Think of a Building Material as a solid material rather than just a surface.
It has a default fill that you see in plan and section and a default surface that you will see on all sides of the element in 3D (OpenGL only).
These Building Materials can be stacked together in a composite to form layered elements for walls, slabs and roofs that have many materials.
The idea is that you will now see each skin in these composites with the correct building material for each of those skins.
However it is possible to over-ride the surface of an element so you don't see the actual Building material but any surface material that you want (as if you have painted that surface.
Now just to complicate things you must be careful of files created in versions before 17. These did not have building materials so they will by default be in 'Legacy' mode when you open them.
This means you will not see the Building Materials but only the surfaces applied to each side and edge of an element.
Go into OPTIONS menu > Project preferences > Construction Elements to turn Legacy mode off.
So in your case either Legacy mode is on or your element has the surface being over-ridden by a brick surface (that has a brick hatch pattern associated with it) or possibly both.
With Legacy mode on the surfaces are controlled just like they were in 16 and before with the 3 long material buttons (5 buttons for beams).
With legacy mode off you can use the small button on the left to over-ride the surface material.
Barry.
One of the forum moderators.
Versions 6.5 to 27
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