Often when someone is building a large MOC, there are hollow areas inside or underneath the creation that will not be seen when the MOC is completed. Many builders use Filler pieces to give structural integrity to this hollow space, thus making the creation stronger for transportation or allowing more structure to be built on top of it.
For example, someone might be making an office building with no interior (no floors, inside walls, etc.) A model of an office building that's hollow due to the lack of an interior could be fragile and might crumble if it were to be picked up by the sides. The builder could make a simple structural lattice inside the building using filler. The filler pieces would most likely be pieces that the builder has no other use for, pieces that the builder has scads of, or pieces in a color that the builder does not find useful. Likewise, someone who is making landscaping that is higher than baseplate level, might use filler as a quick way to raise the level of the landscape and then cover it all with green pieces.
Filler is rarely built solid. It is usually built as some kind of grid or a pattern of pillars that has a lot of hollow area but still provides good support for whatever is built on top of it. There are occasions when a builder uses filler that can actually be seen; this would typically be along the backside of a large MOC that faces away from spectators.
Filler plates are sometimes used by builders making a studs-out mosaic. If someone is making a studs-out mosaic, there may be colors that they have in only bricks or only plates. Therefore the builder will make the mosaic one brick thick. Then they can use two layers of filler plates underneath the plates of the desired color. This allows both bricks and plates to be used to make a mosaic with an even surface.
Common Pieces Used
Common pieces used as Filler include:
- 2X bricks in unwanted colors
- Printed bricks
- Modified bricks, such as ones with click hinge attachments or axle pins
A Note on Using Quatro as Filler
Some builders use Quatro as filler, but this must be done amongst layers of DUPLO bricks. Due to the structure of the bottom of a Quatro brick, it cannot be placed directly on top of a flat layer of LEGO System studs. However, Quatro can be placed on DUPLO studs. Therefore when someone wants to use Quatro pieces as filler, they must place a layer of DUPLO pieces, build Quatro on top of that, then add another layer of DUPLO on top of the Quatro. Once these three layers have been built, regular LEGO System 2X bricks can be put on top of the "DUPLO-Quatro sandwich."