycast hits the top of a block, the normal is (0, 1, 0). If it hits the side, the normal might be (1, 0, 0). By multiplying the normal by half the size of your block and adding it to the hit position, you can "push" the new block out so it sits perfectly on the surface rather than inside it. It's a small detail that makes the whole system feel way more polished.
Optimizing for Performance
If your game is going to have thousands of blocks, you need to think about performance early on. Every part in Roblox has a cost. If you have 10,000 individual parts with shadows and collisions enabled, the frame rate is going to tank.
One trick is to use Folders to organize blocks in the workspace. Another is to turn off CastShadow for smaller decorative blocks. Also, consider using a single script on the server to handle all placement requests rather than having a script inside every single block. The more "static" you can keep your blocks, the better the game will run.
Adding the "Juice"
A functional script is cool, but a fun script is better. This is what developers call "juice." When a block is placed, don't just have it pop into existence. Add a little sound effect—maybe a "thud" or a "click." Add a small particle effect like a puff of dust.
You can also add a tween to the ghost block so it slides smoothly between grid points rather than teleporting instantly. These tiny visual cues tell the player's brain that the system is responding to them, making the building process feel much more satisfying.
Common Pitfalls to Avoid
When you're writing your roblox custom block system script, watch out for these common headaches:
- Z-Fighting: This happens when two parts are in the exact same spot, and their textures flicker. Your snapping math needs to be precise to prevent this.
- Placement Inside Players: Always check if a player is standing where the block is about to be placed. You can use
GetPartsInPartorRaycastParamsto ignore the player's character during the check. - Infinite Range: Make sure you set a maximum distance for your raycast. You don't want players placing blocks on the moon while they're standing on the ground.
Taking it Further
Once you've got the basic placement down, you can start adding more advanced features. Maybe you want different types of blocks? You could send a "BlockType" string through your RemoteEvent. Want to rotate blocks? You can track a rotation variable in your LocalScript and pass that to the server too.
The beauty of a custom system is that it's yours. You aren't limited by the "Stamper Tool" or some ancient library from 2015. You can build a system that supports textures, custom shapes, or even blocks that have their own logic (like a furnace that cooks items or a light that turns on at night).
Writing a roblox custom block system script is a bit of a rite of passage for Roblox developers. It forces you to learn about 3D math, client-server communication, and optimization all at once. It might be frustrating when your blocks start spawning upside down or flying away, but once it clicks, it's one of the most rewarding systems to see in action. Just take it one step at a time—get the raycast working, then the grid, then the server event—and before you know it, you'll have a building system that works exactly the way you want it to.